Ph.D Paola Morasca

Marco Massa

Enrica Laprocina

Kevin Mayeda

Scott Phillips

Luca Malagnini

Daniele Spallarossa

Giovanni Costa

Paolo Augliera

Improved 2-D Attenuation Analysis for Northern Italy Using a Merged Dataset from Selected Regional Seismic Networks

Keywords: seismic attenuation tomography, coda waves, Northern Italy

 were able to resolve only a small area due to limited data coverage. For this reason the interpretation of the attenuation anomalies was difficult given the complexity of the region and the poor resolution of the available data. In order to better understand the lateral changes in the crustal structure and thickness of this region, we selected 770 earthquakes recorded by 54 stations for a total of almost 16000 waveforms derived from seismic networks operating totally or partially in Northern Italy. Direct S-wave and coda attenuation images were obtained using an amplitude ratio technique that eliminates source terms from the formulation. Both direct and early-coda amplitudes are used as input for the inversions and the results are compared. Results were obtained for various frequency bands ranging between 0.3-25.0 Hz, and in all cases show significant improvement with respect to the previous study since the resolved area has been extended and more crossing paths have been used to image smaller scale anomalies. Quality-factor estimates are consistent with the regional tectonic structure exhibiting a general trend of low attenuation under the Po Plain basin and higher values for the western Alps and northern Apennines. The interpretation of the results for the Eastern Alps is not simple, possibly because our resolution for this area is still not adequate to resolve small scale structures.

supporting papers that describe in great detail more of the method. Including this once again in this paper would seem redundant.

-Regards… Paola Morasca, Ph.D.

Response to reviewers' comments for "Improved 2-D Attenuation Analysis for Northern Italy Using a Merged Dataset from Selected Regional Seismic Networks." By Morasca et al. are highlighted in blue.

In addition, we have highlighted changes in the revision as well.

COMMENTS FOR THE AUTHOR:

Reviewer #1: This paper presents a 2-D Attenuation study for Northern Italy. The authors use earthquakes recorded by several stations deployed in the region. This study extends geographically the knowledge of Q distribution especially north eastern of the studied zone. The presented results agree with previously reported. I found this manuscript to be well written and presenting a very complete study. I support its publication in JOSE. I have some minor comments:

1.-Abstract shows too much detail about the networks used for the study. Moreover, authors repeat the same information in "Data" section. I would suggest remove details about networks in "Abstract" section. __________ RESPONSE: __________ OK…, the specific details about the networks have been eliminated from the abstract, but remains in the 'Data' section.

2.-Lines 6 and 9 (page 3) should say (e.g. Akinci et al. 2004...) and (e.g. Mele et al. 1997) __________ RESPONSE: __________ These have been corrected.

3.-Line 35 (page 3) should say . Kissling and Spakman, 1996) __________ RESPONSE: __________ It has been corrected.

4.-In the "Methodology Overview" section I suggest to include a more detailed explanation of the used formulation for inversion. __________ RESPONSE: __________ We address this issue by referring the reader to a number of methodology papers by one of the coauthors (Scott Phillips) who developed the methodology and who co-authored a number of studies that used this same empirical coda envelope tomography methodology. (e.g., [START_REF] Phillips | Amplitude ratio tomography for regional phase Q[END_REF]Phillips et al., 2008;Mayeda et al., 2005).

5.-Figure 2. For the checkerboard test, different colour scale should be used. Maybe a gray scale should be enough to show the results. __________ RESPONSE: __________ We highlighted each of the checkerboard pixels in white to better define them. However, we keep the same colors to coincide with the inversion maximum and minimum Q values. Furthermore, we crop the inversion results to only those areas where we feel we have resolution.

6.-Authors should do a carefully review of "References" section because there are a lot of errors, e.g.:

- Atkinson et al., 2006 should be after Atkinson and Bore, 1995. -For Bocaletti et al.: reference is missing the year of publication.

-Some times used (year) and sometimes ,year __________ RESPONSE: __________ OK…, these have been double-checked and corrected.

Reviewer #2: This paper uses local to regional observed seismic waveforms to reconstruct the regional attenuation structure in Northern Italy. The authors put a lot of effort into homogenizing the various small networks in Italy to obtain a unified data set. I think that his is an applaudable effort and I do hope that many more studies will come out of this effort.

Unfortunately I have severe problems with the methodology they are using and before this is solved I do not think the paper is appropriate for publication.

As I understand the GRL paper from Philips et al, 2005, who developed the method applied, they used regional surface waves (Lg phase) for the tomographic approach. Ray tracing in figure 1 bottom panels suggests that body waves are used in this study? Additionally, very deep events are used (>60km), which definitely will not have a good surface wave train.

__________ RESPONSE: __________

We eliminated deep events and also found a few events to have erroneous depths. Fortunately, the inversion results did not change because those events were insignificant in number. As all our events are now crustal and so we eliminated the cross-sectional map.

On page 8 also direct S-wave arrivals are discussed. Since the author apply a geometrical correction 1/r I assume they are using body waves.

__________ RESPONSE: __________ Yes, we do both a direct wave and coda wave 2-D inversion. However, we fully recognize that this methodology is empirical and there will always be a trade-off between assumed spreading and 1/Q. As in previous studies (e.g., Phillips and Stead, 2008;Phillips et al. 2008;and Mayeda et al. 2005) we used a spreading that was 1/r following [START_REF] Yang | A numerical investigation of Lg geometrical spreading[END_REF].

I think a proper introduction of the methodology is needed with some basis in wave propagation to understand this paper. Right now the methodology overview is far to short (1 page) and it is not clear to the reader what actually is being analyzed.

__________ RESPONSE: __________

We have added a bit more text, but refer the reader to prior work which documents the methodology (i.e., [START_REF] Morasca | 2-D Coda and Direct Wave Tomography in Northern Italy[END_REF]) and thus we give only a general review here in our paper. For example, we also cite the following method paper:

Phillips, W.S., R.J. Stead, G.E. Randall, H.E. Hartse and K. Mayeda, Source effects from broad area network calibration of regional distance coda waves, in Advances in Geophysics, 50, Scattering of Short Period Waves in the Heterogeneous Earth, H. Sato and M.C. Fehler, Editors, 319-351, 2008 If so what is the lateral resolution of their grid and how big is the actual kernel if they allow the early coda as well. I think to use a 'simple' ray approach might be appropriate for the direct wave, but definitely not for the coda part.

__________ RESPONSE: __________

Though we could test a ray "fattening" algorithm, our feeling is that changes would be of 2nd or 3rd order. In our opinion, such a study would be of low benefit and is outside the scope of the paper. Also, the Q is not real Q. Our spreading model is empirical. It's just curve fitting and we hope that the variations qualitatively match what we might expect from the mapped geology.

The previous application of the method in the western U.S. (Phillips and Stead, 2008) and western Alps [START_REF] Morasca | 2-D Coda and Direct Wave Tomography in Northern Italy[END_REF] used predominantly direct Lg which by any measure is not a ray, but a superposition of multiply-reflected shear waves in the crust/upper mantle. The early coda would simply smear this region further laterally. The checkerboard test is admitedly a simple attempt at understanding the resolution of this idealized model with our given station-event distribution. However, we also apply significant spatial smoothing and damping and it is also inherent in the LSQR method, which taken together, mimics broad spatial averaging, consistent with our current ideas on the early local and regional coda.

Equation 1 is in this respect also not helpful. What is actually the cost function that is going to be minimized with LSQR? There are just too many questions centered around this topic, which are not addressed that I can not judge if publication of the results is warranted. __________ RESPONSE: __________ Delving into the actual scattering volume and mechanisms would be well beyond the scope of this study that is based almost soley on empircal constructs. We recognize that our "Q" values are apparent Q for both coda and direct waves.

Associate Editor:

Dear authors, first of all let me apologize for a big delay with processing your paper. It was not easy to find the reviewers, and it took enough time to receive their reports. Today we have got already both reports which allows me to make a decision. Your paper, based on an admirable amount of data, promises a significant contribution to our understanding of the attenuation structure in Northern Italy. Nevertheless, before being acceptable for publication in the J. of Seismology a moderate revision is needed (something between minor and major). The reviewers' comments will guide you toward the better presentation. Please read them carefully.

As for the data and results, there is practically no need to change, but as for the presentation, the requested revision is a 'major' one. It is mainly necessary to extend the methodology section. It would be useful to include even a figure of a typical seismogram, showing the segments that you process; what do you exactly measure on seismograms and/or on their narrow-band filter outputs. Then, in the text, you should explain in simple words how do you proceed from the measured quantities to the quantities used in the inversion. In particular, include exact definition of the 'S-wave amplitude' and 'coda amplitude'. Finally, introduce the inverse problem by the equation relating the data, kernel and the parameters, so that reader could understand what is actually solved by LSQR. Do not hesitate to devote a few new text pages to this extension. It will strongly improve understanding of your paper for less specialized readers.

