John Douglas

Jean-Jacques Serrano

Damien Coraboeuf

Olivier Bouc

Claire Arnal

François Robida

Hormoz Modaressi

Martin Atkinson

Graham Vowles

Ian Holt

RISK ASSESSMENT FOR THE ROAD NETWORK IN THE FRENCH-ITALIAN BORDER REGION USING WEB SERVICES

The EC Framework Programme 6 Integrated Project Open Architecture and Spatial Data Infrastructure for Risk Management (ORCHESTRA, http://www.eu-orchestra.org/) is concerned with the creation of an information technology architecture and associated web services for risk management. Its aim is to improve the interoperability of risk management data and software and influence international software standards (such as those provided by the Open Geospatial Consortium) in the context of the EC INSPIRE directive. In particular, multi-risk and cross-border issues are addressed since these are important within the European context. In order that the outcome of the project is useful in real-life risk management situations a number of pilot projects are being undertaken to test different aspects of the developed service networks. It is also expected that these pilots will also demonstrate the utility of the ORCHESTRA project to end users. This paper discusses the pilot project being undertaken by BRGM, JRC and Ordnance Survey with the collaboration of local partners in the French-Italian border region between Nice and Genoa. The basis of the project is the evaluation of risk to the road network and the impact of road network disruption by, for example: earthquakes, landslides, floods, fires and chemical spills. Road closures can have a dramatic impact on the economic, social and functional life of a region and these impacts can spread far from the site of the blockage and, even, across international borders. The region chosen for this pilot project is especially prone to hazardous events and also, due to the lack of redundancy in the road network within the region, disruption to a major route can have a large effect. This pilot will seek to create a distributed network of web services that will access hazard, route and traffic information held within databases based at the data providers.

INTRODUCTION

In today's rapidly changing world it is becoming more and more important that risk coming from natural and hazards with a human-origin is correctly managed. Increasing urbanisation and the undertaking of high value projects means that, in some areas, risk is increasing. In addition, due to climate change there is some evidence that the hazard level due to weather-related hazards is increasing. As in all aspects of modern life information technology (IT) is playing an increasing role in risk management since it allows the undertaking of tasks and makes others easier to perform.

However, risk management in Europe and elsewhere is made more difficult by interoperability problems due to IT systems that cannot easily communicate with each other. Even those in organisations concerned with the same risk, e.g. two institutes concerned with different aspects of earthquake risk, may be running different computer systems. The problem is even worse when viewed across risk domains, e.g. IT systems concerned with industrial risk cannot often communicate with those concerned with earthquake risk, limiting the ability to undertake true multi-risk analysis. Multi-risk analysis is being increasingly recognised as an important tool because it gives a better overall picture of the level of and the interactions between risks in a region (e.g. [START_REF] Douglas | RISK-NAT (Module 4): Methods and tools for risk evaluation[END_REF]. In order to get around these problems, users often must spend valuable time reformatting files, for example, to the detriment of undertaking more fundamental risk management tasks. In addition, this effort is, in some sense, wasted since, in all probability it will need to be repeated when data is sourced from another organisation or when a new IT system is developed. These interoperability problems limit the work that can be achieved since it is difficult to access and use existing data; data which cost much time and effort to collect.

In Europe, these problems arise not only across different types of risk, but also across regional and national borders. In addition to interoperability difficulties, language differences also become an issue. It is often important to undertake risk management across borders because impacts of events are not limited by human borders and actions undertaken to mitigate risk are also not contained by political boundaries.

In order to address these difficulties on a large scale addressing the whole range of geographical data, an EC directive called INSPIRE (http://inspire.jrc.it) was launched in 2001. It is planned that this directive will be fully implemented by 2013 (implementation will begin in 2007). The directive is concerned with the setting up of a European Spatial Data Infrastructure, to implement metadata rules for data and services, to implement rules for harmonised spatial data specifications (exchange and update, ID systems, thesauri, key attributes, etc.) and to implement rules for network services (upload, discovery, view, download, transformation, etc.).

To develop these ideas the field of risk management, a Framework Programme 6 Integrated Project called Open Architecture and Spatial Data Infrastructure for Risk Management (ORCHESTRA, http://www.eu-orchestra.org) was launched in September 2004. In the following section the ORCHESTRA project is briefly introduced. The following sections are then concerned with the pilot implementation of an ORCHESTRA system in order to aid in undertaking risk assessments for the road network in the border region of France and Italy.

ORCHESTRA

ORCHESTRA involves 15 partners both from the risk management domain and from the IT industry. The purpose of the project is to design a platform-independent open service-oriented architecture (SOA) for risk management purposes, and implement it for Internet-based applications (web services) that can be called via the web to undertake a given process. They could be seen as subroutines located on the web to be used when needed by any user. The 'open' part of the project refers to the fact that the specifications of these services will be made public and free of charge so that they can be implemented by any interested party and can be accessed by all potential users. The actual procedure followed by the web service to perform the required task is not considered and will not be specified; however, the inputs and outputs to the service and the actions it must undertake will be specified. The philosophy behind this approach is that if the specifications are followed then risk management systems using these web services will be fully interoperable without needing to be created by a common entity. This philosophy has key advantages with respect to other approaches. For example, it is expected to favour the emergence of an operational market for risk management services in Europe, it eliminates the need to replace or radically alter the hundreds of already operational IT systems in Europe (drastically lowering costs for users), and it allows users and stakeholders to achieve interoperability while using the system most adequate to their needs, budgets, culture etc. (i.e. it has flexibility).

Interoperability of IT systems is a subject of much research and development in many countries. For example, the Open Geospatial Consortium (OGC) is an organisation that seeks to develop software standards in the field of systems concerned with geographical information. In addition, there are existing and under-development ISO and EC standards. ORCHESTRA bases its architecture on these standards and seeks to influence their development in order to make them applicable to risk management applications.

An additional advantage of the SOA approach in the risk management domain is that it allows applications to be updated with minimal effect on the services that call them. Procedures involved in risk management change often as experience is gained through disasters. Consequently it is important that the systems developed are agile allowing developments to be rapidly implemented in practice.

ORCHESTRA is divided into the following five subprojects (SPs):

• SP1: Management, exploitation, dissemination and consolidation This SP is concerned mainly with project management issues and will not be discussed here.

• SP2: User requirements and policy watch

This SP is concerned with the specification of the risk management requirements to which the develop services and architecture must conform.

• SP3: Open architecture

This SP is mainly concerned with the specification of the system architecture.

• SP4: Risk management services

This SP is mainly concerned with the development of risk management services through the implementation of the selected pilot projects.

• SP5: Community building and training

This SP seeks to develop links between the project and potential end users and to provide training on the use of the developed architecture and services to interested parties. This SP will not be discussed here.

So that it is firmly rooted in risk management applications, the ORCHESTRA project includes a considerable input from experts in different risks, within SP2 in particular. The risks particularly represented by experts involved with the project are: earthquake, systemic/industrial, floods, forest fires and coastal (oil spills etc.). SP2 is concerned with supplying the rest of the project with requirements, from the risk manager point of view, that the ORCHESTRA system must obey in order to be useful for risk management purposes.

The user requirements coming from SP2 have influenced the system architecture being developed by SP3 and the list of services that will be developed within the rest of the project within SP3 and SP4.

In the second half of the ORCHESTRA project four concurrent pilot projects concerned with different aspects of risk management in various European regions are being undertaken within SP4. The aim of these pilot projects is to validate the services and architecture developed during the course of the project and to demonstrate the results of the project to potential end users. In order to build on work already undertaken in earlier workpackages of the project and since it addresses truly multi-risk and cross-border aspects, BRGM, JRC and Ordnance Survey decided to develop a pilot project based on the risk to the road network from different hazardous events, such as earthquakes, landslides and industrial accidents. The following sections briefly introduce the problem and the work planned.

PILOT PROJECT ON RISK ASSESSMENT FOR THE ROAD NETWORK

Road transport plays an important role in the economic, functional and social life of a region. For example, roads enable the transport of commodities from their source (e.g. a factory) to the distribution centre (e.g. a shop) and the consumers from their homes to the distribution centre. Therefore, disruption to a road can have a dramatic impact and lead to extra costs, inconvenience and, within the post-event phase of the disaster cycle, difficulties in accessing affected communities. Roads can be disrupted (i.e. blocked) by a number of different events, for example: direct surface rupture or liquefaction caused by an earthquake, landslides (however caused), fire (forest or otherwise), floods, chemical incidences, avalanches, volcanoes and storms (through the falling of trees). [START_REF] Cova | Transportation Hazards, Chapter 17, Handbook of Transportation Engineering[END_REF] provide a good review of the types of hazard that can affect the road network and also their consequences. By focusing on the effect of an event, where the cause of the event is not specified, the proposed pilot is multi-risk. In addition, many of the services required to undertake the pilot are risk-neutral and can be used for various other projects.

As an example of the large impact that disruption to an important trans-frontier road can have, consider the closure of the Frejus road tunnel in the Alps between France and Italy due to a fire on 4 th June 2005, which killed two people. The average truck traffic each day through the tunnel was 3 800, all of which had to find alternative routes, such as through the Mont Blanc tunnel, which required a detour of about 200km. The tunnel accounts for about four-fifths of the commercial road traffic between France and Italy. The tunnel remained shut for about two months. This closure lead to large additional transportation costs. The three-year closure from 1999 to 2002 of the Mont Blanc tunnel due to another fire, which killed 39 people, was estimated to have cost the Italian economy 2.6 billion euros. [START_REF] Léone | Concept de vulnérabilité appliqué à l'évaluation des risques générés par les phénomènes de mouvements de terrain[END_REF] presents a detailed analysis of the effect of landslides on the road network in the region of Diois in France.

