Dr Tomas Fischer

Tomas Fischer

Tomas Fischer

Josef Horálek

Jan Michálek

Alena Boušková

Tomáš Fischer

The 2008-West Bohemia earthquake swarm in the light of the WEBNET network

Keywords: earthquake swarm, seismic network, seismicity

A swarm of earthquakes of magnitudes up to ML=3.8 stroke the region of West Bohemia/Vogtland (border area between Czechia and Germany) in October 2008. It occurred in the Nový Kostel focal zone, where also all recent earthquake swarms (1985/86, 1997 and 2000) took place, and was striking by a fast sequence of macroseismically observed earthquakes. We present the basic characteristics of this swarm based on the observations of a local network WEBNET, which has been operated in the epicentral area, on the Czech territory. The swarm was recorded by 13 to 23 permanent and mobile WEBNET stations surrounding the swarm epicentres. In addition, a part of the swarm was also recorded by strong-motion accelerometers, which represent the first true accelerograms of the swarm earthquakes in the region. The peak ground acceleration reached 0.65 m/s2. A comparison with previous earthquake swarms indicates that the total seismic moments released during the 1985/86 and 2008 swarms are similar, of about 4E16 Nm, and that they represent the two largest swarms occurred in the West Bohemia/Vogtland region since the ML=5.0 swarm of 1908. Characteristic features of the 2008 swarm are its short duration (four weeks) and rapidity, and consequently the fastest seismic moment release compared to previous swarms. Up to 25 000 events in the magnitude range of 0.5<ML<3.8 were detected using an automatic picker. A total of 9 swarm phases can be distinguished in the swarm, five of them exceeding the magnitude level of 2.5. The magnitude-frequency distribution of the complete 2008 swarm activity shows a b-value close to 1. The swarm hypocenters fall precisely on the same fault portion of the Nový Kostel focal zone that was activated by the 2000 swarm (ML≤3.2) in a depth interval from 6 to 11 km and also by the 1985/86 swarm (ML≤4.6). The steeply dipping fault planes of the 2000 and 2008 swarms seem to be identical considering the location error of about 100 m. Also, focal mechanisms of the 2008 swarm are identical with those of the 2000 swarm, both matching an average strike of 170° and dip of 80° of the activated fault segment. An overall upward migration of activity is observed with first events at the bottom and last events at the top of the of the activated fault patch. Similarities in the activated fault area and in the seismic moments released during the three largest recent swarms enable to estimate the seismic potential of the focal zone. If the whole segment of the fault plane was activated simultaneously it would represent an earthquake of ML∼5. This is in good agreement with the estimates of the maximum magnitudes of earthquakes that occurred in the West Bohemia/Vogtland region in the past.

Thank you for your final review of our manuscript. We have accepted all your suggestions, namely unified the ML notation and corrected other mistakes. The figures are now submitted in high quality form.

I believe now the MS is in good form to be published. Manythanks for enabling us to publish in JOSE.

Yours sincerely, Tomáš Fischer moment release compared to previous swarms. Up to 25 000 events in the magnitude range of 0.5<M L <3.8 were detected using an automatic picker. A total of 9 swarm phases can be distinguished in the swarm, five of them exceeding the magnitude level of 2.5. The magnitude-frequency distribution of the complete 2008 swarm activity shows a b-value close to 1. The swarm hypocenters fall precisely on the same fault portion of the Nový Kostel focal zone that was activated by the 2000 swarm (M L ≤3.2) in a depth interval from 6 to 11 km and also by the 1985/86 swarm (M L ≤4.6). The steeply dipping fault planes of the 2000 and 2008 swarms seem to be identical considering the location error of about 100 m. Also, focal mechanisms of the 2008 swarm are identical with those of the 2000 swarm, both matching an average strike of 170 ° and dip of 80 ° of the activated fault segment. An overall upward migration of activity is observed with first events at the bottom and last events at the top of the of the activated fault patch. Similarities in the activated fault area and in the seismic moments released during the three largest recent swarms enable to estimate the seismic potential of the focal zone. If the whole segment of the fault plane was activated simultaneously it would represent an earthquake of M L ∼5. This is in good agreement with the estimates of the maximum magnitudes of earthquakes that occurred in the West Bohemia/Vogtland region in the past.

Introduction

The region of West Bohemia (Czech Republic) and Vogtland (Saxony, Germany) is well known for its geodynamic activity (Fig. 1). Periodically recurring intraplate earthquake swarms are the most striking features of the region. High flux of mantle derived CO 2 indicated by more than one hundred CO 2 -rich mineral springs and numerous gas vents seem to be a further peculiarity of region. Earthquake swarms are mainly attributed to volcanic areas, geothermal fields, and mid-ocean rifts (e.g. [START_REF] Wyss | Mapping active magma chambers by b values beneath the off-Ito volcano, Japan[END_REF][START_REF] Dreger | Dilational processes accompanying earthquakes in the Long Valley Caldera[END_REF], the worldwide most intensive swarms occur apparently in Iceland, which is a part of the mid-Atlantic plate boundary (e.g. Stefansson et al., 2006[START_REF] Jakobsdóttir | Earthquake swarms at Upptyppingar, north-east Iceland: A sign of magma intrusion? Stud[END_REF]. Intraplate earthquake swarms without active volcanism occur mainly in areas of enhanced crustal fluid activity, particularly in regions of Quaternary volcanism [START_REF] Ibs-Von Seht | Earthquake swarms in continental riftsa comparison of selected cases in America, Africa, and Europe[END_REF]. One of such regions is West Bohemia and Vogtland, where the youngest known volcanic activity occurred at about 0.3-0.5 Ma ago [START_REF] Gögen | Alpha-recoil track dating of Quaternary volcanoes[END_REF]. This region belongs to the western part of the Bohemian Massif where three principal tectonic unitsthe Saxothuringian, Moldanubian, and Teplá-Barrandianare in contact [START_REF] Babuška | Control of paths of quaternary volcanic products in western Bohemian Massif by rejuvenated Variscan triple junction of ancient microplates[END_REF]. It is intersected by an ENE-WSW trending neotectonic structure, the Eger rift, and by the NNW-SSE striking Mariánské Lázně fault. An active fault system trending N-S, the Počátky-Plesná zone, has been detected recently [START_REF] Bankwitz | Structural characteristics of epicentral areas in Central Europe: study case Cheb Basin (Czech Republic)[END_REF]. The area is intruded by several granitic plutons with estimated depth of 10 km [START_REF] Blecha | Geological interpretation of gravity profiles through the Karlovy Vary granite massif (Czech Republic)[END_REF].

In terms of seismicity, the West Bohemia/Vogtland region belongs to the north-south trending Leipzig-Regensburg seismoactive zone [START_REF] Bankwitz | Structural characteristics of epicentral areas in Central Europe: study case Cheb Basin (Czech Republic)[END_REF][START_REF] Korn | Seismicity and seismotectonics of West Saxony, Germanynew insights from recent seismicity observed with the Saxonian seismic network[END_REF] with the largest historical earthquakes reaching macroseismic intensity (on the MSK-64 scale) of VIII in 1346 and VII-VIII in 1366 and 1872 [START_REF] Leydecker | Erdbebenkatalog für die Bundesrepublik Deutschland mit Randgebieten für die Jahre 800 -2004[END_REF]. West Bohemia/Vogtland is typical of a frequent occurrence of weak earthquake swarms, mostly of magnitudes M L ≤ 3.5. Extraordinarily strong swarm activity was observed at the turn of the 19 th and 20 th centuries. Intensive swarms occurred in 1896/97, 1903, 1908/09 with less intensive swarms in between (1900, 1901, 1904, 1906 and 1911). Swarms with macroseismically observed shocks also occurred in 1914occurred in , 1929occurred in , 1962occurred in and 1973occurred in (Grünthal, 1989;;[START_REF] Kárník | Time pattern of the swarm of December 1985 -March 1986 in West Bohemia[END_REF]. The swarm in 1985/86, with two shocks reaching the magnitudes of M L =4.6 and M L =4.2, represents a new revival of intensive swarm activity in this region.

The two probably strongest earthquakes in this region were those of March 6, 1872 and November 3, 1908, both with magnitudes M L =5.0. While the magnitude of the former one was estimated on the base of macroseismic observations, the latter was estimated instrumentally. The macroseismic and instrumental observations were first coupled by [START_REF] Credner | Der vogtländische Erdbebenschwarm vom 13. Februar bis zum 18. Mai 1903 und seine Registrierung durch das Wiechertsche Pendelseismometer in Leipzig[END_REF] for the 1903 swarm but the first earthquake swarm being reliably documented by instrumental observations is that of 1908 [START_REF] Etzold | Die sächsischen Erdbeben während der Jahre 1907-1915[END_REF]. However, it should be noted that instrumental observations of local swarm events were poor due to low sensitivity and also inappropriate, poorly known frequency responses of seismographs operated at that time nearby the epicentral region. E.g., only 13 events of the 1962-earthquake swarm were recorded at the Cheb station. This, together with the fact that neither seismograms nor bulletins from the early stations Cheb and Hof have been handed down, makes the magnitude comparison of the past and recent West Bohemia/Vogtland swarms rather difficult. Proper observations in the region began only after the 1962 swarm when a network of analogue stations VOGTLAND was established in the border area of Saxony [START_REF] Neunhöfer | Local stations and local network[END_REF] [START_REF] Korn | Seismicity and seismotectonics of West Saxony, Germanynew insights from recent seismicity observed with the Saxonian seismic network[END_REF], the Bavarian Seismological Network (http://www.erdbeben-inbayern.de), and KRASNET operated by the Masaryk University in Brno in the period of 1991-2008[START_REF] Nehybka | Seismotectonic analysis of the seismological measurements in the Western Bohemia[END_REF]. Two decades of continuous observations from WEBNET form a basis for the majority of studies of the West Bohemia/Vogtland earthquake swarms.