I am looking forward receiving your revised paper. Jiri Zahradnik difficult given the complexity of the region and the poor resolution of the available data. In order to better understand the lateral changes in the crustal structure and thickness of this region, we selected 770 earthquakes recorded by 54 stations for a total of almost 16000 waveforms derived from seismic networks operating totally or partially in Northern Italy (RSNI, Regional Seismic network of Northwestern Italy; INGV-MI-PV, Istituto Nazionale di Geofisica e Vulcanologia, section of Milan-Pavia; RSNC, Italian National Seismic Network managed by INGV-CNT, Rome; RSFVG, Rete Sismometrica del Friuli-Venezia Giulia; NEIBB, Nord-Est Italy Broadband Network and SDSnet, Swill Digital Seismic Network). Direct S-wave and coda attenuation images were obtained using an amplitude ratio technique that eliminates source terms from the formulation through the choice of damping parameter and geometrical spreading. Both direct and early-coda amplitudes are used as input for the inversions and the results are compared.

Results were obtained for various frequency bands ranging between 0.3-25.0 Hz, and in all cases show significant improvement with respect to the previous study since the resolved area has been extended and more crossing paths have been used to image smaller scale anomalies. Quality-factor

Introduction

The quality factor, Q, is one of the many parameters required for input into seismological models for geophysical applications such as simulation of strong ground motions (Boore, 2003;Atkinson andBoore, 1995, 2006), earthquake hazard analysis (e.g. Akinci et al. 2004, Montaldo et al. 2005), or for a structural and tectonic interpretation (e.g. Mele et al. 1997).

Within Northern Italy, the Q parameter has been defined using one-dimensional models for different sub-regions (e.g., [START_REF] Morasca | Ground Motion Scaling in the Western Alps[END_REF]Drouet et al. 2008, for the Western Alps; Thouvenot, 1983, Castro et al. 2008and Bay et al. 2003 for the Central Alps; Malagnini et al. 2002, Castro et al. 1996and Console and Rovelli, 1981 for the Eastern Alps). The motivation for improving upon the 2-D attenuation model for Northern Italy is several fold. First, the region is characterized by several anomalous zones of anomaly (Kissling, 1993, Kissling and Sparkman, 1996[START_REF] Paul | A threedimensional crustal velocity model of the southwestern Alps from local earthquake tomography[END_REF]. In general, a simple 1-D model is very likely insufficient to describe the seismic wave propagation at the regional scale in this region. For example, within each sub-region strong lateral anomalies exist (Campillo et al. 1993;[START_REF] Paul | A threedimensional crustal velocity model of the southwestern Alps from local earthquake tomography[END_REF]Cattaneo et al. 1999;Kissling, 1993, Kissling andSparkman, 1996), and in some cases the authors had to use complicated 1-D models to describe the attenuation in the sub-region [START_REF] Morasca | Ground Motion Scaling in the Western Alps[END_REF].

A lateral varying attenuation model of seismic attenuation in Italy was proposed by Carletti and Gasperini (2003), that gives a qualitative description that was based upon seismic intensity derived from macroseismic observations. For a more quantitative description, Campillo et al. (1993) computed the Lg/Pn amplitude ratios across the Western Alps and used numerical simulations to model an anomalous propagation of Lg waves across the region.

However, little has been published using amplitude attenuation to produce detailed images of regionally varying Q in Northern Italy. An attempt at estimating attenuation tomographically in this region was carried out by [START_REF] Morasca | 2-D Coda and Direct Wave Tomography in Northern Italy[END_REF]. The authors tested the attenuation tomography method developed by [START_REF] Phillips | Amplitude ratio tomography for regional phase Q[END_REF] combined with the coda wave measurement approach proposed by Mayeda et al. (2003), to correct the observed data. Their results showed that the 2-D path correction produce lower standard deviations when applied to coda wave measurements. For comparison, they also applied the 2-D approach on the direct wave amplitudes, and also in this case the improvement was observed, although less pronouncedthe improvement did not approach coda levels.

The limitation of the study by [START_REF] Morasca | 2-D Coda and Direct Wave Tomography in Northern Italy[END_REF] was in the small resolved area that makes it difficult to derive an interpretation of the observed Q variations as a function of the complex structural setting. Although the authors try to correlate their attenuation results to the known anomalies in the region, they admit acknowledge their difficulties and underline the necessity to enlarge the resolved area for a more significative significant interpretation of the waves propagation in the Northern Italy. In fact, the spatial distribution of events and stations may introduce an azimuthal effect on the amplitudes of the ground motion (Campbell and Bozorgnia, 1994), and the [START_REF] Morasca | 2-D Coda and Direct Wave Tomography in Northern Italy[END_REF] study is mainly based on data recorded in the Western western Alps and Northern northern Apennines with only a few paths coming from the North north eastern side of Italy.

The aim of this study is to provide a more complete understanding of attenuation for Northern northern Italy, as literature is lacking for this region. On a pragmatic level, having a detailed knowledge of the laterally varying attenuation will be an important contribution for seismic risk analysis. This study represents the first time that a dataset of this size has been amassed infor Northern northern Italy, which that includes waveforms and stations from various networks working operating in this area. In fact, while seismic travel-time tomography studies rely on published phase picks in network bulletins, attenuation tomography requires a large amount of waveforms waveform data with good azimuthal coverage.

Tectonic framework

The region we are focusing on is characterized by a combination of different structures interacting with each other. On a large scale, it is possible to consider the relationship between the Alpine chain and the Northern Apennines, two different orogenic structures: ; the first one being related to the subduction of Europe underneath the Adriatic plate, and the second one is a fold-and-thrust belt developed as consequence of the westward subduction of the Adriatic plate (Carminati et al., 2004).

Looking more in detail at the relationship between the Alps and the Northern Apennines, the Ligurian Alps represent the western margin that links the two main structures, where the progressive opposite retreat and arching of the Western western Alps and the Northern northern Apennines have induced tensional forces and a general E-W regional stretching (Vignaroli et al. 2008).

The central area of Northern northern Italy is characterized by the presence of the Po Plain, a wide flexural basin covered by thick Plio-Quaternary sediments, slightly dipping toward the east (Pierdominici et al., 2005). Below the Po Plain different north-verging apenninic arcs face the south verging Alpine tectonic structures (Castellarin and Vai, 1986) showing a complex pattern related to a non-uniform stress field (Pierdominici et al., 2005).

The Northeastern northeastern part of Italy is mainly influenced by the interaction between the Dinarides belt and the Southern Alps, the backthrust of the Alps (Slejko et al., 1987(Slejko et al., , 1989)), reflecting the collision between the Adria microplate and the European plate. As stated by Carminati et al (2004), even this area seems to be affected by the presence of the Apenninic structure, even though less than in the central-western Alps. This indicates a complex interaction of geodynamic fields within this region.

All these interactions within Northern northern Italy result in complicated variations in crustal thickness and strong intracrustal inhomogeneities that are reflected in the anomalous propagation of the seismic waves in this area. Westward, the crust-mantle boundary bathimetry is not completely known, although it seems to be composed by three different surfaces (Kissling, 1993), and the crust is affected by lateral variations, as observed by many authors (Cattaneo et al., 1999;Kissling, 1993;Kissling and Spakman, 1996), with a thickness varying from a minimum of ~ 10 km in the Ligurian sea to ~ 50 km beneath the Alps (Buness et al. 1990). A well known intracrustal anomaly of this area is the Ivrea body, interpreted as a wedge of the Adriatic upper mantle (Kissling 1993;Di Stefano et al., 1999).

In Eastern eastern Italy, the main feature is represented by the transition between the European and Adriatic crust interpreted as a doubling crust (Lippitsch, 2002) or as a transcrustal ramp (Transalp Working Group, 2002). The geometry of the Adriatic crust-mantle boundary shallows rapidly from a depth of 60 km beneath the Dolomites to 30 km or 40 km, depending on the considered model (Ebbing, 2004), beneath the Po Plain.