The geographical location of a pilot based on the effect of disruption to the road network is not important since roads are vulnerable in all regions of the world and also, by their very nature, are often cross-border (local, regional or national frontiers). However, the area chosen for this project is the department of Alpes-Maritimes (06) in France and the Piedmont and Liguria regions in Italy, where BRGM and JRC already have good contacts with potential end users. The cross-border roads in this region, for example the E80 (A8 in France and A10 in Italy) motorway from Nice to Genova, pass through mountainous areas and feature many tunnels and viaducts. These make them particularly susceptible to hazards that block the road because it is not possible to easily bypass or remove the blockage due to access difficulties. In addition, the mountainous relief in the region means the number of alternative routes is quite low. The map in Figure 1 shows the road network in the considered area.

In this pilot it is planned to particularly focus on the following three different situations that feature different types of instrument and data: the motorway (E80) from Nice to Genoa, roads in the valley of Roya and departmental roads (in order to integrate the departmental authorities in the project). This choice of three different levels of roads should allow potential problems with wider-scale implementation of the developed system to be highlighted.

USE CASES

In this pilot project it is planned to implement six general well-defined procedures undertaken by end users (commonly called 'use cases' in IT parlance). It is hoped that these six will test many aspects of the system and will demonstrate its ability to overcome current interoperability problems. Also the selected use cases involve varied interactions between different parties, e.g. different data providers and various end users. Figure 2 shows the relationships between the different use cases that will be considered. The following sections give brief details of these use cases. In order to aid with the implementation of these use cases their details have been transformed to Unified Modeling Language (UML). This pilot project is split into three phrases of about six months each, during which two use cases will be implemented.

ud Use Case Model

Find routes cut by a possible event

Calculate cost of route unav ailability

Motorway company

Regional authority DDE, Regional council, ...

Find the roads exposed to hazard

Hazard expert

Find alternativ e routes after a route is cut

Road transport company

Calculate losses of rev enue

Citizen

Maps of alternative roads

Calculate estimated time of unav ailability

Engineering company

Manages

Find roads cut by a possible event

In this use case, the end user retrieves data on the road network and the possible hazardous events in the region.

Then they simulate the triggering of an event and the system is used to identify which roads will be cut by such an event. This information is stored for use by other use cases.

Find roads exposed to hazard

In this use case, the end user collects data on the road network and maps of the level of hazard for different types of event (earthquake, landslide, flood etc.). From this information they compile an inventory of the roads that are at risk.

Find alternative routes after a route is cut

This use case consists of the following steps. Firstly the user retrieves road and traffic information for the region. Next they collect information on which roads will be blocked by an event (e.g. from the first use case) and they indicate within the road network information where these blockages are. Finally the system calculates alternative routes for different types of traffic and stores these routes for future use.

Calculate estimated time of unavailability

This use case simply retrieves from an external source an estimate of the amount of time that a road will not be usable. This estimate is stored for later use.

Calculate losses of revenue

In this use case, the user compiles information on the length of time that given roads will be unavailable (from the previous use case). Then they search for traffic that uses these roads and from combining these pieces of information arrives at an estimate of the loss of toll revenue due to the closure of the road.

Calculate cost of route unavailability

This use case combines information coming from previous use cases to estimate the overall economic cost of a road being unavailable. In the first step, the user collects information on the road network, the traffic that uses the unavailable roads and how long these roads will be unavailable for. From this information and the list of alterative routes for each traffic type the system computes an estimate of the cost of the road being blocked.

DATA NEEDED

The following types of data have been identified as having to be accessible via web services in order to fully be able to follow the use cases described above. Some of these data will need to be translated into French and Italian. Also due to organisational differences between France and Italy the levels of political divisions may vary between the two countries. It is planned to access these data by using ontologies and schema mappings that can take these variations in structure into account.

Hazard data

Information required on previous hazards will include: location polygon of area affected by previous event, type of previous event (landslide, earthquake etc.), date and time of event and severity of event (e.g. magnitude or macroseismic intensity). Information on many previous hazardous events that affected France are already available at BRGM in a form accessible via web services (e.g. SISFRANCE: http://www.sisfrance.net/ )

Traffic (per road considered)

Traffic information required will include: name of traffic type, number of vehicles of each type of traffic that use a road per hour/day etc. (this will vary with time of day/time of year etc.), average number of people carried by the vehicle type and types of roads that can be used by the traffic type.

Road network (per traffic type considered)

Road network information that will be required includes: name of road, type of road (motorway, national road etc.), name of organisation responsible for road, location of road and their connectivity, type of traffic that can use the road, road capacity (how many vehicles can use the road per hour/day etc.) and average traffic speed for different types of traffic (this varies with time of day/time of year etc.).

Political divisions

This data will be used to know which commune, department, region and country a road or event is within.

Topographical information

This information will be used as a base layer for overlaying other information over.

DIGITAL RIGHTS MANAGEMENT

An important part of this pilot project is a consideration of data security and the handling of access rights to data. In risk management, as in other domains, some data is sensitive and it is important that access to this data is well managed so that it can only be viewed by authorised users. In addition, there are important issues over payment and access rights to some types of information and services. All these important considerations will be investigated and will be implemented within the pilot in the following three step approach.

• Stage 1: Open Access Services -initial integration of services with no authentication and authorisation. The objective of this phase is to build the initial pilot capability and test the implemented services. • Stage 2: Authentication and Authorisation Services -installation and configuration of the ORCHESTRA Service Network Architecture Services for authentication and service authorisation. • Stage 3: Prototype Digital Rights Management Capability -this stage will address the higher risk aspects of implementing technical measures to manage and protect the content owners intellectual property. The content owners requirements will be understood during stage 1 and 2, and key elements of these requirements will be prototyped during this stage.

CONCLUSIONS

As in many aspects of IT, risk management is currently hampered by interoperability difficulties due to a variety of data formats, procedures, platforms etc. These difficulties are being addressed, on a general, scale by the ORCHESTRA project and, for specific cases, by a series of associated pilot projects. This brief article discusses the pilot project being undertaken by BRGM, JRC and Ordnance Survey concerned with assessing the risk to the road network in the France-Italy border region. The complete implementation of this pilot project should be completed by summer 2007.

Figure 1 :

 1 Figure 1: Location of the zone considered in the pilot project.

Figure 2 :

 2 Figure 2: Relationships between the different use cases that will be considered in the pilot project.

ACKNOWLEDGEMENTS

This work has been co-funded by the European Commission's IST programme, under Unit G5 "ICT for the Environment", through the Integrated Project FP6-IST 511678 ORCHESTRA (see http://www.eu-orchestra.org). We thank Steven Hosford for his comments on an early version of this article and José Esteban for his corrections and suggestions.


John Douglas
email: j.douglas@brgm.fr

Hideo Aochi
email: h.aochi@brgm.fr

Peter Suhadolc
email: suhadolc@dst.units.it

Giovanni Costa
email: costa@dst.units.it

ON THE APPLICABILITY OF ONE-DIMENSIONAL CRUSTAL STRUCTURES FOR GROUND-MOTION SIMULATION

) for the incorporation of the effect of crustal structure into empirical ground motion estimation equations is only practical for one-dimensional structures.

The purpose of this article is to investigate when two-dimensional structures provide significantly different results than using an average one-dimensional model. The difference between the 2D and the derived 1D structure is quantified by a single parameter that seeks to characterise how twodimensional the structure is. The maximum size this parameter can be before a 2D structure is required for accurate modelling of ground motions is assessed based on a series of FDM simulations. The purpose of this proposed technique is to provide guidance as to when 2D structures should be used or when 1D structures are sufficient, without the requirement to performing many simulations.

INTRODUCTION

Ground motion simulation methods, such as the finite-difference method (FDM) or the modal summation technique, require a model of the crustal structure through which the seismic waves pass in terms of density, velocity and attenuation parameters, such as Q. The use of such structural models within simulations means that travel path effects, such as the constructive interference of different phases, can be modelled. Currently one-, twoor three-dimensional models are used without much description of the benefits of using a particular dimension of model. Within FDMs, a 2D or 3D structure can be assumed without a significant increase in computational time. One-dimensional models have the advantage of yielding results that are easier to interpret in terms of phases and also they require only one set of simulations for all considered horizontal source and site locations. In addition, the method introduced by [START_REF] Douglas | On the incorporation of the effect of crustal structure into empirical strong ground motion estimation[END_REF] for the incorporation of the effect of crustal structure into empirical ground motion estimation equations is only practical for one-dimensional structures.

The purpose of this article is to investigate when two-dimensional structures provide significantly different results than using an average one-dimensional model. The difference between the 2D and the derived 1D structure is quantified by a single parameter that seeks to characterise how two-dimensional the structure is. The maximum size this parameter can be before a 2D structure is required for accurate modelling of ground motions is assessed based on a series of FDM simulations. The purpose of this proposed technique is to provide guidance as to when 2D structures should be used or when 1D structures are sufficient, without the requirement to performing many simulations.