Showing a few tens of thousands events within the last twenty years, the Nový Kostel (NK) focal zone (centre  50.23ºN, 12.45ºE) was disclosed to dominate the recent seismicity of the whole region.

Besides numerous microswarms, the most significant earthquake activities there were the 1985/86 swarm (M L ≤4.6), a weaker swarm in January 1997 (M L ≤3.0), a medium swarm in autumn 2000 (M L ≤3.3), and an intensive swarm in October 2008 (M L ≤3.8) [START_REF] Fischer | Space-time distribution of earthquake swarms in the principal focal zone of the NW Bohemia/Vogtland seismoactive region: period 1985-2001[END_REF]. In particular, being studied from many aspects, the swarms in 1997 and 2000 represent the milestones in understanding the nature of the West Bohemia/Vogtland earthquake swarms. [START_REF] Fischer | Space-time distribution of earthquake swarms in the principal focal zone of the NW Bohemia/Vogtland seismoactive region: period 1985-2001[END_REF] found that the NK zone displays a pronounced fault plane at the depths between 6 and 11 km. It is oriented almost S-N (strike 169 o N) and steeply dips (dip 80 o ) westward, which corresponds well with the presumed Počátky-Plesná tectonic line (see Fig. 1). The source mechanisms of the 2000 swarm, in contrast to the one in 1997, fits well the geometry of the main NK fault plane. This points to a progressive rupturing of one segment of the main fault plane. Triggering mechanisms acting during such rupturing were investigated on the basis of the 2000 swarm data by [START_REF] Hainzl | Indications for a successively triggered rupture growth underlying the 2000 earthquake swarm in Vogtland/NW-Bohemia[END_REF], [START_REF] Parotidis | Pore-pressure diffusion: A possible mechanism for the earthquake swarms 2000 in Vogtland/NW-Bohemia, central Europe[END_REF], [START_REF] Hainzl | Detecting fluid signals in seismicity data through statistical modelling[END_REF], and [START_REF] Fischer | Slip-generated patterns of swarm microearthquakes from West Bohemia/Vogtland (central Europe): Evidence of their triggering mechanism?[END_REF]. [START_REF] Horálek | Role of crustal fluids in triggering the West Bohemia/Vogtland earthquake swarms: Just what we know (a review)[END_REF] summarized outcomes of the 1997 and 2000 swarm studies. They inferred that faults in this region are in a subcritical steady state as a result of the regional tectonic stress and their portions can be brought to a critical state due to the increase of pore pressure of crustal fluids After reaching instability, a self-organization of the swarm activity prevails and fluids mainly act in the background, i.e., they keep the fault segments with the swarm activity in a critical state.

Brief information on the 2008 swarm was given by [START_REF] Horálek | The West Bohemian 2008earthquake swarm: when, where, what size and data[END_REF]. The aim of our paper is to provide basic characteristics of the 2008 swarm based on the WEBNET observations and compare them with previous swarms in 1985 /86, 1997 and 2000. This involves precise hypocenter locations, space-time distribution, energy release and preliminary focal mechanisms.

WEBNET seismic network and 2008-swarm data

At present WEBNET consists of 13 three-component digital seismic stations and covers an area of about 900 km 2 (Fig. 1). Seismometers are installed, on concrete pillars in 5 m deep vaults. Central station NKC is situated in the centre of the main focal zone NK, further six stations (SKC, VAC, LBC, POC, STC, KVC) surround this zone. The remaining six stations (KRC, KOC, LAC, ZHC, KAC, TRC) are located in minor focal zones. The network layout ensures a proper areal and azimuthal coverage of the focal area, particularly with respect to the NK zone.

Eleven stations are equipped with passive seismometers SM-3, two stations with broadband seismometers Güralp 40-T and one station with the LE-3D sensor (Table 1). WEBNET is formed by two subnets (N and L in our notation), which differ in data acquisition systems and telemetry used.

Subnet N (stations NKC, SKC, VAC, LBC, POC, STC, KRC, ZHC and KAC) is equipped with the Janus-Trident (Nanometrics) data acquisition system and with WaveLan microwave telemetry enabling internet access to each station. Subnet L consists of four stations (NKC, KVC, KOC, LAC) equipped with the Lennartz 5800 PCM data acquisition system with one-way radio telemetry, which enables data transmission from distant stations KOC and LAC. It also facilitates parallel operation of two different seismograph systems, the Güralp 40-T/Janus-Trident and SM-3/5800 PCM at the central station NKC. Two independent subnets with overlapping stations guarantee uninterrupted observations in case of outage of one of them. The data from both subnets are transmitted to the data centre at the TV tower Zelená Hora near Cheb, which is connected to the internet. Station TRC is operated in an autonomous mode due to problems with a telemetry route to the data centre.

The configuration of WEBNET and the parameters of seismograph systems guarantee highquality recording of the West Bohemia/Vogtland events of magnitudes -0.5M L 5 (the dynamic range is 120 dB). All WEBNET stations produce seismograms proportional to the ground velocity in a frequency band of 0.5 -80 Hz (subnet N) or 0.5 -60 Hz (subnet L), and 0.033 -80 Hz at stations equipped with Güralp 40-T seismometers. The sampling rate of all stations is 250 Hz. Subnet N provides both continuous and triggered data while subnet L provides triggered data only.

Additionally, ten seismic vaults consisting of a container with a concrete pillar at about 2 m depth were built to provide the best possible areal and azimuthal coverage of the West Bohemia seismogenic region. During the 2008 swarm they were equipped with LE-3D seismometers and 24-bit Gaia data acquisition systems. The frequency response of these stations is proportional to the ground velocity between 1.0 and 80 Hz and a sampling rate of 250 Hz is used. The basic parameters of the permanent and temporary WEBNET stations are listed in The 2008 swarm was recorded in total by 13 to 23 permanent and temporary WEBNET stations without any significant gaps. All seismograms of permanent stations are unclipped, even at the stations located just in the epicentral area. The triggering threshold of permanent and mobile stations was about M L =-0.5 and 0.5, respectively. Prevailing frequencies of the largest events in the ground velocity seismograms range approximately between 3 and 10 Hz, so that WEBNET seismograms from permanent stations can be regarded as broadband with respect to the frequency band of ground velocity. This is manifested in Fig. 2 accelerograms and those derived from velocigrams at station NKC (Fig. 3) shows that WEBNET seismograms can easily be used for deriving accelerograms. Note that the ALTUS-K2 recordings provide the first proved accelerograms of the West Bohemia earthquakes with macroseismic effects on buildings.

Two types of bulletins of the 2008 swarm are available. First, there are manual readings of the Pand S-wave arrival times and amplitudes from six selected stations (NKC, KVC, LBC, VAC, SKC, KRC), which are suitably distributed to ensure precise hypocentre locations. The seismogram processing is still in progress. These data form the basis for routine FastHypo locations (Hermann, 1979) provided on the interactive map www.ig.cas.cz/en/structure/observatories/webnet/map-ofepicenters and for relative master event locations provided on request. Second, a homogeneous magnitude-time catalogue was generated by the automatic P-and S-wave picker that is based on an algorithm of [START_REF] Fischer | The August-December 2000 Earthquake swarm in NW Bohemia: the first results based on automatic processing of seismograms[END_REF]. This catalogue contains about 25 000 events in the magnitude range -0.5<M L <3.8 with a magnitude of completeness of -0.3 (see Fig. 4), which is by 0.5 lower than that for the 2000 swarm [START_REF] Hainzl | Indications for a successively triggered rupture growth underlying the 2000 earthquake swarm in Vogtland/NW-Bohemia[END_REF]. This enhancement is on account of applying an automatic event detection to continuous seismograms, which was, however, impossible for the 2000-swarm catalogue because only triggered seismograms were available at that time.

Basic characteristics of the 2008 swarm, comparison with previous swarms

The swarm in 2000 and was preceded by an increased microswarm activity in that area [START_REF] Fischer | Post 2000-swarm microearthquake activity in the principal focal zone of West Bohemia/Vogtland: Space-time distribution and waveform similarity analysis[END_REF]. The previous 2000 swarm with five largest events exceeding M L =3 faded away in 2001 in the form of few microswarms (Fig. 5). The period between 2002 and 2003 was characterized by a relative quiescence followed by a seismic unrest starting in 2004. This activity was manifested in a sequence of microswarms with increasing maximum magnitudes reaching M L ~2.0. The hypocenters followed a steeply dipping fault plane, which was the origin of most of the previous swarms and background activity. Thus, the occurrence of the 2008 swarm, which took place at the same fault plane, can be regarded as a culmination of the previous microswarm activity.