In this context, the seismic activity is significant in the northeast part of Italy, along the Southern southern Alps-Po Plain margin between Friuli (Slejko et al., 1987;Slejko et al., 1989;Benedetti et al., 2000;Galadini et al., 2001) and western Lombardia, that was recently struck by the November 24, 2004, Ml=5.2, Salò earthquake, which occurred along the west coast of Lake Garda. The seismic activity tends to decrease westward along the Apennine foredeep, from the Ferrara, to the Emilia and the Monferrato Arcs, even thoughbut it is not negligible (two seismic sequences occurred in the Monferrato area on August 2000 and July 2001, with the largest events of local magnitude 5.1 and 4.6 respectively).

In the Western western Alps, the seismic activity is generally moderate and concentrated along an external belt, corresponding to the Penninic front, and an internal belt, corresponding to the Austro-Alpine front and its southern extension (Cattaneo et al., 1999;Sue et al., 1999).

Data

A substantial amount of effort has gone into collecting and quality checking the data set for this study. Our merged data set is composed of 770 selected earthquakes with local magnitude > 3, each one recorded by at least 2 stations as required by the amplitude ratio tomography methodology of [START_REF] Phillips | Amplitude ratio tomography for regional phase Q[END_REF]. The waveforms (~16,000) have beenwere recorded by 54 stations (Table 1) from the following networks: SDSNet (Swiss Digital Seismic Network), RSNI (Regional Seismic The maps of Figure 1 show the dense ray coverage we obtained with this data set. This is an important point for the aim of this study since it allows robust tomographic inversions with good resolution. Except for few events, most of the rays stay in the first upper 50 km, which means that our 2-D Q results refer to the crust and the uppermost mantle structure.

Methodology overview

The data are corrected for the instrument response, and the horizontal components, filtered for through narrow frequency bands inover a the range 0.3-25 Hz, are considered used to compute both S and coda waves amplitudes to be used as input information forto the tomographical inversions.

For S-wave measurements, the maximum amplitude following the onset of the S-wave arrival is considered for each horizontal component and the average is computed.

For coda-waves measurements, we used the approach empirical formulation of Mayeda et al. (2003;eq. 3) that use an empirical formulation (see eq. 3 of Mayeda et al. 2003) to calibrate narrowband synthetic coda envelopes to fit the observed onesobservations and compute the amplitudes. The amplitudes are defined on the base ofcomputed from the early part of the coda. Traditional coda studies originating with Aki (1969) and Aki and Chouet (1975) were based on very late coda where measurements were taken well past twice the S-wave travel time. In the methodology used in this study to compute coda amplitudes, we intentionally measure the coda immediately following the direct S-wave. This ensures that the early coda is included in all of our measurements. In fact, the parameterization of the coda envelopes developed by Mayeda et al. 2003, is done so that early and late coda are well fit at both local and regional distances.Considering Our use of short, early coda windows, in fact, it is makes it possible to approximate the coda path Q to with the direct ray path, making it possible to apply a tomography method in the tomographic inversion. It is possible to assume an ellipsoidal volume as the coda path to better match scattering theories of the origin of coda waves; however, the actual distribution of energy within the ellipsoid is a matter of discussion (e.g. single scattering, multiple scattering), and beyond the scope of this work. We further believe that incorporating an ellipsoidal path of any type will change results very little since ellipsoids will be extremely elongated, and will be affected by roughly the same tomographic grid nodes as the direct wave paths. Traditional coda studies originating with Aki (1969) and Aki and Chouet (1975) were based on very late coda where measurements were taken well past twice the S-wave travel time. In the methodology used in this study to compute coda amplitudes, we intentionally measure the coda immediately following the direct S-wave. This ensures that the early coda is included in all of our measurements. In fact, the parameterization of the coda envelopes developed by Mayeda et al. 2003, is done so that early and late coda are well fit at both local and regional distances.

For the attenuation tomography modeling, we follow the procedure proposed by [START_REF] Phillips | Amplitude ratio tomography for regional phase Q[END_REF] based on the concept of removing source effects from the inversion by taking the ratio between the singleindividual amplitude measurements and their average value taken over all records associated with event. The source removal algorithm is no better nor worse than algorithms that solve explicitly for source terms, neither does the source term removal eliminate any source-path tradeoffs that might occur for events on outside edges. This class of techniques requires the assumption that the source radiates isotropically, and that the amplitudes, used as input for the inversions, are corrected for the geometrical spreading, since leaving the method to invert for lateral varying Q. We fully recognize that this methodology is empirical and there will always be a tradeoff between assumed spreading and the apparent 1/Q. As in previous studies (e.g., Phillips and Stead, 2008;Phillips et al. 2008;and Mayeda et al. 2005) we used a spreading that was 1/r for the direct waves based on the results of [START_REF] Yang | A numerical investigation of Lg geometrical spreading[END_REF] For the coda, we optimized our inversions and found that a spreading of r -0.7 worked best for all frequency bands as was done in the western Alps by [START_REF] Morasca | 2-D Coda and Direct Wave Tomography in Northern Italy[END_REF]. We reiterate that our results yield apparent 1/Q as our spreading model is also empirical. Because this is essentially empirical curve fitting we hope that our resultant variations qualitatively match what we might expect from the mapped geology. The measured coda and S-waves amplitudes are then corrected assuming 1/r geometrical spreading [START_REF] Yang | A numerical investigation of Lg geometrical spreading[END_REF] and separately used in the following amplitude ratio equation:

A ij -< A ij > j = S i -< S j > j + (P k dx ijk a k -< P k dx ijk a k > j ) log10(e) ( 1
)
where A is log10 amplitude, i, j, k indices are represent site, source and path discretization respectively, S is are the log10 site terms, and dx are the path lengths through a discretized region, a is the discretized attenuation coefficient (a k =/2Q k c), and P k are the raypath sums. The simbols symbols <> indicate thean averages for the same event j. Though we could test a ray "fattening" algorithm to better mimic the spatial sampling of the early coda, our feeling is that changes would be of 2nd or 3rd order and is outside the scope of the paper. Furthermore, the previous application of the method in the western U.S. (Phillips and Stead, 2008)

We solve for each frequency band using sparse matrix methods (LSQR; [START_REF] Paige | Algorithm 583, LSQR: Sparse linear equations and leastsquares problems[END_REF], applying the first difference regularization to the attenuation terms and assuming constrain the site terms to sum equal to zero. For the discretization, we assume cell size 0.1 X 0.1 degrees for all frequency bands.

Results and Discussion

For both S and coda waves, we find that the major structural features of the region are observable at all frequency bands. For each band, the corresponding checkerboard test was computed to identify the area where the results can be properly interpreted and to define the resolution, that is 0.5 degrees for all cases. In general, the well resolved areas are almost the same, except for very low and very high frequency (< 0.7 Hz and > 8.0 Hz) where it is slightly smaller. In figure 2 we show two examples for the frequency bands 1.5-2.0 Hz and 6.0-8.0 Hz for both coda and S waves and the corresponding checkerboard test results. The checkerboard test is admitedly a simple attempt at understanding the resolution of this idealized model with our given station-event distribution.

However, we also apply significant spatial smoothing and damping and it is also inherent in the LSQR method, which taken together, mimics broad spatial averaging, consistent with our current ideas on the early local and regional coda. The results obtained using coda waves are more smoothed than those derived from the direct waves analysis despite the fact that we apply lower smoothing constraints for coda waves since their averaging properties naturally reduce the scattering effects.

In general, the results confirm the observations by [START_REF] Morasca | 2-D Coda and Direct Wave Tomography in Northern Italy[END_REF] with higher Q to the east, corresponding to the Po Plain with respect to the Western Alps region. However, our results allow a more clear interpretation given the enlarged resolved area (Figure 3) that helps to complete the observations of the previous study where the relations between attenuation and geological structures were difficult, especially for the broader regions. In fact, the low attenuation previously observed in the Po Plain, also appeared to extend into the northernmost part of Apennines, a result that is at odds with other studies which characterize the region as high attenuation (Malagnini et al., 2000, Mele et al., 1997). Our current results however, confirm that this sector of the Northern Apennines is characterized by high Q and we show that the expected low Q are present only starting from the south of the Lunigiana-Garfagnana region which we presume is more influenced by the high heat flow in the lower crust beneath Tuscany.

A high attenuation anomaly, which corresponds with the so-called Ivrea body, is observed clearly with the direct waves and to a lesser extent with the coda waves over a broad range of frequencies.

However, the presence of this anomaly using both wave-types and a range of frequency bands implies that the anomaly is real and produced by heterogeneities spanning a range of length-scales, in agreement with Campillo et al. (1993) who observed an extinction of Lg waves in the same zone.