PREVIOUS STUDIES

There are few published studies that examine the difference in simulated ground motions from using a 3D or 2D model as opposed to a 1D model (sometimes referred to as a plane layer model) except for examining near-surface effects such as basin effects (e.g. [START_REF] Bard | A numerical study of the variations of ground motion parameters across twodimensional sediment-filled valleys[END_REF]. [START_REF] Heaton | A study of the strong ground motion of the Borrego Mountain, California, earthquake[END_REF] model the strong-motion displacements recorded at El Centro from the 1968 Borrego Mountain earthquake. They note that the thick sediments under the receiver are not present in the source region and therefore the crustal structure along the travel-path is 2D. However, since the basin structure near El Centro is relatively flat and homogeneous and because the major effect of sediments is to allow post-critical angle multiple reflections to occur near the receiver [START_REF] Heaton | A study of the strong ground motion of the Borrego Mountain, California, earthquake[END_REF] are able to model the displacements (dominated by SH type motion) reasonably accurately by adopting a simple plane layer over a half-space model of the receiver region. They find that it is not necessary to include intermediate and lower crustal structure to model the El Centro strong-motion record. [START_REF] Heaton | Predictability of strong ground motion in the Imperial Valley: Modeling the m4.9, November 4, 1976 Brawley earthquake[END_REF] conduct a similar study to model displacements from a smaller earthquake in the same region and find that a 1D model is sufficient. This paper is also important since the authors find that wave propagation effects (due to the crustal structure) should be included in studies of source spectra and seismic wave attenuation.

To model the same strong-motion displacements as [START_REF] Heaton | A study of the strong ground motion of the Borrego Mountain, California, earthquake[END_REF], [START_REF] Swanger | Simulation of strong-motion displacements using surface-wave modal superposition[END_REF] overcome the problem of a slight change in crustal structure at the source and at the site by using a 1D from a point allow the travel path from source to site. However, they do not provide a objective method for choosing this model. They believe that they are able to accurately model the displacements at El Centro since the crustal between the source and the site is uniform laterally. [START_REF] Dreger | Broadband modeling of local earthquakes[END_REF] investigate the difference in ground motions using a simple 1D structure and a series of relatively simple 2D structures representing the Los Angeles area. They find that: a ridge structure beneath the recording station acts as a low-pass filter; a ridge structure between the source and receiver dramatically alters the motions; and that a basin structure leads to the trapping of energy in the basin which escapes leading to much higher ground motions. [START_REF] Campillo | Attenuation of crustal waves across the Alpine Range[END_REF] model the propagation of waves across the Alps using both a flat layer model and a 2D model and find considerable differences with ground motions from the 2D model being much less than those from the 1D simulation. However, they find that the observed Lg decay is much stronger than that predicted by the simulations.

Similarly [START_REF] Gibson | Numerical simulation of high-and low-frequency Lg-wave propagation[END_REF] model the propagation of Lg waves within a flat layer structure, across the Pyrenees and also within a structure with a Moho step. Again significant differences in ground motions are found between the different simulations but they do not closely match the strong Lg attenuation observed within recorded seismograms. [START_REF] Stidham | Three-dimensional structure influences on the strong-motion wavefield of the 1989 Loma Prieta earthquake[END_REF] compute synthetics for a 1D and 3D velocity model of the San Francisco Bay region where 3D effects are particularly important. They find that there is significant difference in the simulated ground motions and that the ground motions from the 3D model match the observed ground motions from the Loma Prieta earthquake better than those from the 1D model. [START_REF] Kennett | The influence of 3-d structure on the propagation of seismic waves away from earthquakes[END_REF] use a 3D model to simulate ground motions ground motions from two Japanese earthquakes (the 1946 Nankai and the 2000 Tottori-ken Seibu earthquakes) and find that it helps the match between the simulated and observed ground motions. [START_REF] Komatitsch | Simulations of ground motion in the Los Angeles basin based upon the spectral-element method[END_REF] find that for the Los Angeles basin 1D structures are not appropriate and lead to underestimations of the ground motions by up to 20 times because the effect of basin resonance is not able to be modelled. A detailed 3D model of the basin, however, leads to good estimations of the ground motions. [START_REF] Laurenzano | Numerical modeling of the 13 December 1990 M5.8 east Sicily earthquake at the Catania accelerometric station[END_REF] modelled the recorded ground motions at the Catania station from the 13th December 1990 M5.8 east Sicily earthquake using a detailed 2D and a variety of 1D structures and find that the 2D structure leads to time-histories that match the recorded motions much better than those computed using the 1D structures.

UTILITY OF 2D SIMULATIONS

The purpose of this article is to propose criteria that can be used to decide whether the crustal structure in a region is sufficiently one-dimensional that simulations using a one-dimensional model of the crustal structure in the region provide accurate ground motion estimates. The following method is proposed to define such a criterion that allows the utility of 2D simulations to be known by just examining the crustal structure model.

The first step of the proposed method is to consider an earthquake located within a two-dimensional crustal structure and a series of stations located at given distances from the earthquake. Next compute two simulations for each station: one using the 2D structure and the second using a velocity-averaged 1D structure (where velocityaveraged means that at each depth the velocities within the 2D structure are averaged from the horizontal location of the source to the horizontal location of the site). During the velocity-averaging process calculate the standard deviations of the velocities within each layer. Average these standard deviations in some way to compute a number that characterises the 1D-ness of the structure between the source and the site. Repeat these last two steps for different receiver points. Plot the decay curves derived from the 1D and 2D simulations and underneath the computed standard deviations of the averaging process. Examine the decay curves from the 1D and 2D simulations to see where they deviate from one another ad at which value of the standard deviation this occurs. The limit defines the criterion for assessing when 1D simulations are sufficient.

SIMULATION SCHEME

For the simulations here the following simulation scheme was used, which is a similar scheme to that used by [START_REF] Douglas | The importance of crustal structure in explaining the observed uncertainties in ground motion estimation[END_REF] to model ground motions in three French regions. A standard staggered finite difference method (FDM), which is fourth order in space and second order in time [START_REF] Levander | Fourth-order finite-difference P-SV seismograms[END_REF][START_REF] Olsen | Simulation of three-dimensional wave propagation in the Salt Lake Basin[END_REF], is used. A time-variable double-couple point source with a strike of 0• is introduced, using the technique of [START_REF] Olsen | Simulation of three-dimensional wave propagation in the Salt Lake Basin[END_REF] and [START_REF] Graves | Simulating seismic wave propagation in 3D elastic media using staggered grid finite differences[END_REF], at the origin of the model region. The mechanism-independent equation of [START_REF] Wells | New empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement[END_REF] for subsurface fault length was used to estimate the source duration assuming a rupture velocity of 2.5km/s, giving durations of 1.30 s for the source-time functions of the simulated earthquakes of M w 5.0. The source time function is described by a locally supported B-spline function of order 4 (degree 3).

Ground motions are calculated within a halfspace (-10-150 km by -5-25 km by -40-0 km) at spatial grid points with 500 m spacing using a time step of 0.02 s, which is sufficient for the assumed source-time function and the wave velocities in the crustal models. Synthetic seismograms are obtained at different epicentral distances from 0 to 145 km every 5 km in the x direction, perpendicular to the fault strike.

Because the assumed source model (point source with a simple smooth time function) poorly simulates the high frequency content of the motions it was decided to study the decay of peak ground velocity (PGV).

CONSIDERED CRUSTAL STRUCTURE

For this preliminary investigation of the importance of 2D structure, the model for the Pyrenees by [START_REF] Souriau | A tomographic study of the lithosphere beneath the Pyrenees from local and teleseismic data[END_REF] was used for a source at 42.25N-0.25E and stations at regular distances up to 43.25N-1.5E (about 140 km). Figure 1 displays the 2D shear-wave velocity crustal structure considered. The 2D cross section along this path features a high velocity zone that is coincident with the Pyrenees mountains. The model of [START_REF] Souriau | A tomographic study of the lithosphere beneath the Pyrenees from local and teleseismic data[END_REF] is not constrained for depths less than 3km therefore the velocity at 3km is assumed to be constant until the surface. This will probably lead to lower amplitude ground motions that occur in practice in the Pyrenees but this is not important for the purpose of this article. As discussed above the 2D structure between the source and the series of receivers at different distances was velocity-averaged to obtain a series of 1D structures for which to use for the simulations. This averaging was conducted from the source (left-hand side) to the site (right-hand side).

Published density estimates for the Pyrenees are not common. Therefore the commonly adopted relation of [START_REF] Gardner | Formation velocity and density -The diagnostic basics for stratigraphic traps[END_REF]: ρ = 1.741V p 0.25 , where V p is P-wave velocity in km/s and ρ is density in g/cm 3 , was adopted. Minor changes in the density values will not significantly affect the results. Q = 300 is assumed for the entire medium, but, in fact, this leads to no significant anelastic attenuation in the simulations.

RESULTS

Figure 2 shows the simulated PGVs with respect to distance using a 1D structure that was computed by averaging the 2D structure between 0 and 140km and the true 2D model. As can be seen the simulated PGV values from the two simulations do not match after a distance of about 60km where the 2D structure features significant variations in velocity.