Tab. 2. Focal parameters of the strongest (M L ≥3.0) 2008 swarm events, determined from manual Pand S-wave readings using an absolute locating procedure included in the SeisBase package [START_REF] Fischer | SeisBaseprinciples for a program and database for routine analysis of data from local seismic networks[END_REF]. that the ground accelerations in some areas were substantially higher. This concerns particularly those located in the Cheb Basin because the given maximum ground accelerations were measured outside the basin on concrete pillars attached to the bedrock. Near-field static displacement in the order of 1E-5 m was observed by [START_REF] Zahradník | Long-period disturbances in broadband records of weak events at small epicentral distances[END_REF] in the broadband records at NKC. Using an empirical formula log M o = 1.05 M L + 11.3 [START_REF] Hainzl | Indications for a successively triggered rupture growth underlying the 2000 earthquake swarm in Vogtland/NW-Bohemia[END_REF], the seismic moments of the largest events are estimated to be between 1.5E15 and 2E15 Nm. A brief scenario of the 2008 swarm was as follows. The 2008 swarm started at midnight from October 5 to 6 by a short burst of M L ≤2 swarm-like seismicity lasting only few hours (Fig. 6a). In this respect it would rank among the frequent microswarms occurring since 2004 (Fig. 5). This burst of seismicity was followed by a sporadic activity with M L <2 persisting for about 40 hours. Then (in the evening of October 7), a new, more intensive swarm phase occurred, that lasted about 12 hours, succeeded. This phase included M L =2.5 events, some of them felt by people in a broader area. The trembling was accompanied by the acoustic effects and scared the population. The most intensive phase started in the evening of October 9; it lasted two days and included three M L >3.5 shocks. After that, a fairly intensive swarm activity, which included M L =3.8 and 3.7 shocks of October 12 and October 14, persisted till October 15. The period between October 15 and October 27 was characterized by a gradual swarm activity decay, which was broken by its abrupt increase dominated by a M L =3.8 shock of October 28; this shock exhibited the largest ground motions (see Table 3) and also the strongest macroseismic effects in the epicentral area. In total, 9 swarm phases can be distinguished, five of them exceeding the magnitude level of 2.5 (Fig. 6a); the terminating phase P9

Date

with the biggest M L =2.8 event lasted till the end of 2008 (Fig. 6b).

A characteristic feature of the 2008 swarm is its rapidity and high rate of energy release. If one compares the approximate duration of the main swarm period being characterized by swarm phases with M L >2.5 events then one gets about 7 weeks for the 1985/86 swarm, one week for the much weaker 1997 swarm, 10 weeks for the 2000 swarm and only 4 weeks for the 2008 swarm (Fig. 6b).

The rapidity of the 2008 swarm is underlined by the total seismic moment released, which is about three times larger than that of the 2000 swarm. In particular, the cumulative seismic moment released during the 1985/86 and 2008 swarms were similar, about 4.3 E16 Nm and 4.0E16 Nm, compared to 1.4E16 Nm for the 2000 swarm and only about 6E14Nm for the 1997 swarm (Fig. 7).

The magnitude-frequency distribution of the complete 2008-swarm (Fig. 4) shows a b-value close to 1, which is typical of the West Bohemia/Vogtland earthquake swarm; b  1.0 have been observed also for swarms of 1908, 1962, 1985/86 and 1997[START_REF] Neunhöfer | Earthquake swarms in the Vogtland/Western Bohemia region: Spatial distribution and magnitude-frequency distribution as an indication of the genesis of swarms?[END_REF] and for the 2000 swarm by e.g. [START_REF] Fischer | The August-December 2000 Earthquake swarm in NW Bohemia: the first results based on automatic processing of seismograms[END_REF] and [START_REF] Tittel | Magnitudes and time distribution of the swarm earthquakes August-November 2000 in NW Bohemia[END_REF]. Similarly, [START_REF] Hainzl | Indications for a successively triggered rupture growth underlying the 2000 earthquake swarm in Vogtland/NW-Bohemia[END_REF] report b  1.0 for the complete 2000 swarm with a considerable variation during the sequence.

Another noteworthy feature of the 2008 swarm is an abundance of the strongest events (M L >3.5) and a lack of events in magnitude range from 3.0 to 3.5, which is clearly demonstrated in Figs. 4 and6a. The magnitude gap between the strongest and medium events resembles the characteristics of the mainshock-aftershock sequences. Contrary to earthquake swarms, the mainshockaftershock sequences are typical of the occurrence of the mainshock that exceeds the remaining events by 0.5 magnitude unit at minimum. In this respect, the 2008 swarm may confirm the findings of [START_REF] Hainzl | Indications for a successively triggered rupture growth underlying the 2000 earthquake swarm in Vogtland/NW-Bohemia[END_REF] who showed that the 2000 swarm can be represented by a sequence of overlapping aftershock sequences, each of them dominated by a mainshock.

Location of the 2008-swarm events

Similar to our previous studies of activity in the NK zone [START_REF] Fischer | The August-December 2000 Earthquake swarm in NW Bohemia: the first results based on automatic processing of seismograms[END_REF][START_REF] Fischer | Space-time distribution of earthquake swarms in the principal focal zone of the NW Bohemia/Vogtland seismoactive region: period 1985-2001[END_REF][START_REF] Leydecker | Erdbebenkatalog für die Bundesrepublik Deutschland mit Randgebieten für die Jahre 800 -2004[END_REF][START_REF] Fischer | Post 2000-swarm microearthquake activity in the principal focal zone of West Bohemia/Vogtland: Space-time distribution and waveform similarity analysis[END_REF] we obtained the precise locations of the 2008 swarm events by the master-event localization based on the method of Zollo et al. (1995) with the use of a 1-D gradient velocity model of Málek et al. (2000). The method is based on finding the minimum of the variance

  2 00 1/ i V N T T     
of the difference T 0 between the origin times of the master event and located events. Here T 0 i is the origin time calculated from the arrival time and travel time of the phase i and N is the number of phases. Similar to other relative location methods, it is required that the distance of the located events to the master event is much smaller than the hypocentral distance, i. determined the average off-plane coordinate for the hypocenters belonging to individual swarms (X 00 and X 08 ), and its standard deviation ( X00 and  X08 ). By comparing the absolute value of the difference X 00 -X 08 (i.e. the distance of the 2000 and 2008 fault planes) and the mean value of the standard deviations one can learn if the fault planes activated during the two swarms are discernible with respect to the location error. In particular, the predominance of the difference X 00 -X 08 over the uncertainty of the off-plane coordinate ( X00 +  X08 )/2 would indicate distinct fault planes of the two swarms. It turns out that while the difference X 00 -X 08 (thin line) oscillates from 0 to 120 m, the uncertainty of the offplane coordinate ranges between 20 and 90 m (see Fig. 8b). Both opposite and similar trends of the two curves occur. However, in most cells the difference of coordinates is smaller than their standard deviation, which indicates that it is not possible to distinguish the fault planes of the two swarms.

Hence, within the attainable location accuracy of about 100 m, the fault planes of the 2000 and 2008 swarms appear to be the same.

Focal mechanismspreliminary results

To get a first idea of faulting during the 2008 swarm, we estimated fault-plane solutions of 70 events throughout the whole swarm, using [START_REF] Maeda | A method of determining focal mechanisms and quantifying the uncertainty of the determined focal mechanisms for microearthquakes[END_REF] method. We inverted 13 P-wave polarities and amplitudes of the direct P-waves as a minimum, and tested stability of solutions by omitting individual stations or their pairs. Only stable solutions, which are depicted in the form of a composite plot in Fig. 10b, have been taken into account. Resultant focal mechanisms are left-lateral, mostly oblique normal, with a large strike-slip component. Even though they show fair variability implying jagged rupturing along the ruptured fault segment, the general pattern corresponds well to the strike (170 °) and dip (80 ° westward) of the fault segment as well of the whole main focal zone NK. The predominant slip angles of about 30 ° are consistent with those of previous studies of focal mechanisms acting in the NK zone (e.g., [START_REF] Antonini | Variations in the focal mechanisms during the 1985/86 Western Bohemian earthquake swarm sequence -correlation with spatial distribution of foci and suggested geometry of faulting[END_REF][START_REF] Plenefisch | Temporal variations of focal mechanisms in the Novy Kostel focal zone (Vogtland/NW-Bohemia)comparison of the swarms of 1994, 1997 and 2000[END_REF][START_REF] Fischer | Slip-generated patterns of swarm microearthquakes from West Bohemia/Vogtland (central Europe): Evidence of their triggering mechanism?[END_REF]. and minimum ( 3 ) regional tectonic stress in the West Bohemia/Vogtland crust (Švancara et al., 2008).

Such solution confirms our previous idea that the faulting during individual swarms in the region is controlled mainly by the regional tectonic stress [START_REF] Horálek | Role of crustal fluids in triggering the West Bohemia/Vogtland earthquake swarms: Just what we know (a review)[END_REF] and that the N-S leftlateral (sinistral) movements with southward subsidence prevail in the NK. However, deeper understanding of the faulting and forces acting during the swarm requires a detailed study of source mechanisms in the full moment tensor description.