The enlargement of the resolved area with respect to the study of [START_REF] Morasca | 2-D Coda and Direct Wave Tomography in Northern Italy[END_REF], allows us to make more informed statements about the attenuation distribution in the Po Plain and the Eastern Alps. In fact, in the previous study, only a limited part of the Po Plain was observable and it was quite close to the limit of the resolved zone where, from the authors checkerboard tests, it is clear that most of the ray paths were oriented NE-SW, while in this current study this area is well covered by crossing rays from many directions (see figure 1). In all our maps, we notice a clear low attenuation zone within this area, probably due to the presence of Quaternary alluvium that covers this region. The variability (blue spots) inside this general trend of high Q could be interpreted as an effect of the variable thickness of sediments. However, looking at the maps in figure 2 for the direct waves (B and B'), a very high Q anomaly (violet) is shown in each one close to the northern border of the resolved area. Since these anomalous areas are observed in a region where the resolution begins to weaken and they are not observed for the coda analysis, we believe that this is not due to thick sediments but more likely due to noise on a few direct waves measurements.

As for the Eastern Alps we show Q values that are very close to the average (<Q>) in the example of figure 2, but considering the whole range of frequencies analyzed, we noticed a slightly higher attenuation with respect to the average Q for many frequencies higher than 1 Hz (also observable in figure 2A) for coda waves, and, on the contrary, a weak low attenuation for direct waves in the range 1-8 Hz. As stated in many studies (Mayeda et al. 2003;Mayeda et al. 1996) the stability of coda measurements with respect to the S amplitudes suggests that results from coda analysis may be more reliable. Moreover, they are consistent with Castro et al. 1996 andConsole andRovelli, 1981 who observed low values of Q for this region.

The small interstation variability observed for our coda amplitudes when compared to the direct waves measurements, confirms the higher stability for coda measurements. Figure 4A shows an example for the frequency band 2.0-3.0 Hz: comparing the standard deviations for the common events analyzed for some pairs of stations we notice that the values are lower for coda measurements in all cases. To confirm that it happens for all station pairs, we also estimated the average standard deviations for both kind of waves considering 55 station pairs and the results show a reduction of amplitude scatter of 0.14 for coda waves compared to direct waves (Figure 4B).

Given the lack of Q tomography images from other studies for Northern Italy, we try to compare our average coda-derived Q and S-wave Q with other 1-D regional studies. Figure 5 shows how both our coda and S-derived Q represent the average over the whole Northern Italy since all the other curves are the results of regional analysis. For example, Drouet et al. 2008 and[START_REF] Morasca | Ground Motion Scaling in the Western Alps[END_REF] focus their studies on the Western Alps finding very similar results although they apply different methodologies. On the other hand, Walter et al. 2007 observed lower values for station BNI (located in the Western Alps) and a stronger frequency dependence than the other two studies, in good agreement with our coda derived Q values.

In the Eastern Alps the Q(f) behavior does not seem very clear since different authors show different results. For example, Castro et al. 1996 andConsole andRovelli, 1981 obtained low Q values compared to Malagnini et al. 2002. Walter et al. 2007 observed, for station TRI, Qs values very close to Console and Rovelli, 1981 only for low frequencies. This may be due to many factors (Douglas, 2003) such as differences in the data selection, methodology, and geometrical spreading assumptions (e.g. Ford et al., 2008). Also, our tomographic results show differences in this area when we analyze coda or direct waves and similarly, opposite high and low attenuation values have been observed by Carletti and Gasperini (2003) in their study on seismic intensity attenuation.

More agreement is evidenced about the regional frequency dependence of Q. In fact, the studies on the Western Alps (except for Walter et al. 2007

Conclusions

The lateral variation of Q in Northern Italy was studied through tomographic inversions at various frequencies ranging between 0.3 and 25 Hz in order to cover the lack of 2-D attenuation information within the region.

This current study allow us to better define the Q variability beneath Northern Italy, analyzing both coda and direct waves. In both cases the main anomalies have been resolved, even though coda results are more smoothed because of the averaging properties of these scattered waves. The main findings of this work can be summarized as follows:

1) For the North-west Italy our new tomographic results confirm the observations by Morasca et al.

2008 who find lower Q in the Western Alps, characterized by the presence of cristalline massifs, with respect to the west Po Plain, the largest alluvial basin of Northern Italy. Also, the main anomalies such as the Ivrea zone, usually interpreted as a shallow slice of mantle (Kissling, 1993), are resolved in both studies. The low Q region of the Ivrea body is likely due to the strong lateral heterogeneity generated by interwoven mantle and crustal material in agreement with the observed blockage of Lg waves across this zone (Campillo et al., 1993).

2) By significantly increasing the available data and stations for Northern Italy, a remarkable improvement in the tomographic images of this region was possible, allowing a better interpretation of some anomalous Q behavior:

a) The Northern Apennines seems characterized by low attenuation even though it is well known (Castro et al. 1999, Malagnini et al., 2000, Mele et al., 1997) that the propagation is inefficient in the Central and Southern Apennines. Our study shows that to the south of the Lunigiana-Garfagnana region, the attenuation effectively increases as an effect of the presence of high heat flow in Tuscany (Boccaletti et al. 1977). Although these studies (Castro et al. 1999, Malagnini et al. 2000, Mele et al., 1997) show high attenuation also for the Northern Apennines, it is important to underline that the Lunigiana-Garfagnana is the very northern sector of this area, and the previous attenuation studies were based on data recorded in the whole Tuscany but showing a gap for this area. On the contrary, our analisys is based on a large amount of data for this sector since we used raypaths coming from a seismic network (RSNI) that covered this area with many stations.

b) A broad, low attenuation zone corresponds to the Po Plain, perhaps an effect of the thick compact sediments covering the basin. The variability observed especially for the direct waves analysis may be due to the different thickness of the material.

c) Looking at the Northeast of Italy, we observed Q values slightly lower than the average for coda waves in a wide range of frequency and weakly higher than <Q> for the direct waves analysis in a range 1-8 Hz. Given the higher stability of the coda measurements with respect to the direct ones (Mayeda et al. 2003;Mayeda et al. 1996), results from coda analysis should be more reliable, moreover they agree with previous studies by Castro et al. 1996

Figures captions

Figure 1 -Distribution of ray paths, stations (triangles) and events (circles). Gray lines represent rays from events to recording stations. We work on almost 16000 waveforms from 770 earthquakes recorded by 54 stations. are not influenced by the number of common events. On the top the values of the SD averaged over all 55 station pairs, are reported for both coda and direct waves. We see that coda results improve the scatter of a factor 0.14.

Figure 5 -Comparison of our <Q> for coda and direct waves obtained for the whole Northern Italy with some regional relationships for the Western Alps [START_REF] Morasca | Ground Motion Scaling in the Western Alps[END_REF]and Drouet et al. 2008, Walter et al. 2007 for station BNI), Eastern Alps (Castro et al. 1996;Console and Rovelli, 1981;Malagnini et al. 2002;Walter et al. 2007 for station TRI) Central Alps (Bay et al, 2003, Castro et al. 2008) and Apennines (Malagnini et al. 2000).

Improved 2-D Attenuation Analysis for Northern Italy Using a Merged Dataset from Selected Regional Seismic Networks.

Morasca (1) , P., Massa (2) , M., Laprocina (3) , E., Mayeda (4) , K., Phillips (5) , S., Malagnini (2) , L., Spallarossa (1) , D., Costa (3) ,G., and P. Augliera (2)

(1) University of Genova, Italy (2) Istituto Nazionale di Geofisica e Vulcanologia (3) University of Trieste, Italy (4) Weston Geophysical Corporation, Lexington, MA (5) Los Alamos National Laboratory, California, USA

Abstract

A merged, high-quality waveform data set from different seismic networks has been used to improve our understanding of lateral seismic attenuation for Northern Italy.

In a previous study on the same region, [START_REF] Morasca | 2-D Coda and Direct Wave Tomography in Northern Italy[END_REF] were able to resolve only a small area due to limited data coverage. For this reason the interpretation of the attenuation anomalies was difficult given the complexity of the region and the poor resolution of the available data. In order to better understand the lateral changes in the crustal structure and thickness of this region, we selected 770 earthquakes recorded by 54 stations for a total of almost 16000 waveforms derived from seismic networks operating totally or partially in Northern Italy. Direct S-wave and coda attenuation images were obtained using an amplitude ratio technique that eliminates source terms from the formulation. Both direct and early-coda amplitudes are used as input for the inversions and the results are compared.