Figure 3 shows the PGVs simulated using a series of 1D structures with velocities equal to the average from the epicentre to the station (as described above) and also the PGVs from the full 2D simulations. On the same figure the standard deviations (σ) from the velocity averaging process are displayed. The standard deviation seeks to measure how two-dimensional the structure is. This figure shows that the results obtained using the two structures are similar until the averaging process means that the 1D and 2D structures are no longer comparable (measured by σ for the first layer higher than about 0.05) at a distance of about 60km corresponding to the high velocity zone in the 2D structure shown in Figure 1. After this threshold is passed the simulated PGVs begin to diverge although since the 2D structure does not feature large lateral variations in velocity (see Figure 1) the results from the 1D and 2D simulations do not differ by much (the maximum difference is about 40%). One interesting result is that the results from the 1D simulations using the velocities averaged up until the station better match those computed using the 2D structure compared with those computed using the velocities averaged up until the end of the grid at 140km. This shows that for lightly 2D structures like examined here the utility of conducting fully 2D simulations is limited and good results can be obtained using correctly averaged 1D structures.

DISCUSSION

The results obtained above suggest a criterion for assessing the need to conduct two-dimensional as opposed onedimensional wave propagation simulations by simply examining the crustal structure model for the region. However, it should be noted that this criterion is based on the results of simulations using only one crustal structure model and hence they must be verified using a variety of different structures to assess how universal the criterion is. A couple of other open questions are the following.

• Is the standard deviation of the velocities along layers at constant depth a good way to characterise the error in averaging 2D structures? • How can the standard deviation of each layer be averaged to take into account that certain layers are more important than others in modifying ground motions (e.g. for short distances layers below the hypocentre are not important)?

Figure 1 :

 1 Figure 1: Two-dimensional crustal structure for shear-wave velocities (see labels on contour lines) used for simulations. Velocities extracted from the model of Souriau and Granet (1995) along a path 42.25N-0.25E to 43.25N-1.5E.

Figure 2 :

 2 Figure 2: Simulated peak ground velocity using the uniform (averaged up to 140km) velocity-averaged 1D structure and the true 2D structure.

Figure 3 :

 3 Figure 3: Simulated peak ground velocity using the velocity-averaged (averaged every 10km) 1D structure and the true 2D structure (top figure) and the standard deviation (σ) from the velocity averaging process for the five layers.

ACKNOWLEDGEMENTS

This study was funded by internal BRGM research projects. Financial contributions from INTERREG IIIB Alpine Space project SISMOVALP and ProCiv-INGV 2004-06 Executive project S4: 'Stima dello scuotimento in tempo reale e quasi-reale per terremoti significativi in territorio nazionale' are also acknowledged. We are grateful to Annie Souriau for sending us her velocity model for the Pyrenees in electronic form.


John Douglas
email: j.douglas@brgm.fr

STRONG-MOTION RECORDS SELECTION FOR STRUCTURAL TESTING

When structures are tested against earthquake loading, either within laboratories or by computer modelling, observed strong-motion records are commonly used as input. However, the method to select records is often not stated and it is thought that records are often chosen simply because they are well-known. Until the early 1990s, strong-motion data was relatively difficult to obtain and process and, in addition, the amount of, particularly near-source, strong-motion was limited. Therefore using standard records was often the only option. However, since then a number of large well-recorded earthquakes have occurred and also there are many new strong-motion data sources available.

As a snapshot of the state-of-the-practice for record selection for structural testing in Europe the proceedings of the last European Conference on Earthquake Engineering (12ECEE) was used. Using those articles concerned with structural testing where strong-motion records are used, a summary of the most popular records chosen and, if known, why they were selected, is given. It is found that a small subset of 13 well-known records is often used, irrespective of other considerations. The mode of the number of input accelerograms used per study is three.

Strong-motion records within structural testing could be used simply to validate that the shake table or the computer model functions and to show general aspects of the structures' response. For such applications the choice of the strong-motion records is not important and the current method of using well-known records could be maintained. However, more often structural testing is employed in order to understand the characteristics of the modelled structure and to estimate its vulnerability to earthquake shaking. For this purpose a more scientific method of choosing input time-histories is required. The procedure suggested here recognises the observed variety with respect to amplitude, frequency content, duration and shape of earthquake ground motions in order to better understand the characteristics of the structure.

INTRODUCTION

In many articles on earthquake engineering the authors undertake testing of their considered structure within the laboratory and/or by using numerical modelling. In the past it was common to use harmonic signals, simple pulses or white noise as input time-histories since they were easy to apply, their results were relatively straightforward to interpret and strong-motion data was limited (e.g. [START_REF] Newmark | Fundamentals of earthquake engineering[END_REF]. It has now become common to use observed strong-motion records as input to the structural models. For example, about 40% of the papers on structural aspects of earthquake engineering presented at the last European Conference on Earthquake Engineering in London in 2002 used at least one natural (modified or not) strong-motion record. At present, however, most papers focussed on structural modelling do not present their criteria for selecting the strong-motion records they use nor are the records they use optimal for gaining an understanding of the behaviour of the modelled system. Most studies are mainly concerned with the effect of structural parameters on the structure's behaviour rather than on the importance of ground-motion characteristics even though shaking characteristics have a large effect on the structure's response. These effects cannot fully be modelled by simply scaling the same time-history to different values of peak ground acceleration (PGA) as is commonly done.

The aim of this paper is not to propose a new scheme for systemic selection of records for proposed engineering projects [see, e.g., [START_REF] Bommer | The use of real earthquake accelerograms as input to dynamic analysis[END_REF] for a discussion of these procedures] but to encourage authors of studies on the analysis of structures presented at conferences or in journals to be more selective in their choice of input time-histories. A better selection of time-histories would enable significant extra value to be extracted from their work without requiring much extra computational or analysis effort on their part. In addition, it would allow a more accurate assessment of the sensitivity of their results to the ground motion, which is currently impossible when a very limited number of input accelerograms is used.

This article begins with a summary the results of a survey of the studies on structural modelling presented at the last European Conference. Following this, an improved method for the selection of natural accelerograms is proposed and an simple example application is presented.

SURVEY OF TIME-HISTORIES CURRENTLY USED IN STRUCTURAL TESTING

The articles in the Proceedings of the Twelfth European Conference on Earthquake Engineering from 2002 were used as a snapshot of the state-of-the-practice of structural testing for earthquake engineering. The focus here is on analysis of masonry, concrete and steel structures. Analysis of geotechnical structures is not considered. Articles from the theme 'Structural Earthquake Engineering -Analysis & Design' were consulted. In total, 188 articles were included in the survey. Of these, 72 articles (about 40%) used natural accelerograms (modified or not) as input to their analysis. Some studies (e.g. [START_REF] Gomez | Seismic evaluation of a truss bridge with energy isolation devices[END_REF] have used simulated strong-motion records since no records existed for their site of interest and a significant number of others (e.g. [START_REF] Isakovic | Seismic isolation of viaducts in Slovenia[END_REF] have generated synthetic records to match a design spectrum. These studies are not considered further here. The interested reader is referred to [START_REF] Bommer | The use of real earthquake accelerograms as input to dynamic analysis[END_REF] for a discussion of this type of approach.

The mode of the number of acceleration time-histories used per study is three and the range is one to 80 records. Table 1 presents a summary of the strong-motion records commonly employed in structural testing along with basic event, path, site and strong-motion parameters of each record. Many studies use only a single record, often the NS component of the famous El Centro 1940 accelerogram. Independent variables and strong-motion parameters for the records listed in Table 1 were taken from the database of the PEER NGA project (http://peer.berkeley.edu/nga/index.html) or from [START_REF] Ambraseys | Dissemination of European Strong-Motion Data[END_REF] except for the parameters of the Hachinohe record.

Many of the time-histories commonly used are from analogue instruments that can feature significant longperiod noise (e.g. [START_REF] Boore | Processing of strong-motion accelerograms: needs, options and consequences[END_REF]) and hence examining response displacement time-histories produced by such motions could be inappropriate unless the long-period noise present in the records is removed by careful processing.

Due to the high sensitivity of results from structural modelling to the set of input acceleration time-histories used (e.g. [START_REF] Kwon | Sensitivity of analytical vulnerability functions to input and response parameter randomness[END_REF] it is important that a reasonably large set of accelerograms is employed so that the uncertainty in the results can be assessed. However, many of the articles surveyed here used only a single input time-history and hence the sensitivity in the results obtained with respect to ground-motion characteristics cannot be assessed. In addition, the heavy reliance on the El Centro and other commonly chosen records could mean that results obtained are biased and their uncertainty could be underestimated. is the distance to the surface projection of the rupture (Joyner-Boore distance), d e is the epicentral distance, Vs30 is the average shear-wave velocity in the upper 30m, PGV is peak ground velocity and PGD is peak ground displacement. For records from [START_REF] Ambraseys | Dissemination of European Strong-Motion Data[END_REF] and for the Hachinohe record strongmotion parameters are reported for the component with the largest horizontal PGA. For other accelerograms the geometric average of the two orthogonal horizontal components orientated randomly is given.

If a strong-motion record is simply used within an analysis to verify that the numerical (e.g. finite element analysis) or experimental test works then the selection of which time-history to use is not important. However, in order to fully test the procedure it is suggested that ground motion records that produce large demands on the structure are selected in order that the simulation reaches the nonlinear range.

In a few studies, which accelerograms are used as input is not stated at all. This information is required by readers of the article in order that the results can be properly interpreted and also so that the experiments undertaken can be repeated by future researchers, which is an important requirement in scientific experiments.

Similarly some studies simply state the name of the earthquake or recording site when introducing the timehistories used. In strong-motion databanks there are often numerous accelerograms of the same earthquake from different stations and multiple records from the same station of various earthquakes. Therefore this lack of precision does not help to understand the results of the study nor to repeat it. Also often the use of a strongmotion record is reported without stating which component of horizontal motion was used. Since usually two perpendicular horizontal components are recorded that can display significantly different motions it is important to state which component is chosen.