Size of the 2008 swarm and seismic potential of the area

As mentioned in Section 3, the 2008 swarm was exceptional for its rapidity and its total seismic moment released. It logically raised a question of its size with respect to the earlier West Bohemia/Vogtland swarms. However, such comparison is an intricate problem due to poor data from the historical local seismic stations. To compare roughly the 2008 swarm size with that of the earlier swarms we used studies of [START_REF] Kárník | Time pattern of the swarm of December 1985 -March 1986 in West Bohemia[END_REF], [START_REF] Neunhöfer | Historical results from near stations[END_REF] and [START_REF] Neunhöfer | Earthquake swarms in the Vogtland/Western Bohemia region: Spatial distribution and magnitude-frequency distribution as an indication of the genesis of swarms?[END_REF]. They had evaluated the magnitudes of 120 West Bohemia/Vogtland swarms and microswarms from a period between 1908 and 1999 using the Iida formula (designed for the magnitude estimate of local events if observations from homogeneous set of stations are not available). As a measure of size we took the magnitude of the largest swarm earthquake. We could infer that only three The WEBNET formula M L = log A smaxlog 2 + 2.1 log R + C -1.7 (A smax is the maximum absolute value of the total amplitude of the S-wave ground velocity measured in m/s, R is the hypocentral distance in km, and C is the station correction) is calibrated to the magnitudes estimated by PRU, the primary station of the Czech Regional Seismological Network. Note that the magnitude estimations of the largest 2008 events reported by some international seismological agencies or data centres are higher as compared to our magnitudes M L . For example, the magnitude of the event on [START_REF] Hainzl | Indications for a successively triggered rupture growth underlying the 2000 earthquake swarm in Vogtland/NW-Bohemia[END_REF] estimated the total area of the main cluster of the 2000 swarm to 9 km 2 ; the area of the 2008 swarm estimated in a similar way amounts to 12 km 2 . With respect to the fact that also the known hypocenters of the 1985/86 swarm occurred on the same fault plane segment, one can speculate that the main hypocenter cluster of the 2000 and 2008 swarms represents the largest fault area capable of brittle deformation in the NK zone. Accordingly, if the cumulative seismic moment of the 2008 swarm (4E16 Nm) would be released in a single mainshock, the maximum expected M L magnitude would amount to 5.0, which is the same as the maximum magnitude estimates of the activity at the break of 20 th century.

However, the precise location of the historical seismicity is uncertain. In particular, the macroseismic data concerning the activity in a period of 1872 -1908 and the rough locations of the 1962 swarm put the historical epicenters to about 15 km to the north, to the Klingenthal -Kraslice area (e.g. [START_REF] Neunhöfer | Earthquake swarms in the Vogtland/Western Bohemia region: Spatial distribution and magnitude-frequency distribution as an indication of the genesis of swarms?[END_REF], see Fig. 1. On the other hand, the agreement between the maximum magnitude estimate of a particular fault segment corresponding to NK zone and the maximum magnitude estimated from historical macroseismic observations in a nearby area could be considered as a prove of the reliability of the estimates.

Discussion

The increasing microswarm activity in the period of 2004-2008 (see Fig. 5), which showed a pronounced migration to the south and downwards from a depth of 10 to 13 km, foreshadowed the oncoming of a larger earthquake swarm. In our recent paper, which was accepted one month before the 2008 swarm onset [START_REF] Fischer | Post 2000-swarm microearthquake activity in the principal focal zone of West Bohemia/Vogtland: Space-time distribution and waveform similarity analysis[END_REF], we indicated the probability of its soon occurrence stating that -the frequent recurrence of microearthquake swarms emerging since 2004

indicates a new activation of the area after the 2000 swarm. Nevertheless, in spite of a relatively large number of events, the seismic energy of this activity is negligible compared to the energy released during a large swarm, similar to that of the year 2000‖. And further, -Thus one could speculate that if the microswarm activity before the 2000 swarm had a causal relationship to a generation of the 2000 swarm, the recent microswarms could be viewed as a precursor to a future swarm in larger depths in the southern part of the focal zone.‖ Our assumption that the gradually growing microearthquake swarm activity since 2004 was a precursor to the intensive swarm of October 2008 has been proven, indeed. However, in contrast to the speculation of greater depth of the new swarm deduced from apparent depth-ward migration of the preceding microearthquake swarms, the 2008 swarm occurred surprisingly in the same depth range as the 2000 swarm.

The recurrence interval of 8 years between the 2000 and 2008 swarms appears to be rather short compared to the intervals between the other strongest swarms (1903, 1908, 1962, and 1985/86).

This relates to the question of tectonic loading of the activated fault segment. According to [START_REF] Horálek | Role of crustal fluids in triggering the West Bohemia/Vogtland earthquake swarms: Just what we know (a review)[END_REF], the faulting during individual swarms is mostly controlled by the regional tectonic stress. However, the faults are brought to a critical state by pore pressure increase. The moment tensors of most events show prevailing double-couple component, the volumetric part occurs only exceptionally on unfavourably oriented faults [START_REF] Horálek | Moment tensors of the January 1997 earthquake swarm in West Bohemia (Czech Republic): double-couple vs. non-double-couple events[END_REF]. Thus it could be of interest to evaluate the mean slip along the whole activated fault segment to learn the rough rate of tectonic loading. Taking the total seismic moments of the 2000 and 2008 swarms (1.4E16 and 4E16 Nm, see Fig. 8), shear modulus of 30 GPa and the total fault area of the main cluster of the swarms (9 and 12 km 2 ) one arrives at the average slip of 5 cm and 11 cm for the 2000 and 2008 swarms, respectively. By comparing the average slip (10 cm) with the inter-swarm period of 10 years (1986 -2000 and 2000 -2008), one would get a tectonic loading rate of ~1 cm/y if the released slip would equal the cumulative loading. Being aware of a large uncertainty of this guess, such a rate appears rather fast for a small intra-continental fault segment extending to about 10 km 2 . Furthermore, aseismic creep of the same rate should be considered in the neighbouring parts of the fault. Creeping parts of faults are commonly observed, e.g. the Parkfield segment of the San Andreas Fault. Such behavior caused by a decreased friction coefficient is even more likely in the case of fluid-permeated faults, which is indeed the case of West-Bohemia/Vogtland due to high CO 2 flux.

The relatively fast recurrence interval and apparent high loading rate may be questioned if taking into account the known activity in the period from 1872 to 1908, whose location is estimated from macroseismic data at about 15 km to the north. If this was true then the fault portion of about 25 km length would experience a stepwise strain release with a burst of activity in the Klingenthal-Kraslice area at the break of 20 th century and a recent burst of activity in the Nový Kostel area at the break of 21 st century some 100 years later. (Note in Fig. 1, that the Klingenhal-Kraslice area occurs at the prolongation of the NK focal zone to the north; it exhibits a repeated microswarm activity which usually precedes or follows the swarms in the NK focal zone). Because the available records do not allow to estimate reliably the location of the historical seismicity, we are missing the information on the previous activation of the NK focal zone. Hence, the recurrence interval in the NK focal zone would definitely exceed 130 years, which is the period of fairly reliable earthquake swarm observations. In such a case the tectonic loading rate would drop to less than 1 mm/y, which agrees well with the loading rates expected within the continents. It should be noted that further 100 km to the north the seismoactive belt extends along the Leipzig-Regensburg zone with earthquakes reaching the magnitude of 2.8 within the last decade [START_REF] Korn | Seismicity and seismotectonics of West Saxony, Germanynew insights from recent seismicity observed with the Saxonian seismic network[END_REF]. Thus we believe that a complex analysis of the recent and historical seismic activity in a broader area of West-Bohemia/Votland, Saxony to the north and Bavaria to the south could improve our understanding of the seismogenic processes, whose effects are recently concentrated in West Bohemia.

The 2008 swarm represents a high quality data set providing the opportunity to deepen our understanding of the triggering and driving forces of the seismic activity in the area. Among others, further studies will probably address the analysis of focal mechanisms in a full moment tensor description, spatio-temporal and seismo-statistical studies, determining the seismic source parameters including finite source models. A paleoseismological study could contribute to revelation of the enigma of a possible high loading rate and creeping parts of the seismoactive fault in the NK focal zone.

Conclusions

-The 2008-earthquake swarm occurred in the period from 6 th October to the beginning of December -The similarities in the activated fault areas and in the seismic moment released during the three recent largest swarms point to a limited seismic potential of the focal zone, which would amount to an earthquake of M L ∼5 if the whole fault plane segment was activated simultaneously.

Figure captions

Fig. 1. West Bohemia/Vogtland region with the earthquake epicentres [START_REF] Horálek | Role of crustal fluids in triggering the West Bohemia/Vogtland earthquake swarms: Just what we know (a review)[END_REF], WEBNET seismic stations and principal faults (EG -Eger rift, MLF -Mariánské Lázně fault). The maximum compression striking 160° (Švancara et al., 2008) 5). The color-coding is proportional to event order. Note that both the swarms have initiated at the bottom of the fault patch (dark-blue colors) and terminated at the topmost tail on the right. A similar time period of 128 days is displayed for both swarms.

  showing a very good fit of seismograms of the M L =3.8 event recorded by the Güralp 40-T/Janus-Trident and SM-3/5800 PCM seismographs at station NKC. A part of the 2008 swarm was also recorded by strong-motion accelerometers ALTUS K2 (Kinemetrics). The accelerometers were operated in cooperation with GFZ Potsdam from October 22, 2008 until the beginning of February 2009 at the WEBNET stations NKC, KVC, SKC, and VAC. This enabled a comparison of accelerometer recordings with accelerograms obtained by numerical differentiation of WEBNET velocity seismograms. Quite good fit between the ALTUS-K2