Results were obtained for various frequency bands ranging between 0.3-25.0 Hz, and in all cases show significant improvement with respect to the previous study since the resolved area has been extended and more crossing paths have been used to image smaller scale anomalies. Quality-factor estimates are consistent with the regional tectonic structure exhibiting a general trend of low attenuation under the Po Plain basin and higher values for the western Alps and northern

Apennines. The interpretation of the results for the Eastern Alps is not simple, possibly because our resolution for this area is still not adequate to resolve small scale structures.

Introduction

The quality factor, Q, is one of the many parameters required for input into seismological models for geophysical applications such as simulation of strong ground motions (Boore, 2003;Atkinson andBoore, 1995, 2006), earthquake hazard analysis (e.g. Akinci et al. 2004, Montaldo et al. 2005),

or for a structural and tectonic interpretation (e.g. Mele et al. 1997).

Within Northern Italy, the Q parameter has been defined using one-dimensional models for different sub-regions (e.g., [START_REF] Morasca | Ground Motion Scaling in the Western Alps[END_REF]Drouet et al. 2008, for the Western Alps; Thouvenot, 1983, Castro et al. 2008and Bay et al. 2003 for the Central Alps; Malagnini et al. 2002, Castro et al. 1996and Console and Rovelli, 1981 for the Eastern Alps). The motivation for improving upon the 2-D attenuation model for Northern Italy is several fold. First, the region is characterized by several anomalous zones (Kissling, 1993, Kissling and Spakman, 1996[START_REF] Paul | A threedimensional crustal velocity model of the southwestern Alps from local earthquake tomography[END_REF]. In general, a simple 1-D model is very likely insufficient to describe the seismic wave propagation at the regional scale in this region. For example, within each sub-region strong lateral anomalies exist (Campillo et al. 1993;[START_REF] Paul | A threedimensional crustal velocity model of the southwestern Alps from local earthquake tomography[END_REF]Cattaneo et al. 1999;Kissling, 1993, Kissling andSpakman, 1996), and in some cases the authors had to use complicated 1-D models to describe the attenuation in the sub-region [START_REF] Morasca | Ground Motion Scaling in the Western Alps[END_REF].

A lateral varying attenuation model of seismic attenuation in Italy was proposed by Carletti and Gasperini (2003), that gives a qualitative description that was based upon seismic intensity derived from macroseismic observations. For a more quantitative description, Campillo et al. (1993) computed the Lg/Pn amplitude ratios across the Western Alps and used numerical simulations to model anomalous propagation of Lg waves across the region.

However, little has been published using amplitude attenuation to produce detailed images of regionally varying Q in Northern Italy. An attempt at estimating attenuation tomographically in this region was carried out by [START_REF] Morasca | 2-D Coda and Direct Wave Tomography in Northern Italy[END_REF]. The authors tested the attenuation tomography method developed by [START_REF] Phillips | Amplitude ratio tomography for regional phase Q[END_REF] combined with the coda wave measurement approach proposed by Mayeda et al. (2003), to correct the observed data. Their results showed that the 2-D path correction produce lower standard deviations when applied to coda wave measurements. For comparison, they also applied the 2-D approach on the direct wave amplitudes, and also in this case the improvement was observed, although the improvement did not approach coda levels.

The limitation of the study by [START_REF] Morasca | 2-D Coda and Direct Wave Tomography in Northern Italy[END_REF] was in the small resolved area that makes it difficult to derive an interpretation of the observed Q variations as a function of the complex structural setting. Although the authors try to correlate their attenuation results to the known anomalies in the region, they acknowledge their difficulties and underline the necessity to enlarge the resolved area for a more significant interpretation of the wave propagation in the Northern Italy.

In fact, the spatial distribution of events and stations may introduce an azimuthal effect on the amplitudes of the ground motion (Campbell and Bozorgnia, 1994), and the [START_REF] Morasca | 2-D Coda and Direct Wave Tomography in Northern Italy[END_REF] study is mainly based on data recorded in the western Alps and northern Apennines with only a few paths coming from the north eastern Italy.

The aim of this study is to provide a more complete understanding of attenuation for northern Italy, as literature is lacking for this region. On a pragmatic level, having a detailed knowledge of the laterally varying attenuation will be important for seismic risk analysis. This study represents the first time that a dataset of this size has been amassed for northern Italy that includes waveforms and stations from various networks operating in this area. In fact, while seismic travel-time tomography studies rely on published phase picks in network bulletins, attenuation tomography requires a large amount of waveform data with good azimuthal coverage.

Tectonic framework

The region we are focusing on is characterized by a combination of different structures interacting with each other. On a large scale, it is possible to consider the relationship between the Alpine chain and the Northern Apennines, two different orogenic structures; the first one being related to the subduction of Europe underneath the Adriatic plate, and the second one is a fold-and-thrust belt developed as consequence of the westward subduction of the Adriatic plate (Carminati et al., 2004).

Looking more in detail at the relationship between the Alps and the Northern Apennines, the Ligurian Alps represent the western margin that links the two main structures, where the progressive opposite retreat and arching of the western Alps and the northern Apennines have induced tensional forces and a general E-W regional stretching (Vignaroli et al. 2008).

The central area of northern Italy is characterized by the presence of the Po Plain, a wide flexural basin covered by thick Plio-Quaternary sediments, slightly dipping toward the east (Pierdominici et al., 2005). Below the Po Plain different north-verging apenninic arcs face the south verging Alpine tectonic structures (Castellarin and Vai, 1986) showing a complex pattern related to a non-uniform stress field (Pierdominici et al., 2005).

The northeastern part of Italy is mainly influenced by the interaction between the Dinarides belt and the Southern Alps, the backthrust of the Alps (Slejko et al., 1987(Slejko et al., , 1989)), reflecting the collision between the Adria microplate and the European plate. As stated by Carminati et al (2004), even this area seems to be affected by the presence of the Apenninic structure, even though less than in the central-western Alps. This indicates a complex interaction of geodynamic fields within this region.

All these interactions within northern Italy result in complicated variations in crustal thickness and strong intracrustal inhomogeneities that are reflected in the anomalous propagation of the seismic waves in this area. Westward, the crust-mantle boundary bathimetry is not completely known, although it seems to be composed by three different surfaces (Kissling, 1993), and the crust is affected by lateral variations, as observed by many authors (Cattaneo et al., 1999;Kissling, 1993;Kissling and Spakman, 1996), with a thickness varying from a minimum of ~ 10 km in the Ligurian sea to ~ 50 km beneath the Alps (Buness et al. 1990). A well known intracrustal anomaly of this area is the Ivrea body, interpreted as a wedge of the Adriatic upper mantle (Kissling 1993;Di Stefano et al., 1999).

In eastern Italy, the main feature is represented by the transition between the European and Adriatic crust interpreted as a doubling crust (Lippitsch, 2002) or as a transcrustal ramp (Transalp Working Group, 2002). The geometry of the Adriatic crust-mantle boundary shallows rapidly from a depth of 60 km beneath the Dolomites to 30 km or 40 km, depending on the considered model (Ebbing, 2004) beneath the Po Plain.

In this context, the seismic activity is significant in the northeast part of Italy, along the southern Alps-Po Plain margin between Friuli (Slejko et al., 1987;Slejko et al., 1989;Benedetti et al., 2000;Galadini et al., 2001) and western Lombardia that was recently struck by the November 24, 2004, Ml=5.2, Salò earthquake, which occurred along the west coast of Lake Garda. The seismic activity tends to decrease westward along the Apennine foredeep, from the Ferrara, to the Emilia and the Monferrato Arcs, but it is not negligible (two seismic sequences occurred in the Monferrato area on August 2000 and July 2001, with the largest events of local magnitude 5.1 and 4.6 respectively).

In the western Alps, the seismic activity is generally moderate and concentrated along an external belt, corresponding to the Penninic front, and an internal belt, corresponding to the Austro-Alpine front and its southern extension (Cattaneo et al., 1999;Sue et al., 1999).

Data

A substantial amount of effort has gone into collecting and quality checking the data set for this study. Our merged data set is composed of 770 selected earthquakes with local magnitude > 3, each one recorded by at least 2 stations as required by the tomography methodology of [START_REF] Phillips | Amplitude ratio tomography for regional phase Q[END_REF]. The waveforms (~16,000) were recorded by 54 stations (Table 1) from the following networks: SDSNet (Swiss Digital Seismic Network), RSNI (Regional Seismic network of The maps of Figure 1 show the dense ray coverage we obtained with this data set. This is an important point for the aim of this study since it allows robust tomographic inversions with good resolution. Except for few events, most of the rays stay in the upper 50 km, which means that our 2-D Q results refer to the crust and the uppermost mantle structure.