Some studies are conducted to investigate the damage caused to a structure during a previous earthquake using strong-motion records that were recorded at the structure itself (e.g. [START_REF] Bessason | The south Iceland earthquakes of June 2000 -recorded response of retrofitted base isolated steel arch bridge[END_REF], at a site close by (e.g. [START_REF] Akbas | Seismic behaviour and push-over analysis of M.Inan viaduct[END_REF] or have characteristics of ground motions that were thought to have occurred at the site. This method for time-history selection is obviously only an option if the study is conducted after a damaging earthquake during which accelerograms were recorded.

Geographical selection criteria

One criterion adopted by many authors is geographical proximity of the earthquake to the site of the considered structure, although often this preference for records from close to home is not explicitly stated. Geographical proximity would be a important criterion if earthquake ground motions display high regional variability. However, this variation has not yet been shown to be real for many regions and therefore adopting regional proximity as a criterion is not currently justified and will often mean choosing from a small subset of available records. Douglas (2004a) considers ground motions from five regions of Europe (the Caucasus region, central Italy, Friuli, Greece and southern Iceland) and by applying analysis of variance to data binned into similar magnitude and distance intervals concludes that there is little evidence for inter-regional variations in ground motions within Europe. [START_REF] Douglas | Use of analysis of variance for the investigation of regional dependence of strong ground motions[END_REF] applies a similar method to data from Europe and California and finds that ground motions could have, on average, slightly higher amplitudes in California than in Europe and therefore perhaps it is best not to mixed data from these two regions. On the other hand, [START_REF] Ambraseys | Equations for the estimation of strong ground motions from shallow crustal earthquakes using data from Europe and the Middle East: Horizontal peak ground acceleration and spectral acceleration[END_REF] find that predictions from their ground-motion model derived using only data from Europe and the Middle East matches predictions of models derived using Californian data and also peak ground accelerations observed during the Parkfield 2004 earthquake.

Ground motions in stable continental regions (SCRs) are richer in high-frequency motions than those in more seismically active regions (e.g. [START_REF] Campbell | Prediction of strong ground-motion using the hybrid empirical method and its use in the development of ground-motion (attenuation) relations in eastern North America[END_REF] and hence an analysis made for a structure in such a region should ideally use input accelerograms from SCRs. However, at present there is little ground-motion data of engineering significance from such regions (e.g. [START_REF] Lubkowski | An evaluation of attenuation relationships for seismic hazard assessment in the UK[END_REF]. One of the few studies to use records from SCRs in the 2002 proceedings is the study by Tremblay & Bouatay (2002), who use such records since the study is concerned with structures in Canada, part of which is an SCR.

Ground motions from subduction-zone earthquakes, especially shallow thrust events occurring on the interface between the two plates, display different characteristics to those from shallow crustal earthquakes (e.g. [START_REF] Atkinson | Empirical ground-motion relations for subduction-zone earthquakes and their application to Cascadia and other regions[END_REF] and hence only records from the tectonic type of events likely to be important for the considered structure should be selected. [START_REF] Amadio | The effect of repeated seismic actions on structures[END_REF] select three accelerograms (El Centro 1940, Romania 1977and Mexico City 1985) with varying frequency contents (shown by differences in their response spectra) in order to investigate the effect of frequency content on structural response. Similarly other authors have chosen records with varying characteristics (e.g. [START_REF] Aziz | Nonlinear secondary systems interactions in buildings[END_REF] although neither the selection procedure nor which characteristics vary are usually stated. Some studies (e.g. [START_REF] Liao | Earthquake responses of steel moment resisting frames subjected to near-fault ground motions[END_REF]) make a distinction between near-field and far-field records because these two types of motions induce different types of response in the modelled structure.

Selection based on ground-motion characteristics

De [START_REF] De Matteis | Seismic performance of MR steel frames stiffened by light-weight cladding panels[END_REF] and other authors select sets of a handful of time-histories that have an average response spectrum similar to Eurocode 8 or other design spectra.

A number of studies make the (sometimes unstated) choice of selecting 'severe' records that will apply large demands to the modelled system. The method used for assessing the severity of these records, however, is not often explicitly stated. Most of the commonly-used records are not associated with strong-motion parameters that are the worst case.

Unspecified selection criteria

For many studies, no reason is given in for choosing the selected records listed in Table 1. Therefore it is assumed that the authors of these study used these records because they are commonly employed. In fact, [START_REF] Elghazouli | Seismic performance of concentrically braced steel frames[END_REF] explicitly states that he uses the El Centro 1940 record because it is often used for analysing structures. This finding that much work is still based on the El Centro 1940 record and that it is not necessarily an appropriate choice has been noted many times in the past, for example [START_REF] Bolt | Estimating seismic ground motion[END_REF] notes that the values of PGV and PGD from this record are less than would be expected for the associated magnitude and distance of this record. Many of the records listed in Table 1 were, at the time of their recording, exceptional records due to being recorded close to large magnitudes earthquakes and/or with high values of amplitude-based strong-motion parameters, such as PGA. However, most of the records listed in Table 1 are more than 25 years old and are no longer the severest motions recorded nor are they anymore the only accelerograms from the near-source of large earthquakes.

Selection criteria based on seismic hazard

If the location of the modelled structure is known or the study is conducted for a typical type of building for a region then the selected time-histories should be representative of the hazard level at the site. Very few studies in the sample set include a discussion on the applicably of the chosen records with respect the seismic hazard at the site under consideration. For example, all except one of the records listed in Table 1 are from earthquakes with M w ≥6.7 and consequently the use of such records to analyse structures in regions where such large earthquakes are thought not to occur (e.g. many regions of Europe) is not appropriate since they will impose greater demands on the structure (particularly with respect to the duration of shaking) than could occur in practice. One study that does consider the hazard level to select accelerograms is the article by Tremblay & Bouatay (2002) who selects records based on tectonics and the magnitude-distance ranges of design events. Di [START_REF] Di Donna | Advanced earthquake protection systems for high voltage electic equipment[END_REF] state that they use the Sturno record from the Irpina 1980 earthquake since it is considered representative of the 500-year ground motions for the considered site. In order to be able to consider the magnitude-distance couples of design events based on a probabilistic seismic hazard analysis, disaggregation of the hazard (e.g. [START_REF] Bazzurro | Disaggregation of seismic hazard[END_REF] would have to be performed, which is not yet common practice in Europe.

Modifications to selected time-histories

A number of studies (e.g. [START_REF] Colombo | Pseudodynamic response of dual RC structure with different designs[END_REF] account for variations in shaking intensity simply by scaling the selected records to different values of PGA. However, the severity of ground motion is not solely a function of the amplitude but also the frequency content, the duration of shaking and how and how much energy is imparted to the structure. The method proposed below allows the importance of these different characteristics to be assessed.

A few studies modify the natural accelerograms in other ways. For example, [START_REF] Sextos | Effect of analysis complexity on the calculated ductility demand of R/C bridge piers[END_REF] deconvolve the Kallithea record from the Athens 1999 earthquake, which was from a soil site, down to bedrock. Some studies modified the selected time-histories to conform to a seismic building code (e.g. [START_REF] Colombo | Pseudodynamic response of dual RC structure with different designs[END_REF].

PROPOSED METHOD

In the past couple of decades, many new sources of earthquake strong-motion records have been developed: on the Internet, e.g. ISESD (http://www.isesd.cv.ic.ac.uk), COSMOS (http://db.cosmos-eq.org/scripts/default.plx) or the PEER NGA database (http://peer.berkeley.edu/nga/index.html), or on CD ROMs (e.g. [START_REF] Seekins | Digital Data Series DDS-7. 'Digitized strong-motion accelerograms of North and Central American earthquakes[END_REF][START_REF] Ambraseys | Dissemination of European Strong-Motion Data[END_REF]. These are the source of many thousands of potential input accelerograms to structural analysis. However, structural testing is an time-consuming and expensive exercise therefore the number of input time-histories used should be kept to a minimum but at the same time the selected records should allow the behaviour of the structures to be investigated and also the sensitivity of this behaviour to variations in ground motions to be understood. This section reports a method developed during the VEDA project [START_REF] Douglas | Selection of strong-motion records for use as input to the structural models of VEDA[END_REF]. This project seeks to derive fragility curves for earthquake risk evaluation in France using not only amplitude as the sole ground-motion intensity measure but also other parameters such as duration. The following section shows an application of this selection procedure.

Currently, most modelling of the response of structures to earthquakes is conducted using one-dimensional horizontal excitation and therefore only horizontal components are considered here and, in addition, the two horizontal components from the same triaxial strong-motion record are considered independently.

In order that a efficient set of input accelerograms is selected some ideas from the theory of Design of Experiments (DOE) (e.g. http://www.itl.nist.gov/div898/handbook/pri/section1/pri1.htm) are employed. Well design experiments maximize the amount of information that can be obtained for a given amount of experimental effort.

Since one purpose of structural testing should be to decide which characteristics of strong ground motions are important for the modelled structures, the experimental design required has a screening objective. The primary purpose of these experiments is to select or screen out the few important main effects from the many less important ones. These designs are also termed main effects designs.