  2008 swarm has been the strongest seismic activity in the West Bohemia/Vogtland region since the severe swarm of 1985/86 and surprised both the public and seismological community by its fast sequence of macroseismically observed earthquakes. It took place, similar like the previous swarms in 1985/86, 1997 and 2000, in the main NK focal zone. It arose quite soon after the previous intensive

  e. the rays are almost identical. Events with P-and S-arrival times from 4 stations at least were relocated. Because the station configuration of WEBNET has changed since 2000, it was not possible to use the same master event for the 2008 swarm hypocenters as it was for the 2000 ones. Hence, we chose a new master event (event of M L =1.5 from October 14, 2008, 10:44), which was recorded during the full operation of the WEBNET network. Its position 50.2155°N, 12.4481°E and depth 9580 m was determined by the relative localization using the master event from the 2000 swarm. In the next we will address the relative position of the 2000 and 2008 earthquake swarms. In this context we use the term fault plane in its general sense; fault segment and the smaller patch are areas delimited by hypocenters upon the fault plane. As mentioned above, the hypocenters of the 2008earthquake swarm occurred in the same NK focal zone as the 1985/86 and 2000 swarms. Furthermore, the 1985/86 and 2000 swarms shared a common fault plane, striking NNW (strike/dip 169°/80°) in a depth range from 6.5 to 10.5 km (see Figs 4 and 5 in Fischer and Horálek, 2003). However, the real extent of the fault plane activated by the 1985/86 swarm could not be determined due to missing data from the first swarm period. It was also shown that the 1997 swarm took place in two differently oriented fault plane segments belonging to the main fault. As follows from Fig 5 and 8a, the 2000 and 2008 swarm hypocenters appear to share a common fault plane. i.e. it is hard to discern if the fault planes activated by the 2000 and 2008 swarms are identical or distinct. To quantify the possible separation of the activated fault planes we subdivided the fault patch in a number of cells and

Fig 9

 9 Fig 9 shows the comparison of the space-time distribution of the 2000 and 2008 swarm hypocenters (2080 and 2020 M L >0.5 events, respectively). Both the 2000 and 2008 swarms show a

  Fig. 11. The patterns indicate subhorizontal P-axes in the NW-SE direction and nearly horizontal Taxes in the NE-SW direction. This is consistent with the orientation of the axes of the maximum ( 1 )

  swarmsof 1903, 1908 and 1985/86 exceeded the magnitude threshold of M L =4.0 and that other swarms occurring in this period reached or possibly slightly exceeded the magnitude M L=3.0 (swarms in 1929 and 1962). This implies that the 2008 swarm has ranked among the four strongest swarms in the West Bohemia/Vogtland region since 1903; only the swarms of 1908 (M L ∼5.0), and 1985/86 (M L =4.6) were much more intensive in terms of magnitudes of the largest earthquakes. The size of the 1903 swarm is uncertain;[START_REF] Neunhöfer | Historical results from near stations[END_REF] give the magnitude of the largest earthquakes slightly exceeding M L =4.0 whereas according to[START_REF] Kárník | Time pattern of the swarm of December 1985 -March 1986 in West Bohemia[END_REF] the 1903-swarm size is similar to that of 1985/86.Undoubtedly, the accuracy of the magnitude determination and comparability of the magnitudes of the recent and earlier seismicity are of paramount importance and we are aware of great uncertainties of the magnitude estimations due to various factors, e.g. different instrumentations, lack of stations, low quality of records, directivity or different ways of magnitude estimations. Let us give two particular examples: (i) the strongest earthquake of the 1908 swarm, which is generally assumed to be of M L ∼5.0, was evaluated by[START_REF] Neunhöfer | Earthquake swarms in the Vogtland/Western Bohemia region: Spatial distribution and magnitude-frequency distribution as an indication of the genesis of swarms?[END_REF] as M L =4.4; (ii) the magnitude of the largest event in the 1997 swarm was evaluated by[START_REF] Neunhöfer | Earthquake swarms in the Vogtland/Western Bohemia region: Spatial distribution and magnitude-frequency distribution as an indication of the genesis of swarms?[END_REF] as M L =2.3 while WEBNET gives a value of M L =3.0.

  October 8, 2008, 08:08:46 UTC (M L =3.7 by WEBNET) is estimated by NEIC with M L =4.1, by LDG with M L =4.4, by GRF with M L =4.1; nevertheless station PRU gives a magnitude of M L =3.7.A worth noting issue arising from this comparison is the fact that no swarm activity since 1903 has exceeded the magnitude of M L =5.0. This relates to a question of the seismic potential of the area.Interestingly, the three recent large swarms(1985/86, 2000 and 2008) occurred in a single focal zone of Nový Kostel and further, the hypocenters of 2000 and 2008 swarms occupy almost the same fault area.

  2008 in the Nový Kostel area of West Bohemia /Vogtland. The swarm included 9 events of M L >3.0 with two largest events of magnitude M L =3.8, which has made it the most intensive swarm since the M L ≤4.6 swarm in 1985/86.-The swarm was recorded by 13 to 23 permanent and mobile WEBNET stations without gaps.Additionally, the period from October 22, 2008 until the beginning of February 2009 was also recorded by strong-motion accelerometers ALTUS K2 (Kinemetrics) deployed at four WEBNET stations. The maximum ground motions in the 2008 swarm observed by the WEBNET correspond to the displacement of 0.25 mm, velocity of 9.5 mm/s and acceleration of 0.65 m/s 2 . By comparing the accelerograms with the differentiated WEBNET velocigrams we have shown that WEBNET seismograms can be easily used for deriving accelerograms.-Up to 25 000 events in the magnitude range -0.5<M L <3.8 were detected using an automatic picker A total of 9 swarm phases can be distinguished in the swarm, five of them exceeding the magnitude level of 2.5. The magnitude-frequency distribution of the complete 2008-swarm shows a b-value close to 1.-The 2008 swarm showed an excess of the strongest events (M L >3.5) in contrast to the lack of events in the magnitude range from 3.0 to 3.5. This feature is in accord with our previous finding based on the 2000 swarm analysis which indicates that the West-Bohemia swarms can be regarded as a set of multiple mainshock-aftershock sequences -Comparison with the previous earthquake swarms in the NK area has shown that the 1985/86 and 2008 swarms were similar in terms of the seismic moment (4E16 Nm) released, compared to 1.4E16 Nm for the 2000 swarm and only 6E14 Nm during the 1997 swarm. -The 2008 swarm has shown the fastest seismic moment release due to its short duration. The main part of the swarm lasted only 4 weeks, compared to 8 weeks during the 1985/86 swarm and 10 weeks during the 2000 swarm. -The hypocenters fall precisely on the same fault portion of the NK focal zone that was activated by the 2000 swarm and most probably also by the 1985/86 swarm in a depth interval from 6 to 11 km. The steeply dipping fault planes of the 2000 and 2008 swarms appear identical within the bounds of the location error of about 100 m. -Also, the focal mechanisms of the 2008 swarm are identical with the focal mechanisms of the 2000 swarm, both matching the average strike 170 ° and dip 80 ° of the activated fault segment. -The hypocenters of the first swarm phase occurred (similar to the 2000 swarm) at the bottom of the activated fault patch. In general, an upward migration took place in comparison with the 2000 swarm, which exhibited a counter-clockwise migration of hypocenters. -Considering the magnitude of the largest earthquake as a measure of the swarm size, the 2008 swarm has ranked among four strongest swarms in the West Bohemia/Vogtland region since 1903 (the beginning of the instrumental observations). Only the swarms in 1908 (M L ∼5.0), and 1985/86 (M L =4.6) were more intensive in terms of magnitudes of the largest earthquakes.

Fig. 4 .

 4 Fig. 4. Cumulative magnitude frequency distribution for the swarms of 1985/86 (blue), 1997 (light blue), 2000 (green) and 2008 (red).

Fig. 5 .

 5 Fig. 5. Top: Magnitude-time distribution of seismicity in the focal zone NK from 1991 to 2008. Note the dominance of the 2000 (green) and 2008 swarms (dark red). Bottom: Depth cross-section of focal zone NK along the main fault plane NK (right) and perpendicular to it (left). Color coding is proportional to the origin time. The rectangle indicates the fault area shown in Fig. 8.

Fig. 7 .

 7 Fig. 7. Cumulative seismic moment released during the earthquake swarms of 1985/86 (blue), 1997 (light blue), 2000 (green) and 2008 (red). The vertical scale for the 1997 swarm (right axis) is 10 times magnified compared to the other swarms (left axis).

Fig. 8 .

 8 Fig. 8. The 2000 (a) and 2008 (b) swarm activity in the magnitude-time plot (top) and fault plane view (bottom) (same projection as in Fig.5). The color-coding is proportional to event order. Note that both the swarms have initiated at the bottom of the fault patch (dark-blue colors) and terminated at the topmost tail on the right. A similar time period of 128 days is displayed for both swarms.

Fig. 9 .

 9 Fig. 9. a) Cross section of the fault segment of the 2000 (circles) and 2008 (crosses) swarms; the horizontal coordinates are rotated 13° clockwise. b) Difference of the mean off-plane coordinate of the 2000 and 2008 swarm events (solid line) compared to the mean standard deviation of the off-plane coordinate of the 2000 and 2008 swarm events (dashed line). The mean and standard deviation are measured in rectangular cells of 150 m size along the fault plane.

Fig. 10 .

 10 Fig. 10. Focal mechanisms for swarm in years 2000 (left) and 2008 (right), lower hemisphere projection is used. The 2000 swarm mechanisms are taken from[START_REF] Fischer | Slip-generated patterns of swarm microearthquakes from West Bohemia/Vogtland (central Europe): Evidence of their triggering mechanism?[END_REF] and correspond to all 133 events with M L >= 1.7. For the 2008 swarm the preliminary mechanisms of 29 selected events with M L >= 2.0 were determined. All mechanisms are left-lateral with a NS oriented fault plane.