Methodology overview

The data are corrected for the instrument response, and the horizontal components, filtered through narrow frequency bands over the range 0.3-25 Hz, are used to compute both S and coda waves amplitudes as input to the tomographic inversions.

For S-wave measurements, the maximum amplitude following the onset of the S-wave arrival is considered for each horizontal component and the average is computed.

For coda-wave measurements, we used the empirical formulation of Mayeda et al. (2003;eq. 3) to calibrate narrowband synthetic coda envelopes to fit observations and compute amplitudes. The amplitudes are computed from the early part of the coda. Traditional coda studies originating with Aki (1969) and Aki and Chouet (1975) were based on very late coda where measurements were taken well past twice the S-wave travel time. In the methodology used in this study to compute coda amplitudes, we intentionally measure the coda immediately following the direct S-wave. This ensures that the early coda is included in all of our measurements. In fact, the parameterization of the coda envelopes developed by Mayeda et al. 2003, is done so that early and late coda are well fit at both local and regional distances.Our use of short, early coda windows makes it possible to approximate the coda path with the direct ray path in the tomographic inversion. It is possible to assume an ellipsoidal volume as the coda path to better match scattering theories of the origin of coda waves; however, the actual distribution of energy within the ellipsoid is a matter of discussion (e.g. single scattering, multiple scattering), and beyond the scope of this work. We further believe that incorporating an ellipsoidal path of any type will change results very little since ellipsoids will be extremely elongated, and will be affected by roughly the same tomographic grid nodes as the direct wave paths.

For the attenuation tomography modeling, we follow the procedure proposed by [START_REF] Phillips | Amplitude ratio tomography for regional phase Q[END_REF] based on the concept of removing source effects from the inversion by taking the ratio between individual amplitude measurements and their average value taken over all records associated with event. The source removal algorithm is no better nor worse than algorithms that solve explicitly for source terms, neither does the source term removal eliminate any source-path tradeoffs that might occur for events on outside edges. This class of techniques require the assumption that the source radiates isotropically, and that the amplitudes, used as input for the inversions, are corrected for the geometrical spreading, leaving the method to invert for lateral varying Q We fully recognize that this methodology is empirical and there will always be a tradeoff between assumed spreading and the apparent 1/Q. As in previous studies (e.g., Phillips and Stead, 2008;Phillips et al. 2008;and Mayeda et al. 2005) we used a spreading that was 1/r for the direct waves based on the results of [START_REF] Yang | A numerical investigation of Lg geometrical spreading[END_REF] For the coda, we optimized our inversions and found that a spreading of r -0.7 worked best for all frequency bands as was done in the western Alps by [START_REF] Morasca | 2-D Coda and Direct Wave Tomography in Northern Italy[END_REF]. We reiterate that our results yield apparent 1/Q as our spreading model is also empirical. Because this is essentially empirical curve fitting we hope that our resultant variations qualitatively match what we might expect from the mapped geology. The measured coda and S-waves amplitudes are then corrected assuming 1/r geometrical spreading [START_REF] Yang | A numerical investigation of Lg geometrical spreading[END_REF] and separately used in the following amplitude ratio equation:

A ij -< A ij > j = S i -< S j > j + (P k dx ijk a k -< P k dx ijk a k > j ) log10(e) ( 1
)
where A is log10 amplitude, i, j, k indices represent site, source and path discretization respectively, S are the log10 site terms, and dx are the path lengths through a discretized region, a is the discretized attenuation coefficient (a k =/2Q k c), and P k are the raypath sums. The symbols <> indicate anaverage for event j. Though we could test a ray "fattening" algorithm to better mimic the spatial sampling of the early coda, our feeling is that changes would be of 2nd or 3rd order and is outside the scope of the paper. Furthermore, the previous application of the method in the western U.S. (Phillips and Stead, 2008) used predominantly direct Lg which by any measure is not a ray, but a superposition of multiply-reflected shear waves in the crust/upper mantle. The early coda would simply smear this region further laterally. For more details on the methodology we address the reader to [START_REF] Phillips | Amplitude ratio tomography for regional phase Q[END_REF]Phillips et al. 2008.

We solve for each frequency band using sparse matrix methods (LSQR; [START_REF] Paige | Algorithm 583, LSQR: Sparse linear equations and leastsquares problems[END_REF], applying first difference regularization to the attenuation terms and constrain the site terms to sum to zero. For the discretization, we assume cell size 0.1 X 0.1 degrees for all frequency bands.

Results and Discussion

For both S and coda waves, we find that the major structural features of the region are observable at all frequency bands. For each band, the corresponding checkerboard test was computed to identify the area where the results can be properly interpreted and to define the resolution, that is 0.5 degrees for all cases. In general, the well resolved areas are almost the same, except for very low and very high frequency (< 0.7 Hz and > 8.0 Hz) where it is slightly smaller. In figure 2 we show two examples for the frequency bands 1.5-2.0 Hz and 6.0-8.0 Hz for both coda and S waves and the corresponding checkerboard test results. The checkerboard test is admitedly a simple attempt at understanding the resolution of this idealized model with our given station-event distribution.

However, we also apply significant spatial smoothing and damping and it is also inherent in the LSQR method, which taken together, mimics broad spatial averaging, consistent with our current ideas on the early local and regional coda. The results obtained using coda waves are more smoothed than those derived from the direct waves analysis despite the fact that we apply lower smoothing constraints for coda waves since their averaging properties naturally reduce the scattering effects.

In general, the results confirm the observations by [START_REF] Morasca | 2-D Coda and Direct Wave Tomography in Northern Italy[END_REF] with higher Q to the east, corresponding to the Po Plain with respect to the Western Alps region. However, our results allow a more clear interpretation given the enlarged resolved area (Figure 3) that helps to complete the observations of the previous study where the relations between attenuation and geological structures were difficult, especially for the broader regions. In fact, the low attenuation previously observed in the Po Plain, also appeared to extend into the northernmost part of Apennines, a result that is at odds with other studies which characterize the region as high attenuation (Malagnini et al., 2000, Mele et al., 1997). Our current results however, confirm that this sector of the Northern Apennines is characterized by high Q and we show that the expected low Q are present only starting from the south of the Lunigiana-Garfagnana region which we presume is more influenced by the high heat flow in the lower crust beneath Tuscany.

A high attenuation anomaly, which corresponds with the so-called Ivrea body, is observed clearly with the direct waves and to a lesser extent with the coda waves over a broad range of frequencies.

However, the presence of this anomaly using both wave-types and a range of frequency bands implies that the anomaly is real and produced by heterogeneities spanning a range of length-scales, in agreement with Campillo et al. (1993) who observed an extinction of Lg waves in the same zone.

The enlargement of the resolved area with respect to the study of [START_REF] Morasca | 2-D Coda and Direct Wave Tomography in Northern Italy[END_REF], allows us to make more informed statements about the attenuation distribution in the Po Plain and the Eastern Alps. In fact, in the previous study, only a limited part of the Po Plain was observable and it was quite close to the limit of the resolved zone where, from the authors checkerboard tests, it is clear that most of the ray paths were oriented NE-SW, while in this current study this area is well covered by crossing rays from many directions (see figure 1). In all our maps, we notice a clear low attenuation zone within this area, probably due to the presence of Quaternary alluvium that covers this region. The variability (blue spots) inside this general trend of high Q could be interpreted as an effect of the variable thickness of sediments. However, looking at the maps in figure 2 for the direct waves (B and B'), a very high Q anomaly (violet) is shown in each one close to the northern border of the resolved area. Since these anomalous areas are observed in a region where the resolution begins to weaken and they are not observed for the coda analysis, we believe that this is not due to thick sediments but more likely due to noise on a few direct waves measurements.

As for the Eastern Alps we show Q values that are very close to the average (<Q>) in the example of figure 2, but considering the whole range of frequencies analyzed, we noticed a slightly higher attenuation with respect to the average Q for many frequencies higher than 1 Hz (also observable in figure 2A) for coda waves, and, on the contrary, a weak low attenuation for direct waves in the range 1-8 Hz. As stated in many studies (Mayeda et al. 2003;Mayeda et al. 1996) the stability of coda measurements with respect to the S amplitudes suggests that results from coda analysis may be more reliable. Moreover, they are consistent with Castro et al. 1996 andConsole andRovelli, 1981 who observed low values of Q for this region.

The small interstation variability observed for our coda amplitudes when compared to the direct waves measurements, confirms the higher stability for coda measurements. Figure 4A shows an example for the frequency band 2.0-3.0 Hz: comparing the standard deviations for the common events analyzed for some pairs of stations we notice that the values are lower for coda measurements in all cases. To confirm that it happens for all station pairs, we also estimated the average standard deviations for both kind of waves considering 55 station pairs and the results show a reduction of amplitude scatter of 0.14 for coda waves compared to direct waves (Figure 4B).