In the terminology of black box process models of DOE, the structural model is the process and the estimated damage parameters from these models are the outputs (responses). The controlled inputs (factors) are split into the parameters defining the structural model (strength of concrete etc.) and the input ground motions. At this stage, there are no uncontrolled inputs (co-factors) since everything can be set by the experimenter. (In reality, the parameters required for the structural model are not known exactly.) Since there is an infinite variety of possible earthquake ground motions it is useful to characterise them using a number of scalar strong-motion parameters that approximately measure different properties of the motions (amplitude, frequency content, duration, energy etc.). Hence the set of strong-motion parameters becomes the controlled inputs to the process. The benefit of this approach is that it is then easier to understand the results of the structural modelling with respect to properties of the input accelerograms. However, since these strong-motion parameters do not perfectly characterise the ground motions (no small set of scalars can hope to fully characterise the true complexity of ground motions) the use of strong-motion parameters introduces uncontrolled factors (co-factors) due to the complexity of the motions not measured by the strong-motion parameters chosen. The strong-motion parameters chosen now define the main effects in the model. Since it is not known a priori which strong-motion parameters are the most appropriate to model the demand for the considered structure, a screening study is required. It is hoped that the result of this screening process will be a preliminary list of which strong-motion parameters are useful to investigate as possible measures of the seismic intensity for further investigation of the structure's response.

Selection based on seismic hazard of region

The first stage of the proposed method is to apply a magnitude-distance filter to exclude records from magnitudes and distances that are not possible for the considered region by considering the seismogenic sources in the region in terms of their location and maximum magnitude or by making use of disaggregation results. Also records from small earthquakes and great distances should be excluded since these will not lead to significant damage to modelled structures unless the structures are particularly weak.

After this first step it is likely that there will still be too many strong-motion records to use all as input to the structural models because of the long run times of numerical models and the high cost of shake-table tests. Therefore an additional selection procedure must be employed to further reduce the total number of selected accelerograms. This additional step is based on selecting records with different characteristics measured by the values of various strong-motion parameters.

Selection based on strong-motion parameters

The selection procedure proposed here is to use a two-level factorial technique where for each strong-motion parameter selected records are chosen to fall within two intervals: either high or low value bins. Figure 1 graphically shows the approach for three strong-motion parameters. One record is chosen to be at each corner of the cube. This experimental design then allows the effect of each strong-motion parameter on the damage sustained to the structure to be investigated but also the interaction effects due to the combined effects of two parameters, for example amplitude and duration. A full factorial run consisting of two-levels for each strong-motion parameter becomes large when only a few strong-motion parameters are considered (e.g. 2 6 =64 records for six strong-motion parameters). In addition, it is important to produce a number of independent sets of strong-motion records in order that the variability due to the uncontrolled factors, not captured by the strong-motion parameters, can be studied. One type of experimental design that can reduce the required number of records is a fractional factorial design. This choice assumes that interaction effects can be neglected. The most commonly used fractional factorial design is one that uses only half the amount of experiments. Since 2 3 =8 records is about the maximum possible for each experiment because it is necessary to undertake repeat experiments, this leads to a maximum choice of four strong-motion parameters (2 4 /2=8 records). In addition, the choice of more strong-motion parameters would cut up the available data too much and would make it impossible to find records for each corner of the cube, leaving some bins empty.

So that an efficient set of strong-motion parameters (i.e. the smallest set that allows the effect of different aspects of the motions on the structure's behaviour to be investigated) is used it is preferable if a set of poorly correlated parameters is chosen. Ideally the chosen parameters should be orthogonal to each other so that the same characteristics of the motions (e.g. amplitude or duration) is not measured twice by two different parameters.

The following section shows the application of this approach for the selection of time-histories for the VEDA project [START_REF] Douglas | Selection of strong-motion records for use as input to the structural models of VEDA[END_REF].

EXAMPLE APPLICATION

Selection based on seismic hazard of region

The geographical scope of VEDA is France although it is hoped that the results obtained will be useful for the definition of fragility curves for use in other countries. Metropolitan France has a seismic hazard that is thought to be characterised by earthquakes of magnitudes (M L ) less than or equal to 6.3 (except for distant earthquakes occurring in the Italian Apennines) [START_REF] Marin | A probabilistic approach to seismic hazard in metropolitan France[END_REF]. Therefore accelerograms from small and moderate earthquakes with magnitudes less than about 6.3 are most appropriate for this project. In addition, [START_REF] Marin | A probabilistic approach to seismic hazard in metropolitan France[END_REF] consider that the average focal depths in France are less than or equal to 12km. In view of this, the CD ROM of strong-motion records developed by [START_REF] Ambraseys | Dissemination of European Strong-Motion Data[END_REF] has been chosen as the source of data for this project since it provides a large set of data mainly from moderate (M w <6.5) shallow (h<30km) earthquakes that occurred within Europe and the Middle East. The data provided by [START_REF] Ambraseys | Dissemination of European Strong-Motion Data[END_REF] has also been thoroughly validated within various projects leading up to the publication of the CD ROM and consequently it provides a reliable source of data. Finally, the strong-motion records contained on the CD ROM have been individually processed using bandpass filters based on the signal-to-noise ratio of the record and therefore the acceleration, velocity and displacement time-histories should be free of noise within the passband of the filters used. Search criteria of 5.3≤M w ≤6.3, d e ≤30km and focal depths≤30km using the database of [START_REF] Ambraseys | Dissemination of European Strong-Motion Data[END_REF] returned 210 horizontal components. This is obvious too many to use all as input to complex numerical models. Therefore the next stage is to reduce this number using criteria based on strong-motion parameters.

Selection based on strong-motion parameters

Another constraint that is important for future use of the fragility curves developed in VEDA is that the strongmotion parameters used to characterise the intensity of the ground motions can be predicted with accuracy when conducting a earthquake risk assessment using the fragility curves. In practice, this means that reliable and consistent (with respect to the seismotectonics of metropolitan France and the magnitude and distance range considered) ground motion estimation equations (GMEE) (e.g. [START_REF] Douglas | Earthquake ground motion estimation using strong-motion records: A review of equations for the estimation of peak ground acceleration and response spectral ordinates[END_REF] for the prediction of the chosen strong-motion parameters must be available. This reduces the number of possible strong-motion parameters that can be used to select the set of records since reliable GMEEs are available for few strong-motion parameters.

Unlike parameters for amplitude or duration, pure measures of the frequency content such as the mean period, T m , or predominant spectral period, T p , [START_REF] Rathje | Empirical relationships for frequency content parameters of earthquake ground motions[END_REF] will not cause a monotonic increase in damage with increasing values of the parameter. For example, holding other characteristics of the motion constant an increase in the amplitude of ground motions will invariably lead to an increase in damage hence it can be used as a measure of the seismic intensity along the x-axis of fragility curves. However, an increase in T m holding other characteristics constant does not necessarily increase the damage but its effect will depend on the natural period of the structure and other structural parameters. Thus it is not useful to use such pure frequency content measures within a factorial design as proposed here. [START_REF] Bommer | Correlations between duration and number of effective cycles of earthquake ground motion[END_REF] find that duration and the effective number of cycles within ground motions are, in general, very poorly correlated. Therefore it would be useful to select strong-motion records with varying durations and numbers of effective cycles in order to investigate the effect of both these ground-motion characteristics on the structural damage. However, since there are currently no useable GMEEs to predict number of cycles of earthquake ground motions, number of cycles is not chosen as a strong-motion parameter for the selection procedure. [START_REF] Aochi | Testing the validity of simulated strong ground motion from the dynamic rupture of a finite fault, by using empirical equations[END_REF] present a table (their Table VII) listing the correlation coefficients between a number of strong-motion parameters based on the data of [START_REF] Ambraseys | Equations for the estimation of strong ground motions from shallow crustal earthquakes using data from Europe and the Middle East: Horizontal peak ground acceleration and spectral acceleration[END_REF]. They find that relative significant duration (τ SR ) (using the interval between 5 and 95% of total Arias intensity, AI) is very weakly correlated to PGA, PGV, AI and response spectral acceleration (S a ) at periods from 0.1 to 2.0s. In addition, a GMEE exists for the prediction of this parameter [START_REF] Abrahamson | Empirical ground motion models[END_REF]. Hence it is a good choice of parameter for characterising the duration of earthquake ground motions for VEDA. [START_REF] Aochi | Testing the validity of simulated strong ground motion from the dynamic rupture of a finite fault, by using empirical equations[END_REF] also find that AI is quite strongly correlated with amplitude measures: PGA, PGV and S a at periods from 0.1 to 2.0s (correlation coefficients between 0.60 and 0.80). Therefore it is not an ideal choice as a selection strong-motion parameter but it does measure the energy input into the structure and also has been found to be useful for the prediction of structural damage in earlier studies (e.g. Bommer et al., 2004). However, it was found that within the dataset used here that, due to the high correlation between AI and S d at 0.1 and 1.0s, it was not possible to find strong-motion records within every bin. Therefore AI was dropped as a strong-motion parameter for selecting records.

PGA and S a are also strongly correlated especially at short periods. [START_REF] Aochi | Testing the validity of simulated strong ground motion from the dynamic rupture of a finite fault, by using empirical equations[END_REF] report correlations between PGA and S a between 0.63 (for 2.0s) and 0.93 (for 0.1s). However, the combination of two amplitude measures at two separated periods provides a measure of both the amplitude and frequency content of a strongmotion record.