  Fig 2 Click here to download line figure: JOS-Fig2-fin.eps

  Fig 3 Click here to download line figure: JOS-Fig3-fin.eps

  Fig 6 Click here to download line figure: JOS-Fig6II.eps

  Fig 7 Click here to download colour figure: JOS-Fig7.eps

  . A new stage of digital seismic observations in West Bohemia was initiated with the strong 1985/86 swarm. The first permanent digital seismic station NKC was established in the main focal zone Nový Kostel in 1989 and became a basis for the present WEBNET network. Further WEBNET stations were set up gradually from 1991 to 2004. Nowadays, West Bohemia and Vogtland are among the best-monitored seismically active areas in Europe. Besides WEBNET, which is jointly operated by the Institute of Geophysics and the Institute of Rock Structure and Mechanics of the ASCR in Prague, the broader seismogenic region is monitored by the Saxonian network SXNET

Table 1

 1

	; further details can be found at

  The maximum observed ground motions at the WEBNET stations there were 9.5E-3 m/s for ground velocity, 2.6E-4 m for displacement and 0.65 m/s 2 for acceleration, see Table3. It is very likely

	2008-10-09 22:20:37.91	50.215	12.445	9.6	3.5
	2008-10-10 03:22:05.26	50.213	12.447	9.5	3.6
	2008-10-10 03:22:06.87	50.208	12.443	9.2	3.2
	2008-10-10 08:08:46.22	50.213	12.448	9.7	3.7
	2008-10-10 11:18:41.62	50.221	12.443	9.6	3.3
	2008-10-12 07:44:56.31	50.213	12.447	9.4	3.8
	2008-10-14 04:01:36.31	50.217	12.444	9.7	3.0
	2008-10-14 19:00:33.10	50.213	12.448	8.9	3.7
	2008-10-28 08:30:11.50	50.216	12.446	7.6	3.8
	During this swarm, nine M L ≥3.0 earthquakes (see Table 2) and about 100 M L ≥2.0 earthquakes
	occurred; six largest events with magnitudes of M L ≥3.5 were observed macroseismically in a broader
	area of West Bohemia (a brief overview of macroseismic observations was given in Horálek et al.,
	2009).
	Origin time	Latitude	Longitude	Depth	M L
	UTC	[°N]	[°E]	[km]

  is indicated in the lower right corner. The solid line indicates the Czech-German border, squares mark towns in West Bohemia, SE Saxony and NE Bavaria. The German KTB superdeep borehole is indicated in the lower left corner. The Nový Kostel and Kraslice-Klingenthal focal zones are indicated by dashed rectangle and ellipse, respectively. A gray rectangle indicates the Počátky-Plesná fault zone. Seismograms of the M L = 3.8 event recorded by the Güralp 40-T/Janus-Trident and SM-3/5800 PCM seismographs at station NKC of a M L =3.8 event; the time is measured relative to 28-10-2008 8:30:12.5 UTC. Both seismograms are sampled at 250 Hz and band-pass filtered in the range of 0.2 -40 Hz. Amplitude is normalized to the maximum value recorded at this station. 40T is derived from the original velocigram. Both are band-pass filtered between 0.1-25 Hz. Amplitude is normalized to the maximum value recorded at this station.

	Fig. 2. Fig. 3. WEBNET CMG-40T (fs=250 Hz) and ALTUS-K2 (fs=200 Hz) accelerograms from station
	NKC of a M L =3.8 event; the time is measured relative to 28-10-2008 8:30:12.5 UTC. Accelerogram
	for CMG-

Acknowledgments. We are grateful to our colleagues Petr Jedlička and Jaroslav Štrunc for maintetance of WEBNET and Zuzana Procházková and Hanka Čermáková for precise seismogram analyses. The presented work was supported by the Norway Grants project A/CZ0046/2/0015 and by the Czech research projects MSM0021620855 and AV0Z30120515.


MD Hanne Elberling
email: hanneelberling@yahoo.com

MD, PhD Allan Linneberg

MD, PhD Else Marie Olsen

PhD Robert Goodman

MD Anne Mette Skovgaard

Dr Med Sci

The prevalence of SDQ-measured mental health problems at age 5-7 years and identification of predictors from birth to preschool age in a Danish birth cohort: The Copenhagen Child Cohort 2000

Keywords: child, mental health problems, prevalence, SDQ, birth cohort intervals, p: parents, t: pre-school teachers

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Background

Psychiatric disorders with their origin in childhood are considerable contributors to the global burden of diseases [START_REF] Costello | 10-year research update review: the epidemiology of child and adolescent psychiatric disorders: I. Methods and public health burden[END_REF],

and half of all lifetime cases of any psychiatric disorder seem to have their beginning before the age of 15 years [START_REF] Kessler | Lifetime prevalence and age-of-onset distributions of DSM-IV disorders in the National Comorbidity Survey Replication[END_REF].

Data from longitudinal studies point to the persistency of psychopathology from childhood through pre-adolescence and adolescence although the individual symptom patterns seem to change over time [START_REF] Angold | Preschool psychopathology: lessons for the lifespan[END_REF][START_REF] Anselmi | Continuity of behavioral and emotional problems from pre-school years to pre-adolescence in a developing country[END_REF][START_REF] Costello | Prevalence and development of psychiatric disorders in childhood and adolescence[END_REF][START_REF] Costello | 10-year research update review: the epidemiology of child and adolescent psychiatric disorders: II. Developmental epidemiology[END_REF].

Epidemiological data on the frequency and distribution of mental health problems at different ages in childhood are essential for service planning and prevention and futhermore as a useful first step in the investigation of the significance of psychopathology in childhood [1;3].

Standardized measures are essential to ensure the validity of case-definition and to facilitate comparison of child mental health problems and risk factors across countries and over time [1;3;7]. The Strengths and Difficulties Questionnaire, SDQ, is a standardized questionnaire used to assess mental health problems and resultant impairment in children and adolescents. It was developed and evaluated in Great Britain [START_REF] Goodman | The Strengths and Difficulties Questionnaire: a research note[END_REF][START_REF] Goodman | The extended version of the Strengths and Difficulties Questionnaire as a guide to child psychiatric caseness and consequent burden[END_REF][START_REF] Goodman | Using the Strengths and Difficulties Questionnaire (SDQ) to screen for child psychiatric disorders in a community sample[END_REF][START_REF] Goodman | Psychometric properties of the strengths and difficulties questionnaire[END_REF] and extensive use and evaluation in many countries and different cultures has increased its usefulness for comparisons between countries [7;12;13].

Prevalence studies using the SDQ have found functionally impairing psychopathology in 10-15% of 5-15 year olds children and adolescents [14;15]. The patterns of mental health problems vary across cultures and over time [3;4;6;15] Relatively lower scores of SDQ-measured problems have been found in children from Nordic countries in contrast to other industrialized countries [13;15;16].

The Copenhagen Child Cohort 2000 (CCC2000) is a prospective study of mental health problems and psychopathology in a Danish birth cohort.. The overall aim of the CCC2000 is to study the presentation, developmental pathways and risk mechanisms of psychopathology longitudinally from birth [START_REF] Skovgaard | The Copenhagen County child cohort: design of a longitudinal study of child mental health[END_REF].

The present study is a follow-up of CCC2000 to estimate the 6-month prevalence, distribution and comorbidity of SDQmeasured mental health problems in children aged 5-7 year An additional aim was to investigate possible predictors of mental health problems of this age . Information on putative risk factors was obtained from Danish National Registers.

Methods

Study population

The Copenhagen Child Cohort 2000, CCC2000 is a birth cohort that consists of all children born in 16 municipalities in the county of Copenhagen in a one-year period from 1 st of January to 31 st of December 2000, comprising a total of 6,090 children. Children not living in Denmark at the time of the investigation were excluded. There was no other criteria for exclusion.

The CCC2000 covers 9% of children born in Denmark during the year 2000 and is representative of the general population of Danish children born that year with regard to the distribution of gender and birth parameters. The methodology of the CCC2000 baseline study cohort has been described in more detail elsewhere [START_REF] Skovgaard | The Copenhagen County child cohort: design of a longitudinal study of child mental health[END_REF].

Recruitment

Information about the child and current place of residence for this wave of the study was obtained from the Danish Civil Register. Out of 6,090 children, 6072 (99.7%) children could be identified in the Danish National Registers (18 children (0.3%) died before they could enter the follow-up study). A total of 174 children were not eligible for the SDQ investigation: 136 (0.2%) lived abroad, 29 (0.5%) had no known address and 9 (0.1%) were missing from the Danish Civil Register. Thus, a total of 5,898 (97%) children were eligible for the SDQ investigation at the 5-7 year follow-up.

The study was carried out from August 2005 to April 2007. The parents were mailed an invitation letter with information about the study and were asked to complete and return an enclosed copy of the SDQ. In addition, parents were asked to relay an SDQ to the child"s pre-school teacher as well. A reminder was mailed after 3 weeks to parents who had not responded. The parents and pre-school teachers returned their questionnaires separately.

Parents who had problems answering the questionnaire because of difficulties with the Danish language were offered help with the translation of the questions.

Instrument

Mental health problems were measured by the Strengths and Difficulties Questionnaire, SDQ, which is a 25 item screening questionnaire in which parents and pre-school teachers report on how the child has been in the past 6-months [8;9;11]. The items are divided in five scales of five items, each, generating scores for emotional symptoms, conduct problems, hyperactivity/inattention, peer relationship problems, and pro-social behaviour. The five scales are each scored from 0-10 and classified as "normal", "borderline", or "abnormal".

A total difficulties score is calculated as the sum of scores of the conduct, hyperactivity, emotional and peer problems scales [8;18]. The extended version of the SDQ used in this study includes questions on whether the respondent believes that the child "had difficulties in one or more of the following areas: emotions, concentration, behaviour or being able to get along with other people". If a problem is reported, the respondent is asked additional questions about distress, social impairment, burden and chronicity [START_REF] Goodman | The extended version of the Strengths and Difficulties Questionnaire as a guide to child psychiatric caseness and consequent burden[END_REF].