Given the lack of Q tomography images from other studies for Northern Italy, we try to compare our average coda-derived Q and S-wave Q with other 1-D regional studies. Figure 5 shows how both our coda and S-derived Q represent the average over the whole Northern Italy since all the other curves are the results of regional analysis. For example, Drouet et al. 2008 and[START_REF] Morasca | Ground Motion Scaling in the Western Alps[END_REF] focus their studies on the Western Alps finding very similar results although they apply different methodologies. On the other hand, Walter et al. 2007 observed lower values for station BNI (located in the Western Alps) and a stronger frequency dependence than the other two studies, in good agreement with our coda derived Q values.

In the Eastern Alps the Q(f) behavior does not seem very clear since different authors show different results. For example, Castro et al. 1996 andConsole andRovelli, 1981 obtained low Q values compared to Malagnini et al. 2002. Walter et al. 2007 observed, for station TRI, Qs values very close to Console and Rovelli, 1981 only for low frequencies. This may be due to many factors (Douglas, 2003) such as differences in the data selection, methodology, and geometrical spreading assumptions (e.g. Ford et al., 2008). Also, our tomographic results show differences in this area when we analyze coda or direct waves and similarly, opposite high and low attenuation values have been observed by Carletti and Gasperini (2003) in their study on seismic intensity attenuation.

More agreement is evidenced about the regional frequency dependence of Q. In fact, the studies on the Western Alps (except for Walter et al. 2007

Conclusions

The lateral variation of Q in Northern Italy was studied through tomographic inversions at various frequencies ranging between 0.3 and 25 Hz in order to cover the lack of 2-D attenuation information within the region.

This current study allow us to better define the Q variability beneath Northern Italy, analyzing both coda and direct waves. In both cases the main anomalies have been resolved, even though coda results are more smoothed because of the averaging properties of these scattered waves. The main findings of this work can be summarized as follows:

1) For the North-west Italy our new tomographic results confirm the observations by [START_REF] Morasca | 2-D Coda and Direct Wave Tomography in Northern Italy[END_REF] who find lower Q in the Western Alps, characterized by the presence of cristalline massifs, with respect to the west Po Plain, the largest alluvial basin of Northern Italy. Also, the main anomalies such as the Ivrea zone, usually interpreted as a shallow slice of mantle (Kissling, 1993), are resolved in both studies. The low Q region of the Ivrea body is likely due to the strong lateral heterogeneity generated by interwoven mantle and crustal material in agreement with the observed blockage of Lg waves across this zone (Campillo et al., 1993).

2) By significantly increasing the available data and stations for Northern Italy, a remarkable improvement in the tomographic images of this region was possible, allowing a better interpretation of some anomalous Q behavior:

a) The Northern Apennines seems characterized by low attenuation even though it is well known (Castro et al. 1999, Malagnini et al., 2000, Mele et al., 1997) that the propagation is inefficient in the Central and Southern Apennines. Our study shows that to the south of the Lunigiana-Garfagnana region, the attenuation effectively increases as an effect of the presence of high heat flow in Tuscany (Boccaletti et al. 1977). Although these studies (Castro et al. 1999, Malagnini et al. 2000, Mele et al., 1997) show high attenuation also for the Northern Apennines, it is important to underline that the Lunigiana-Garfagnana is the very northern sector of this area, and the previous attenuation studies were based on data recorded in the whole Tuscany but showing a gap for this area. On the contrary, our analisys is based on a large amount of data for this sector since we used raypaths coming from a seismic network (RSNI) that covered this area with many stations.

b) A broad, low attenuation zone corresponds to the Po Plain, perhaps an effect of the thick compact sediments covering the basin. The variability observed especially for the direct waves analysis may be due to the different thickness of the material.

c) Looking at the Northeast of Italy, we observed Q values slightly lower than the average for coda waves in a wide range of frequency and weakly higher than <Q> for the direct waves analysis in a range 1-8 Hz. Given the higher stability of the coda measurements with respect to the direct ones (Mayeda et al. 2003;Mayeda et al. 1996), results from coda analysis should be more reliable, moreover they agree with previous studies by Castro et al. 1996 andConsole andRovelli, 1981, who observed low Q in this region. However, Malagnini et al. 2002 show different results for the same area, highlighting an unclear picture of the propagation in this region. We think that the boundary position of this sector of Northern Italy could require additional information from the surrounding networks of Austria and Slovenia to add some constraints to better estimate the attenuation in this area of Italy.

Figures captions

Figure 1 -Distribution of ray paths, stations (triangles) and events (circles). Gray lines represent rays from events to recording stations. We work on almost 16000 waveforms from 770 earthquakes recorded by 54 stations. are not influenced by the number of common events. On the top the values of the SD averaged over all 55 station pairs, are reported for both coda and direct waves. We see that coda results improve the scatter of a factor 0.14.

Figure 5 -Comparison of our <Q> for coda and direct waves obtained for the whole Northern Italy with some regional relationships for the Western Alps [START_REF] Morasca | Ground Motion Scaling in the Western Alps[END_REF]and Drouet et al. 2008, Walter et al. 2007 for station BNI), Eastern Alps (Castro et al. 1996;Console and Rovelli, 1981;Malagnini et al. 2002;Walter et al. 2007 for station TRI) Central Alps (Bay et al, 2003, Castro et al. 2008) and Apennines (Malagnini et al. 2000).

  network of Northwestern Italy), INGV-MI-PV (seismic network managed by INGV section of Milano-Pavia), RSNC (Italian national National seismic Seismic network Network managed by INGV-CNT, Rome), RSFVG (Rete Sismometrica del Friuli-Venezia Giulia), NEIBB (Nord-Est Italy Broadband Network). The two stations of the NEIBB (TRI and VINO) are managed both by OGS (Istituto Nazionale di Oceanografia e Geofisica Sperimentale) and by the Departement of Earth Sciences of the University of Trieste.

  who show results only for station BNI) show that the quality factors vary slowly with frequency, while on the contrary, all the studies on the Eastern sector of Northern Italy indicate a strong frequency dependence of Q. For the Central Alps, the results obtained byBay et al., (2003) seem very similar to those obtained byMalagnini et al. 2002 for the Eastern Alps. However, Qs values obtained byCastro et al. 2008 for the Central-Eastern Alps, seem to be the closest to our Northern Italy averaged results.Since our study also includes the Northern Apennine, we observe low attenuation only to the south of the Lunigiana Garfagnana. In figure5we show the 1-D attenuation function proposed for the Apennines byMalagnini et al. 2000. Their results show low Q values at all analyzed frequencies (0.25-5.0 Hz) as well as a very low frequency dependence. It is important to note that their study is based on the entire Apennine belt, and the influence of the Q values relative to the Northern sector of the chain on their relationship is small compared to the whole Apenninic data set.

Figure 2 -

 2 Figure 2 -Results for the frequency bands 1.5-2.0 Hz and 6.0-8.0 Hz. A and A' are the maps obtained from coda waves analysis; B and B' from S-waves; C and C' are the corresponding checkerboard tests showing the resolution area and the dimension of the anomalies we are able to resolve (0.5 X 0.5 degrees). The input model is shown on the right corner of each checkerboard map and the low and high attenuation input cells are Q=500 and Q=50 respectively.

Figure 3 -

 3 Figure 3 -Map showing the dimension of the resolved area (contiuous black line) obtained in this work compared to the resolution limits of the previous study by Morasca et al. 2008 (dashed black line). The improvement is noticeable especially to the east since we can cover the whole Po Plain and part of the Eastern Alps.