Table 2 lists the strong-motion parameters chosen to undertake the selection of records for VEDA along with the minimum and maximum values of these parameters within the set of records with 5.3≤M w ≤6.3, d e ≤30km and focal depths≤30km and the ranges of the low and high bins used for selecting the records. The ranges of the low and high bins were chosen in order to have sufficient numbers of records within each bin. A more powerful test of which strong-motion parameters are useful for estimating damage would be obtained if the ranges of the bins used was minimised so that the scatter in the strong-motion parameters within each bin was reduced. However, due to the limited amount of data this was unfortunately not possible so large ranges had to be adopted. The databank of records was searched to find records that fall within the different bins. Within each bin a random record from those within the bin was chosen to form part of the strong-motion selection. Since only three strong-motion parameters are used to undertake the selection the fractional factorial approach described above does not need to be followed. This will also allow the investigation of interaction effects between different characteristics of the motions. Hence sets of eight strong-motion records were selected for each independent input dataset. It was possible to find four independent sets of eight records (32 records in total) using this approach. These four sets will be used as input to the models of VEDA.

Once structural testing has been performed using these selected accelerograms it is planned to use the method of, for example, [START_REF] Rabinowitz | Logic trees, sensitivity analyses, and data reduction in probabilistic seismic hazard assessment[END_REF] to interpret the results and find which characteristics of the ground motions are most important for understanding the behaviour of the structures considered. These results will then be used to construct fragility curves.

CONCLUSIONS

This article presents a survey of the current state-of-the-practice for natural accelerogram selection for use as input to structural testing. It shows that even though the quantity and quality of strong-motion data has increased dramatically over the past 25 years and also that access to this data has been considerably improved through the creation of CD ROMs and Internet sites, many authors continue to select from a limited set of well-known accelerograms (e.g. El Centro 1940). Since the use of a common set of strong-motion records (e.g. El Centro 1940 and Petrovac 1979) is firmly imbedded within earthquake engineering practice, a more appropriate set of standard strong-motion records to use could be proposed. A common set would also allow comparisons to be easily made between different studies.

In contrast to this possible approach, this article presents a simple selection procedure leading to a small set (of probably eight or sixteen records) of accelerograms that can be used to efficiently investigate the behaviour of modelled structures under different types of earthquake loading.

Figure 1 :

 1 Figure 1: A 2 3 two-level, full factorial design with factors X1, X2 and X3. Arrows show increasing values of the factors (from http://www.itl.nist.gov/div898/handbook/pri/section3/pri3331.htm).

Table 1 : Strong-motion records most commonly employed in structural testing

 1

	Earthquake	dd/mm/yyyy Station	M w	d f	Vs30	PGA	PGV	PGD
					(km)	(m/s)	(g)	(cm/s)	(cm)
	Imp.Valley	19/05/1940	El Centro #9	7.0	6	213	0.25	30	17
	Kern County 21/07/1952	Taft-Lincoln Sch.	7.4	38	385	0.17	16	9
	Takochi-oki 16/05/1968	Hachinohe	7.9	165	-	0.23	18	7
				(M s )	(d e )
	Friuli	06/05/1976	Tolmezzo	6.5	7	1021	0.34	20	3
	Tabas	16/09/1978	Tabas	7.4	8	712	1.04	66	34
	Montenegro 15/04/1979	Petrovac	7.0	3	464	0.46	39	5
	Montenegro 15/04/1979	Ulcinj-Albatros	7.0	11	1083	0.21	26	9
	Montenegro 15/04/1979	Ulcinj-Olimpik	7.0	13	399	0.29	35	9
	Northridge	17/01/1994	Newhall-Fire St.	6.7	3	269	0.67	84	26
	Northridge	17/01/1994	Rinaldi-Rec. St.	6.7	0	282	0.66	109	30
	Kobe	16/01/1995	Kobe JMA	6.9	1	312	0.71	76	18
	Chi-Chi	20/09/1999	TCU075	7.6	1	573	0.29	59	59
	Chi-Chi	20/09/1999	TCU102	7.6	2	714	0.25	87	76
	where: d f

Table 2 : Strong-motion parameters and ranges of low and high bins used for selecting records. Also given are the minimum and maximum values of the strong-motion parameters in the subset of Ambraseys et al. (2004) used.

 2

	Parameter	Minimum	Maximum	Low bin range	High bin range
	S d at 0.1s in cm	0.0035	0.37	≤0.03	>0.03
	S d at 1.0s in cm	0.059	16.7	≤0.5	>0.5
	τ SR is s	2.56	24.48	≤10	>10
	Where S d is elastic response spectral displacement for 5% damping.

Tremblay, R.,Bouatay, N. (2002), Loading protocols for the seismic testing of ductile bracing members in concentrically braced steel frames, Proceedings of the Twelfth European Conference on Earthquake Engineering, Paper no. 480.

ACKNOWLEDGEMENTS

Some of the work presented in this article was developed during the ANR project 'Seismic vulnerability of structures: A damage mechanics approach' (VEDA). The rest was funded by internal BRGM research projects. I thank Caterina Negulescu for her comments on the first version of this paper.


John Douglas

ESTIMATING STRONG GROUND MOTIONS AT GREAT DEPTHS

In this paper, ratios of peak ground acceleration (PGA), velocity (PGV) and displacement (PGD) and response spectral acceleration (SA) of the ground motion at the surface and at different depths are estimated using shear-wave velocity profiles of rock sites, information on Q in the upper crust and recorded ground motions from rock sites. The results are compared with those reported in the literature for those sites with strongmotion instruments at great depths. As expected estimated ground motions at depth are, in general, lower than those on the surface. Also it is found that the estimated ratios do not show much variation with depth except for high-frequency strong-motion parameters for profiles with high attenuation (Q=10) where ground motions at great depths can be higher than those at the surface. However, it is unlikely that such a low value of Q will occur in practice at depths down to 2km. When details of the near-surface velocity and attenuation structure are not known it is found that the commonly used factor of two (accounting for the effect of the free surface) is a reasonable conservative first estimate at all depths.

Introduction

Large-scale engineering projects, such as tunnels, require estimates of the earthquake ground motions at depth to which the construction maybe subjected to during its operating life. Even though it is believed that underground structures are significantly less vulnerable to earthquake shaking than structures on the surface, estimates of the ground shaking to be design against need to be provided to the design engineers. Recently proposed (and constructed) rail tunnels in the Alps include sections at depths greater than 1km where there is little observed data available on which to base estimates of the motions expected. Most boreholes containing strong-motion instruments are less than 100m deep and most of these are at soil sites, whereas at great depths the tunnels pass through bedrock. The advent of KiK-Net in Japan with instruments at up to 2km will provide valuable observational data but since at each KiK-Net site there is only one station at the surface and one at depth it is not possible to study how ground motions vary with depth. For tunnelling projects the depth of the tunnel varies along its path therefore consistently derived ground motions estimates are required for many depths.

At present, in the Alps, a number of long deep base rail tunnels have just been completed, are under construction, or are in the design stage. Examples of these tunnels are: the Gotthard base tunnel and the Lötschberg base tunnel in Switzerland, the Brenner base tunnel in Austria and the Lyon-Turin tunnel connecting France and Italy. Since the level of seismicity in the Alps is reasonable high, these tunnels need to be designed to withstand earthquakes. Underground structures have been found to be less vulnerable to damage from earthquakes than those on the surface (e.g. [START_REF] Hashash | Seismic design and analysis of underground structures[END_REF]. Usually, seismic design loads for underground structures are characterized in terms of deformations and strain imposed by the surrounding ground (e.g. [START_REF] Hashash | Seismic design and analysis of underground structures[END_REF] but, as with surface structures, inertial forces caused by ground accelerations are also considered in their design. Hence, estimates of the ground motions to be expected at depths are required. At present, however, there is little information available on which to base these estimates.

Ground motions at depths are likely to be considerably different to those at the surface due to these factors (e.g. [START_REF] Fukushima | Site dependent attenuation relations of seismic motion parameters at depth using borehole data[END_REF]:

• the effect of the free surface, in theory, means that the motion at depth is two times lower than that at the surface; • rock at the surface is, in general, softer (lower wave-velocities), than those at depth therefore the motion at the surface will have been amplified with respect to the motion at depth; • the attenuation of high frequencies is less at depth than at the surface (e.g. [START_REF] Abercrombie | Near-surface attenuation and site effects from comparison of surface and deep borehole recordings[END_REF]); • interference between the ascending and descending waves and diffraction phenomena can also diminish the motions at depth.

Previous studies

In the literature there exist a number of publications on ground motions at depth, however, most of these are based on observations at less than 100m and also from soil sites. Recent tunnels include sections at much greater depths and within rock therefore many of these previous studies are of limited use for this study. At beginning of 2001 a survey of borehole instruments operating at depth was conducted (the results can be accessed at: http://www.crustal.ucsb.edu/~steidl/research/ArrayTable.htm). It lists a number of rock borehole sites with instruments operating at depths of greater than 100m, namely: Whitshell Pinarig, Manitoba (Canada) (maximum depth of 420m within granite) ; Vineyard Canyon, Parkfield, (USA) (maximum depth of 198m) ; Tomioka (Japan) (maximum depth of 950m within sandstone); Shiroyama (Japan) (maximum depth of 239m) ; Oroville (USA) (maximum depth of 475m within fractured rock) ; Joaquim North, Parkfield (USA) (maximum depth of 198m) ; Iwaki (Japan) (maximum depth of 330m within sandstone) ; Cajon Pass (USA) (maximum depth of 2900m within soft rock). Also the KiK-Net network in Japan (http://www.kik.bosai.go.jp/kik/index_en.shtml) includes many instruments at depths greater than 100m. Four previous studies on the observed ratio between the motion at depth and on the surface are those by De Luca et al. (1998), [START_REF] Fukushima | Site dependent attenuation relations of seismic motion parameters at depth using borehole data[END_REF], [START_REF] Mcgarr | Development of ground-motion prediction equations relevant to shallow mining-induced seismicity in the Trail Mountain area, Emery County, Utah[END_REF] and [START_REF] Shimizu | Earthquake-related ground motion and groundwater pressure change at the Kamaishi Mine[END_REF].