A computerised algorithm brings together the answers from parents and pre-school teachers on symptoms and impact to estimate the probability of mental health problems [10;14;19]. The algorithm makes separate predictions of conductoppositional problems, hyperactivity-inattention problems and emotional problems and generates an overall prediction regarding probable, possible or unlikely mental health problems [START_REF] Goodman | Using the Strengths and Difficulties Questionnaire (SDQ) to screen for child psychiatric disorders in a community sample[END_REF]. In summary, the algorithm predicts disorders on the basis of the combination of high symptom and impact scores. Probable conduct-oppositional and emotional disorders can be predicted on the basis of high symptoms and impact reported by just one respondent (since these disorders can be situational). Conversely, a probable hyperactivity-inattention disorder is only predicted when there is supporting evidence from both the parent and preschool teacher (since ADHD and hyperkinesis are expected to be pervasive across situations).

Using this algorithm to combine data from different informants has been shown to identify psychiatric diagnoses in children with a specificity of 80-95%, a sensitivity of 63%-85% and the positive and negative predictive value of respectively 54% and 96% in a general population [START_REF] Goodman | Using the Strengths and Difficulties Questionnaire (SDQ) to screen for child psychiatric disorders in a community sample[END_REF]. The SDQ has been used in both industrialized counties and in less developed countries [START_REF] Goodman | Predicting type of psychiatric disorder from Strengths and Difficulties Questionnaire (SDQ) scores in child mental health clinics in London and Dhaka[END_REF], suggesting that the questionnaire can be used in different cultural settings.

The SDQ has been translated into Danish and used in general population studies of Danish children, however with no available psychometric data [START_REF] Obel | Strengths and Difficulties Questionnaire (SDQ-Dan). A new instrument for psychopathologic screening of children aged 4-16 years[END_REF].

Since the transition from preschool to school does not happen until the age of 7 in Denmark, a few SDQ questions were modified, substituting "kindergarten" for "school", and "pre-school teacher" instead of teacher".

(For further information, see www.sdqinfo.com.)

In this study children were defined as having impairing mental health problems if they were classified as having "probable" mental health problems by the SDQ algorithm; they are contrasted with the rest of the sample who were classified by the algorithm as having "unlikely" or "possible" mental health problems. Mental health problems are further described using the total difficulties score and specific SDQ symptom and impact scores, all generated using standard procedures [8;9;18].

Register data

Data from Danish National Registers were used for attrition analysis and to obtain data on perinatal adversities and putative psycho-social risk factors.

In Denmark, all citizens have a unique personal identification number which follows the person through out life. The personal ID number is used by all central registers and authorities including the health care system. Furthermore, it is possible to obtain valid information on the socioeconomic status of the parents of each individual child.

Data from the Medical Birth Register included birth weight, gestational age, apgar score (a method of evaluating the physical condition of a newborn infant shortly after delivery. Normal apgare score is 10), birth complications and serious malformation. Data on hospital admissions included diagnoses on mental health disorders (ICD-10 F-diagnoses) obtained from the National Patient Register and the Psychiatric Central Register. Data on socio-economic variables in the family were obtained from Integrated Database for Labour Market Research. Overall, data on 99.7% of the birth cohort were obtained via these registers.

Ethics

The project was approved by the Scientific Ethics Committee of Copenhagen County (KA-05103) and the Danish Data Protection Agency (2007-41-1361).

Statistical analysis

The chi square test was used to evaluate differences between groups. Prevalence estimates were given with 95% confidence intervals (CI). A weighting procedure based on direct standardization [START_REF] Rothman | Modern epidemiology[END_REF] was applied to take account of the way in which responders differed from the target population in health and socioeconomic risk factors.

For example, a weighted estimate of the prevalence of mental health problems in the sample studied (responders) assuming the same gender distribution as in the entire sample (including both responders and non-responders).

Test of Marginal Homogeneity was used for analyses of the differences in SDQ scores categorized as normal, borderline and abnormal in children with 2 informants.

Because of non-homogeneity of variances, the Wilcoxon Signed Rank test was used to investigate the differences in median SDQ symptom scores derived from parent and pre-school teachers. Kappa coefficients were calculated to investigate the agreement in SDQ symptom scores between parents and preschool teachers.

Children with "probable" mental health problems as predicted by the SDQ algorithm were compared on perinatal adversities and socioeconomic risk factors with children where mental health problems were predicted to be "possible" or "unlikely". Subsequently, predicted mental health problems were dichotomised as probable or not (i.e. combining "possible" and "unlikely") and multivariate logistic regression was performed, including independent variables which were found to be associated with mental health problems in univariate analyses (p<0.1). The multiple logistic regression analysis estimated the independent effect of the variables while controlling for effect of child gender and maternal age.

Differences in mental health problems by gender and by the number of SDQ informants are reported as risk ratio, RR with 95% CI.

Statistics were performed with the statistical program SAS, version 9.2 (SAS Institute Inc, Cary, NC, USA)

Results

Of 5,898 children eligible for the SDQ study, questionnaires were completed on a total of 3,501 (59%). Parent questionnaires were obtained on 3,349 children (57%) and pre-school teacher questionnaires on 2,594 children (44%).

Questionnaires from both parents and pre-school teachers were obtained on 2,442 children (41%). Information from parents only was available on 907 children; and information from pre-school teachers only was available on 152 children. The gender distribution was 1,775 (51%) boys and 1,726 (49 %) girls. The mean age (SD) was 5.4 (0.3).

Table 1 shows background characteristics of the CCC 2000 cohort. The following characteristics were over-represented among non-responders: parents who were not born in Denmark; low household income; younger mothers; low maternal education; parents living separately at the time of birth; and changed family composition in the first 5 years of life.

No differences in the distribution of perinatal adversities were found, except for a small, but statistically significant difference in mean birth weight (3,535 grams in children with SDQ data as compared with 3,449 grams in nonparticipants).

Table 1 about here

Prevalence of mental health problems

The 6-month prevalence of mental health problems predicted by the SDQ algorithm is shown in table 2. Among the 3,501 participants, 168 were classified as having mental health problems, which corresponds to an overall 6-month prevalence estimate of 4.8% for participants. A further 513 children (14.7%) were classified as "possible" cases and 2,781 (79.4%) children were classified as "unlikely."

The analyses were repeated using weights to adjust for differential non-response, with only a slight increase in estimated 6-month prevalence from 4.8% to 5.0% (95%CI 4.2-5.8). The weighting procedure is described in the section on statistical analysis.

Among the different types of mental health problems, conduct problems were the most common, found in 3%, followed by. emotional problems, found in 1.5%, and hyperactivity/inattention in 0.7%. Boys were at a higher risk of having at least one mental health problem as compared to girls: RR 2.0 and at a significantly higher risk in all types of mental health problems except for emotional problems.

Table 2 about here

Table 3 shows a comparison of parent and pre-school teacher ratings in the 2,442 children with information from both a parent and a pre-school teacher. Among these children, 3.6% (N=88) had a total difficulties score in the abnormal range as judged by parent report, but significantly more children, namely 5.9 % (N=143), had abnormal scores as judged by pre-school teacher report (table 3). When looking at measures of impact, a total of 114 children (4.8 %) had an abnormal impact score according to their parent report, whereas 147 children (6.2%) had an abnormal impact according to their preschool teacher report. The most common mental health problems reported by parents were emotional problems (7.4% in abnormal range), whereas problems of hyperactivity/ inattention were the most common reported by pre-school teachers (9.0% in abnormal range). The agreement of parents and the pre-school teachers in their reports on child mental health problems was in general low, as indicated by a Kappa value ranging from 0.09 to 0.29 Children with information from preschool teachers only were more likely to have an abnormal high total difficulties score compared to children with SDQ completed by both parents and pre-school teachers: RR 3.2 (95%CI 2.2-4.7). However, no difference in risk was seen regarding children with parent reports only.

There was no significant difference in gender for children with missing reports from parents and the preschool teacher.

Table 3 about here

Co-occurrence of mental health problems at child age 5-7 years Among children with mental health problems, 7% had overlap across two main SDQ subgroups, primarily due to the co-occurrence of conduct problems in more than a third of the children with hyperactivity/inattention problems. (figure 1).

Fig 1 about here

Predictors of mental health problems at 5-7 years

Table 4 shows perinatal and socioeconomic variables associated mental health problems in children aged 5-7 years. The only significant child-related factors were being a boy and having been diagnosed at hospital with a mental health 10 disorder in the first four years of life. No significant associations were found with regard to markers of perinatal adversities or illness.. Socioeconomic factors significantly associated with mental health problems at 5-7 years were immigrant parents, teenage mothers, less educated mothers, lower household income, parents living separately at the time of the birth, and the child being exposed to changes in family composition in their first 5 years of life No significant association was found with regard to paternal age, parity at birth, the social status of the living areas at birth.

Table 4 about here

All variables significantly (p=<0.1) associated with mental health problems (p<0.1) were included in multivariate models (logistic regression) adjusted for gender and maternal age (table 5). Male gender was the only characteristics of the child that was significantly and independently associated with mental health problems, OR 2.11. Overall, gestational age was not significantly associated with mental health problems, although children with gestational age below 31 weeks were associated with higher risk of mental health problems compared to children born after the 36 th week.

Several features or correlates of social and economic disadvantage predicted mental health problems in the multivariate model: young maternal age, low household income at the time of birth and the child living without any biological parents or with a single parent in the first year of life.