Figure 4 -

 4 Figure 4 -A) Examples of coda (gray circles) and direct (white circles) amplitude measurements of common events at some pairs of stations for the frequency band 2.0-3.0 Hz. All the shown amplitudes are corrected for the attenuation. Notice that coda waves reduce the scatter in all cases. B) Standard deviations for the same frequency band (2.0-3.0 Hz) considering 55 station pairs for both coda (triangles) and direct (circles) waves analysis.The plot shows as the standard deviations

  Northwestern Italy), INGV-MI-PV (seismic network managed by INGV section of Milano-Pavia), RSNC (Italian National Seismic Network managed by INGV-CNT, Rome), RSFVG (Rete Sismometrica del Friuli-Venezia Giulia), NEIBB (Nord-Est Italy Broadband Network). The two stations of the NEIBB (TRI and VINO) are managed both by OGS (Istituto Nazionale di Oceanografia e Geofisica Sperimentale) and by the Departement of Earth Sciences of the University of Trieste.

  who show results only for station BNI) show that the quality factors vary slowly with frequency, while on the contrary, all the studies on the Eastern sector of Northern Italy indicate a strong frequency dependence of Q. For the Central Alps, the results obtained byBay et al., (2003) seem very similar to those obtained byMalagnini et al. 2002 for the Eastern Alps. However, Qs values obtained byCastro et al. 2008 for the Central-Eastern Alps, seem to be the closest to our Northern Italy averaged results.Since our study also includes the Northern Apennine, we observe low attenuation only to the south of the Lunigiana Garfagnana. In figure5we show the 1-D attenuation function proposed for the Apennines byMalagnini et al. 2000. Their results show low Q values at all analyzed frequencies (0.25-5.0 Hz) as well as a very low frequency dependence. It is important to note that their study is based on the entire Apennine belt, and the influence of the Q values relative to the Northern sector of the chain on their relationship is small compared to the whole Apenninic data set.

Figure 2 -

 2 Figure 2 -Results for the frequency bands 1.5-2.0 Hz and 6.0-8.0 Hz. A and A' are the maps

Figure 3 -

 3 Figure 3 -Map showing the dimension of the resolved area (contiuous black line) obtained in this

Figure 4 -

 4 Figure 4 -A) Examples of coda (gray circles) and direct (white circles) amplitude measurements of

Figure1

  Figure1Click here to download high resolution image

Table 1 -

 1 Stations used in this study.

	Station	Latitude	Longitude	H (m) Network
	ASO2	45.80480	11.91800	221.0 INGV-MI-PV
	BACM	44.27830	10.07216	490.0 RSNI
	BAD	46.23400	13.24300	640.0 RSFVG
	BAG3	45.82280	10.46650	807.0 INGV-MI-PV
	BOB_	44.76792	9.44782	910.0 RSNC
	CAE	46.00900	12.43800	870.0 RSFVG
	CODM	44.39083	9.85000	616.0 RSNI
	CTLE	45.27630	9.76220	66.0 INGV-MI-PV
	DIX_	46.08050	7.40810	2410.0 SDSNet
	DOI_	44.50415	7.24665	1039.0 RSNC
	DRE	46.17300	13.64400	1750.0 RSFVG
	EMV_	46.06310	6.89930	2220.0 SDSNet
	FENM	45.03017	7.06267	1000.0 RSNI
	FNVD	44.16782	11.12290	950.0 RSNC
	GENL	44.40570	8.96970	80.0 RSNI
	GRAM	44.49130	10.06580	871.0 RSNI
	GROG	43.42618	9.89201	118.0 RSNC
	LAB2	45.48030	9.23210	125.0 INGV-MI-PV

  used predominantly direct Lg which by any measure is not a ray, but a superposition of multiply-reflected shear waves in the crust/upper mantle. The early coda would simply smear this region further laterally. For more details on the methodology we address the reader to[START_REF] Phillips | Amplitude ratio tomography for regional phase Q[END_REF] Phillips et al. 2008.

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  and Console and Rovelli, 1981, who observed low Q in this region. However,Malagnini et al. 2002 show different results for the same area, highlighting an unclear picture of the propagation in this region. We think that the boundary position of this sector of Northern Italy could require additional information from the surrounding networks of Austria and Slovenia to add some constraints to better estimate the attenuation in this area of Italy.

	Morasca, P., Malagnini, L., Akinci, A., Spallarossa D. (2006). Ground Motion Scaling in the
	Western Alps. Journal of Seismology, Vol. 10 n. 3 pp 315-333. Yang, X. (2002). A numerical investigation of Lg geometrical spreading. Bull. Seism. Soc. Am.,	Formatted: Font: Not Italic Formatted: Font: Not Italic Formatted: Font: Not Italic
	92, 3067-3079.	Formatted: Font: Not Italic
	Morasca, P., Mayeda, K., Gök, R., Phillips W. S., and L., Malagnini (2008). 2-D Coda and Direct	Formatted: Font: (Default) Times New Roman
	Wave Tomography in Northern Italy. Bull.Seis.Soc.Am. 98, pp.1936-1946	Formatted: Font: Not Italic Formatted: Font: Not Italic
	Paige, C. C., and M. A. Saunders (1982). Algorithm 583, LSQR: Sparse linear equations and least-	Formatted: Font: (Default) Times New Roman Formatted: Font: (Default) Times New Roman
	squares problems, ACM Transactions on Mathematical Software, 8, 195-209.	Formatted: Font: Not Italic Formatted: Font: (Default) Times New Roman, Not Italic
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Formatted: Font: Not Italic Formatted: Font: Not Italic Formatted: Font: Not Italic Formatted: Font: Not Italic Paul A, Cattaneo M, Thouvenot F, Spallarossa D, B´ethoux N, Fr´echet J (2001) A threedimensional crustal velocity model of the southwestern Alps from local earthquake tomography. J Geophis Res 106(B9):19367-19389 Phillips, W. S., Hartse, H. E. and J. T. Rutledge (2005), Amplitude ratio tomography for regional phase Q. Geophys. Res. Lett. Vol. 32, no 21, L21301,doi: 10.1029/2005GL023870. Phillips, W.S. and R.J. Stead (2008). Attenuation of Lg in the western US using the USArray. Geophys. Res. Lett. Vol. 35, L07307, doi:10.1029/2007GL032926, 2008 Phillips, W.S., R.J. Stead, G.E. Randall, H.E. Hartse and K. Mayeda (2008), Source effects from broad area network calibration of regional distance coda waves, in Advances in Geophysics, 50, Scattering of Short Period Waves in the Heterogeneous Earth, H. Sato and M.C. Fehler, Editors, 319-351.

Table 1 -

 1 Stations used in this study.

	Station	Latitude	Longitude	H (m) Network
	ASO2	45.80480	11.91800	221.0 INGV-MI-PV
	BACM	44.27830	10.07216	490.0 RSNI
	BAD	46.23400	13.24300	640.0 RSFVG
	BAG3	45.82280	10.46650	807.0 INGV-MI-PV
	BOB_	44.76792	9.44782	910.0 RSNC
	CAE	46.00900	12.43800	870.0 RSFVG
	CODM	44.39083	9.85000	616.0 RSNI
	CTLE	45.27630	9.76220	66.0 INGV-MI-PV
	DIX_	46.08050	7.40810	2410.0 SDSNet
	DOI_	44.50415	7.24665	1039.0 RSNC
	DRE	46.17300	13.64400	1750.0 RSFVG
	EMV_	46.06310	6.89930	2220.0 SDSNet
	FENM	45.03017	7.06267	1000.0 RSNI
	FNVD	44.16782	11.12290	950.0 RSNC
	GENL	44.40570	8.96970	80.0 RSNI
	GRAM	44.49130	10.06580	871.0 RSNI
	GROG	43.42618	9.89201	118.0 RSNC
	LAB2	45.48030	9.23210	125.0 INGV-MI-PV
	MABI	46.05490	10.51400	1853.0 INGV-MI-PV
	MAIM	43.91420	10.49150	290.0 RSNI
	MAL3	46.29130	9.86570	2030.0 INGV-MI-PV
	MAR2	45.73970	10.11750	600.0 INGV-MI-PV
	MDI_	45.76972	9.71600	954.0 RSNC
	MER2	45.67250	9.41820	350.0 INGV-MI-PV
	MI50	46.01070	9.29100	219.0 INGV-MI-PV
	MI52	45.60800	10.52600	123.0 INGV-MI-PV
	MI55	45.60620	10.21550	210.0 INGV-MI-PV
	MI61	45.63750	9.93380	253.0 INGV-MI-PV
	MMK_	46.05050	7.96400	2210.0 SDSNet
	MONC	44.06000	7.92000	420.0 RSNC
	MONE	44.07950	7.75500	1320.0 RSNI
	MRGE	45.75000	7.08000	1600.0 RSNC
	MUGI	45.92190	9.04170	878.0 SDSNet
	NEGI	43.84770	7.70466	640.0 RSNI
	NEGR	45.49760	10.94820	167.0 INGV-MI-PV
	POPM	44.04500	10.75710	300.0 RSNI
	RONM	43.88130	7.59810	300.0 RSNI
	RORM	44.11210	8.06620	390.0 RSNI
	RORO	44.11217	8.06617	260.0 RSNI
	ROTM	44.84930	8.35260	221.0 RSNI
	SALO	45.61770	10.52510	544.0 INGV-MI-PV
	SARM	44.18410	10.40100	1040.0 RSNI