De [START_REF] De Luca | Site response study in Abruzzo (Central Italy): Underground array versus surface stations[END_REF] show Fourier spectral ratios between ground motions at the surface and those at depth (depths between 400m and 1410m) under a mountain (Massif Gran Sasso, Italy). The mountain is composed of relatively soft limestone. They find that these ratios are not strongly affected by depth. The maximum ratio is about four at a frequency of about 5Hz. [START_REF] Fukushima | Site dependent attenuation relations of seismic motion parameters at depth using borehole data[END_REF] study the motions at three sites in Japan: Iwaki (instruments on the surface, at 20, 70, 130, 200 and 330m), Tomioka (instruments on the surface, at 6, 100, 251, 660 and 950m) and Etchujima (instruments on the surface, at 40 and 100m). The Iwaki site is composed of sandstone until a depth of about 300m and then granite at greater depths. Shear-wave velocities are between 840m/s at the surface and 2.8km/s at depth. Tomioka is composed of sandstone, siltstone and tuff with shear-wave velocities between 520 at the surface to 2.8km/s at depth. The site at Etchujima is underlain by sand and clays with shear-wave velocities between 110 and 460m/s. [START_REF] Fukushima | Site dependent attenuation relations of seismic motion parameters at depth using borehole data[END_REF] provide estimates of the average ratio between the maximum acceleration at depth and the maximum acceleration on the surface from 20 earthquakes for Iwaki, 16 earthquakes for Tomioka and 23 earthquakes for Etchujima. These ratios show that the ratio between the PGA at depth and at surface does not change greatly with varying depth, e.g. for Tomioka the ratio varies between 0.33 and 0.23 for depths between 100 and 950m.

McGarr & Fletcher ( 2005) study the ground motions induced by local (less than 10km) coal mining activity. All stations considered are on hard rock (shear-wave velocities greater than 1.5km/s). They calculate the average ratio between the PGA on the surface (eight stations) and the PGA at about 600m (one station) and find a factor of 2.2, which they attribute to the effect of the free surface. [START_REF] Shimizu | Earthquake-related ground motion and groundwater pressure change at the Kamaishi Mine[END_REF] also study ground motions in a mine (Kamaishi Mine, Japan) at the surface and at depths of 140, 315 and 615m. The mine is in a region of Paleozoic and Mesozoic sedimentary formation and Cretaceous granite. They calculate the ratio between PGA on the surface and at depth for 41 earthquakes and find that the ratio varies with depth: at a depth of 615m they find ratios between two and four and for depths of 140 and 315m a ratio between 140 and 315m.

Proposed method

The following estimation method based on models of generic rock profiles of velocity and attenuation characteristics is proposed here. The two generic rock profiles (one for very hard rock, V s,30 =2.8km/s, and on for soft rock, V s,30 =618m/s) proposed by [START_REF] Boore | Site amplifications for generic rock sites[END_REF] were used for shear-wave velocity and density values. Accurate Q estimates for the upper few kilometres of the crust are difficult to obtain because there are few deep borehole recording sites. [START_REF] Abercrombie | A summary of attenuation measurements from borehole recordings of earthquakes: The 10 Hz transition problem[END_REF] provides a review of Q estimates for this depth range. Most Q values seem to fall between 10 and 50 therefore these values were chosen in order to bracket the most likely range of values. Abercrombie's examples show that Q varies with depth but this has been neglected here for simplicity. Also neglected in this method is the effect of the interference between the ascending and descending waves and diffraction phenomena. [START_REF] Boore | SMSIM -Fortran programs for simulating ground motions from earthquakes: Version 2.0 -A revision of OFR 96-80-A[END_REF] provides a computer program (site_amp) for the calculation of amplification due to the presence of a profile of varying shear-wave velocity and intrinsic attenuation. This program was used to compute the amplification curves presented here by computing the expected amplification at the surface and at different depths (the generic profiles were used up until the depth under consideration). The shear-wave velocity at the base of the site profile was assumed to be 3.5km/s and the density was assumed to be 2.8g/cm 3 , which are common choices for these variables. The angle of incidence has been chosen as 90°. The effect of the free surface is included by multiplying the ratios obtained by two.

Fourier amplitude spectral ratios

Figure 1 shows the Fourier amplitude spectral ratios for the four different profiles (generic rock with Q=10 and Q=50 and generic very hard rock with Q=10 and Q=50) computed using the method outlined above for depths between 0 and 2200m in steps of 200m and frequencies between 0.1 and 20Hz (periods between 0.05 and 10s).

The spectral ratios show that except for high frequencies (>5Hz) ground motions are of lower amplitude that those at the surface due to the effect of the free surface and also site amplification due to the presence of lower shear-wave velocities near the surface than at depth. This is in agreement with the observational results presented above where ground motions at depth are lower than those observed at the surface. Ground motions at softer sites (e.g. the generic rock site) show larger spectral ratios due to higher site amplifications due to lower shear-wave velocities. Depth has a limited effect on the level of the amplification due to lower shear-wave velocities near the surface since the strong velocity gradient that leads to significant amplification is confined to the top few hundred metres.

For high frequencies, however, ground motions at depth can be significantly higher than those at the surface due to lower attenuation at depth. The amplitude of high frequency ground motions can be expected to increase with increasing depth due to lower attenuation, as observed by, for example, [START_REF] Abercrombie | Near-surface attenuation and site effects from comparison of surface and deep borehole recordings[END_REF]. (1998) with respect to shear-wave velocity and Q are not known and are unlikely to be similar to those of the generic rock site.

Ratios of strong-motion parameters

The individually processed strong-motion data released on CD ROM by [START_REF] Ambraseys | Dissemination of European Strong-Motion Data[END_REF] were used to assess the expected ratio between strong-motion parameters on the surface and at varying depths using the spectral ratios derived above. All the records from rock sites (330 horizontal components) on the CD ROM were used. Their Fourier amplitude spectra were computed and divided by the spectral ratios estimated for each of the four generic rock profiles to obtain estimated Fourier amplitude spectra at depth. These Fourier amplitude spectra were then inverse Fourier transformed to obtain the predicted ground acceleration at depth. This assumes that the phase spectrum of the motions does not change with depth. For each pair of observed surface acceleration timehistory and estimated acceleration time-history at depth the ratio between different strong-motion parameters was computed.

Figures 23456789show the estimated mean and mean plus and minus one standard deviation ratios between strong-motion parameters on the surface and at depth constructed using the 330 input ground motions.

The figures show that except for high frequency ground-motion parameters (PGA and SA at 0.1s) for profiles with high attenuation (Q=10) the ground motions at depth are smaller than those at the surface by factors up to about four reflecting the findings for spectral ratios estimated above. In addition the ratios of ground-motion parameters are not strong affected by depth. For PGA and SA at 0.1s for profiles with high attenuation ground motions at depth can be significantly higher than those at the surface, up to a factor five for PGA at 2km for the soft rock profile with Q=10. It is unlikely that such a factor will be observed in reality because such low values of Q are expected only close to the surface and not over 2km. In general the effect of the free surface is the most important factor in the assessment of motions at depth and a factor of two seems to be a reasonable conservative estimate of the corrective factor for assessing ground motions at depth in the absence of detailed near-surface velocity and, especially, attenuation information.

Conclusions

In this short article it is found that strong ground motions at depth are, in general, lower than those on the surface. Also it is found that the estimated ratios do not show much variation with depth except for high-frequency strong-motion parameters for profiles with high attenuation (Q=10) where ground motions at great depths can be higher than those at the surface. However, such a low value of Q is unlikely to occur in practice at depths down to 2km. When details of the near-surface velocity and attenuation structure are not known it is found that the commonly used factor of two (accounting for the effect of the free surface) is a reasonable conservative first estimate at all depths.

Figure 1 .

 1 Figure 1. Estimated Fourier amplitude spectral ratios between ground motions at surface and at varying depths estimated for the generic very hard rock site (left-hand side) and generic rock site (right-hand side) for two Q values (black Q=10 and red Q=50).

Figure 2 .

 2 Figure 2. Estimated mean and mean plus and minus one standard deviation ratios between PGAon surface and at varying depths for the four considered profiles.

Figure 3 .

 3 Figure 3. Like Figure 2 for PGV.

Figure 4 .

 4 Figure 4. Like Figure 2 for PGD.

Figure 5 .

 5 Figure 5. Like Figure 2 for SA at 0.1s.

Figure 6 .

 6 Figure 6. Like Figure 2 for SA at 0.2s.

Figure 7 .

 7 Figure 7. Like Figure 2 for SA at 0.5s.

Figure 8 .

 8 Figure 8. Like Figure 2 for SA at 1.0s.

Figure 9 .

 9 Figure 9. Like Figure 2 for SA at 2.0s.

Acknowledgements

This article was inspired by seismic hazard assessment work undertaken for the Lyon-Turin (LTF) tunnel by BRGM. The work present in this paper was funded by internal BRGM research projects. I thank Pascal Dominique, Agathe Roullé and Hideo Aochi for discussions.