Table 5 about here

Discussion

In this 5-7 years follow-up of the birth cohort CCC 2000, we found a 4.8% 6-month prevalence of mental health problems assessed by parent and preschool teacher SDQs. The participation rate was 59% and analyses of attrition showed that some psycho-social risk factors were more frequent in non-participants. However, estimates of prevalence, weighted according to differential non-response only increased the prevalence from 4.8% to 5.0%. The prevalence estimates in the present study correspond closely to the 5% prevalence reported by a Danish study of parent SDQ scores in 4,971 children at 7 years of age though this study did not include teacher reports or questions about impact [START_REF] Christensen | The first 7 years of life[END_REF].

Conduct problems were the most common mental health problem, found in 3%. Problems of hyperactivity and inattention were found in 0.7 %, with nearly half of these children also having conduct or emotional problems.

The results from the present Danish study correspond to findings in studies from other Nordic countries, both with regard to the SDQ total difficulties mean scores reported by parents and with regard to the specific symptom scores [13;15;16], whereas SDQ studies from other countries in Europe have found higher estimates of prevalence, ranging from 6.3%-9.8%. [18;23-25].

A study of 5855 children aged 5-10 years from Great Britain [START_REF] Youth In Mind Goodman | SDQ -information for researchers and professionals about the Strengths and Difficulties Questionnaire[END_REF] thus showed a total difficulties score of 9.6 % (teacher reports) and 9.9 % (parent reports), contrasting with the lower rate found in the present study of 5.9 % (preschool teachers reports) and 3.6 % (parents reports). Similarly, the frequency of abnormal symptom scores was lower in all areas in Danish children when compared to British children. In particular with regard to hyperactivity/inattention, where the proportion of Danish children with abnormal symptom scores was 5.6 % (parent reports) and 9.0 % (preschool teacher reports), whereas the corresponding figures in British children were 16 % (parents reports) and 13.8 % (teachers reports).

In a cross-cultural comparison of findings from prevalence studies from Norway and Great Britain, it was shown that a lower prevalence of emotional problems in Norwegian children compared to British children could be explained by Norwegian parents under-reporting emotional symptoms, whereas a lower prevalence of conduct problems and hyperactivity/inattention in Norwegian children was considered to reflect a real advantages in child mental health in Nordic countries [START_REF] Heiervang | The Nordic advantage in child mental health: separating health differences from reporting style in a cross-cultural comparison of psychopathology[END_REF].

As compared with children in many previously studies, the present CCC2000 sample were younger and had more equal access to social and educational facilities. This could partly explain a lower prevalence [3;13;15]. It is noteworthy that we found an even lower rates of conduct problems and hyperactivity/ inattention problems than the study of Norwegian children. It is also striking that our rate of hyperactivity/inattention problems was so much lower than the 6.4% prevalence of ADHD reported by a study of Danish children at age 8-10 years [26]. Our low prevalence rates should be interpreted in the context of the study. The mean age of the CCC 2000 children in the present study was 5.4 years, and only 4.3% of the children had started school at the time of the SDQ investigation. Instead, they spent their day-time in kindergarten, mostly engaged in free play and outdoors activitiesa setting that is much freer from discipline and rules than school. By contrast, children in the majority of European countries start school or school-like day-care before the age of 5 years, and the study from Norway SDQ investigated children at age 8-10 years in school settings. Thus if mental health problems are precipitated or aggravated by the external pressures and demands of school, this could contribute to our lower prevalence. In addition, hyperactivity/inattention symptoms may be under-detected in a kindergarten setting since pre-school teachers have less opportunity than school teachers to observe children engaged in tasks that require detailed knowledge and concentration.

In the CCC2000 study we found low agreement between parents and teachers reports, with low Kappas for all scales of mental health problems. In addition, there were significant differences in the means SDQ scores from parents and preschool teachers for all SDQ scores other than the peer problem score. Pre-school teachers were reporting more problems than parents in all areas of mental health other than emotional problems and peer problems. Particularly given the low agreement between parents and pre-school teachers found in the present study, the rate of problems may have been underestimated in the quarter of the study population with just one informant.

In conclusion, the relatively low prevalence in Danish pre-school children might thus be substantive and reflect a population at relatively low risk load and at an age characterised by a low exposure to environmental stress and , possibly also reflect methodological shortcomings (underdetection).

Factors associated with mental health problems at age 5-7 years

Male gender was associated with an overall risk of mental health problems and with a higher risk of conduct problems and hyperactivity/inattention which is in accordance with findings in studies of children at other ages and from other cultures [14;16]. Socioeconomic risk factors for mental health problems at child age 5-7 years included young maternal age, low household income, and the child living with a single parent or without any biological parents at the time of birth. Previous studies have found socioeconomic risk factors associated with in particular emotional and conduct disorders [START_REF] Heiervang | Psychiatric disorders in Norwegian 8-to 10-year-olds: an epidemiological survey of prevalence, risk factors, and service use[END_REF]. In the CCC2000 study where the majority (94%) of child mental health problems were emotional or conduct problems, we found socioeconomic risk factors being the only significant predictors.

Corresponding associations have been reported in studies of Norwegian and the British children, although only male gender and family type associated with higher risk of conduct problems in the Norwegian study, whereas overall mental health problems were associated with markers of poverty [13;15;16].

Even though this describes predictors and not causal mechanism, it is still a useful reminder of the need for mental health services to focus on children in families with well known socioeconomic risks [27].

Limitations

The main limitation of the study concerns the relatively low rate of participation (59%), particularly among disadvantaged groups where children might plausibly have been more prone to develop mental health problems. This bias may have resulted in underestimation of the prevalence of mental health problems. However, use of data from National registers made it possible to calculate the weighted prevalence adjusting for non-response, and this did not result in any substantial changes in prevalence.

Another limitation concern the possible informant bias as 26% of the children had missing information from one informant, as has been discussed above.

Finally, the Danish version of SDQ has been forward-and backward-translated as recommended by The World Health Organization [28] but the SDQ has not yet been validated on Danish populations. However, measures of validity and reliability have been established in other European countriesvalidation studies from Norway and Sweden in particular are relevant since Denmark is quite similar to other Nordic countries in language and culture [START_REF] Obel | The Strengths and Difficulties Questionnaire in the Nordic countries[END_REF].

Conclusions

In a general population study of children with mean age 5.4 years from the Copenhagen Child Cohort 2000, mental health problems measured by the Strengths and Difficulties Questionnaire were found in 5% with conduct problems being the most common.

Male gender and several socio-economic risk factors measured from birth to child age 4 were associated with higher risk of mental health problems in children age of 5-7 years.

The result from the study replicate findings from other general population studies regarding the possible advantage of mental health in Nordic countries when focusing on conduct and hyperactivity/inattention problems.

The relatively low SDQ problems scores found in the present study might reflect the low-stress context provided by Danish kindergartens, or the greater difficulty of detecting significant hyperactivity and inattention in such a setting.

Further planned follow ups of this cohort will demonstrate whether the transition to formal schooling results in a substantial increase in overt mental health problems, particularly hyperactivity-inattention. .

  and adolescents in Germany: results of the BELLA study within the National Health Interview and Examination Survey. Eur Child Adolesc. Psychiatry, 17 Suppl 1, 22-33.[25] Shojaei,T., Wazana,A., Pitrou,I., & Kovess,V. (2008) The strengths and difficulties questionnaire: validation study in French school-aged children and cross-cultural comparisons. Soc Psychiatry Psychiatr. Epidemiol.

	[26] Petersen,D.J., Bilenberg,N., Hoerder,K., & Gillberg,C. (2006) The population prevalence of
	child psychiatric disorders in Danish 8-to 9-year-old children. Eur Child Adolesc. Psychiatry,
	15, 71-78.
	[27] Friedman RJ & Chase-Landsdale PL. Chronic Adversities. [In Rutter& Taylor (eds) Child and
	Adolescent Psychiatry], 261-276. 2002. Oxford Blackwell Science.
	Ref Type: Generic
	[28] World Health Organization. Process of translation and adaptation of instruments. 2009.
	http://www.who.int/substance_abuse/research_tools/translation/en/index.html.
	Ref Type: Generic
		Hyperactivity/inattention,
		N=24
	Conduct problems,
	N=105
		N=13
	N=9
		N=2
	N=95	N=1	N=48
			Emotional
			problems, N=51

Table 1

 1 Characteristics of the entire birth cohort CCC2000 at the 5-7 years follow-up and attrition analyses based on register data on socioeconomic and perinatal variables

	Figure1

Changes in family composition in the first 5 years of life:

  ISCED= The International Standard Classification of Education (ISCED) designed by UNESCO ** Likelihood Ratio Chi-square test probability for comparison of responders and non-responders

					<0.0001
	Yes:	944 (15.6 %)	484 (13.8 %)	460 (18 %)	Missing=16
	Household income at child
	birth:
	1. Quartile (lowest)	1,497	711 (20.4 %)	786 (31.3 %)	<0.0001
	2. Quartile	1,497	861 (24.8 %)	636 (25.3 %)	Missing= 83
	3. Quartile	1,497	925 (26.6 %)	572 (22.8 %)
	4. Quartile (highest)	1,498	981 (28.2 %)	517 (20.6 %)

*

Acknowledgements

The study was supported by The Copenhagen County Research Foundation (now called The Capital Region of Denmark), The Health Insurance Foundation, Mrs C. Hermansens Memorial Fund, The Foundation of Butcher Max Wørzner and wife, The Psychiatric Foundation of 1967, The Tryg Foundation, The Augustinus Foundation, The Danish Association for Mental Health

Conflicts of interests

None

Robert Goodman is director and part owner of Youth in Mind, which provides no-cost software and websites related to the SDQ.