Journal of Seismology 8: 41–56, 2004. © 2004 Kluwer Academic Publishers. Printed in the Netherlands.
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Use of microtremor measurement for assessing site effects in Northern Belgium – interpretation of the observed intensity during the MS = 5.0 June 11 1938 earthquake F. Nguyen1 , G. Van Rompaey2 , H. Teerlynck1 , M. Van Camp2 , D. Jongmans3,1 & T. Camelbeeck2 1 Laboratory
of Engineering Geology and Applied Geophysics, Li`ege University, Bat. B19, 4000 Li`ege, Belgium, e-mail:
[email protected]; 2 Royal Observatory of Belgium, Avenue Circulaire, 3, 1180 Bruxelles, Belgium, e-mail:
[email protected]; 3 LIRIGM, Grenoble University, France, e-mail:
[email protected]
Received 1 May 2002; accepted in revised form 14 March 2003
Key words: 1938, dispersion curve, H/V, microtremor, Nakamura, site effects, surface waves inversion
Abstract To provide quantitative information on site effects in the northern part of Belgium, forty-seven H/V microtremor measurements were performed with 5 second seismometers over an area of about 15.000 km2. Most of the results show a northward regular increase of the fundamental period in agreement with the augmentation of the Mesozoic and Cainozoic soft sediments thickness from a few meters 40 km south of Brussels to 900 m at the Netherlands-Belgium border. The measured resonance frequency values were consistent with theoretical computations performed at different sites on the basis of existing information and shallow seismic experiments. At one particular site (Uccle) where borehole data were available, microtremor measurements using an array of four seismological stations with different apertures allowed to obtain the low frequency part of the Rayleigh wave dispersion curve, extending the range covered by the analysis of surface waves artificially generated. The Vs profile derived from the surface wave inversion corroborates the 1 Hz natural frequency of the site. Comparison of these results with the macroseismic information concerning the MS = 5.0 1938 earthquake which occurred 50 km west of Brussels, confirmed the hypothesis that the geological structure of the Brabant massif is likely to control damage distribution during such an earthquake. Comparison between the intensity map of the 1938 earthquake and the resonance period of sediments obtained by our microtremor study shows a clear relation between the two parameters. During the 1938 earthquake, site effects played a prominent role due to the dimension of the source whose corner frequency was about 1 Hz.
Introduction As many regions in north-western Europe, Belgium is characterised by a low to moderate seismic activity. The country experienced two damaging earthquakes during the last twenty years, in Liège on November 8, 1983 (MS = 4.7) and in Roermond (The Netherlands) on April 13, 1992 (MS = 5.4). Recent investigations (Melville et al., 1996; Camelbeeck et al., 2000) on strong historical earthquakes suggest that since the 14th century three earthquakes of magnitude of or
greater than MS = 6.0 occurred in the southern North Sea (May 21, 1382), in the Channel (the April 6, 1580) and the northern part of the Ardenne (September 18, 1692). The spatial distribution of the seismic activity is shown in figure 1A which includes the instrumental seismicity recorded since 1911 and the major historical earthquakes since 1350. The regional seismic hazard for Belgium has been recently assessed (Figure 1B), considering a 475 years return period and rock conditions, as required by Eurocode 8 (Leynaud et al., 2000; Mihailov et al., 2001). Peak ground ac-
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Figure 1. A: Seismic activity in and around Belgium from instrumental seismicity (1911–1998) and large historical earthquakes (1350–1910). B: Regional seismic hazard map for a return period of 475 years (Leynaud et al., 1999).
43 celerations were computed using an attenuation law (Ambraseys et al., 1996) with one standard deviation (84 percentiles) to account for the large uncertainty on the strong motion data. The calculated seismic hazard is higher in three main areas, the Roer Graben and Liège regions (L), the Hainaut zone (H) and the Pas de Calais zone (PC), corresponding to the source zones with the strongest known activity. However, the computed peak acceleration values do not consider site effects resulting from the presence of soft sedimentary layers overlaying the rock. In Eurocode 8, local geology is taken into account in the response spectra proposed for different soil conditions (Vs in the last 30 meters). Site effects could be significant in Belgium where the northern part of the country (mainly the London-Brabant Massif) is covered by Mesozoic and Cainozoic soft deposits whose thickness progressively increases northward, from a few meters 40 km south of Brussels to more than 900 m near Antwerpen. In this massif, a damaging earthquake of magnitude MS = 5.0 occurred on June 11 1938. Its epicenter was located 50 km to the west of Brussels. The data collected at the Royal Observatory of Belgium at the time of the event (official inquiry, personal communications, mails have been restudied in order to better evaluate local intensities. In parallel, site effects in the same region have been studied using microtremor measurements. Our goal is to evaluate regional site effects from the records and 1D modelling and to infer their impact on the intensity spatial distribution during the 1938 earthquake.
Geological setting The geology of Belgium consists of Palaeozoic rocks unconformably overlaid with almost flat lying layers of Mesozoic and Cainozoic soft rocks and sediments in most part of the country (Figure 2). This basement, generally folded, is made up of Lower Palaeozoic massifs and Upper Palaeozoic formations. The Lower Palaeozoic massifs are composed of slates and quartzites which were deformed during the Caledonian orogeny (about 410 My ago). The main massif is the London-Brabant Massif (Brabant Anticlinorium) in north Belgium, which is mostly overlaid by slightly tilted Mesozoic and Cainozoic soft rocks and sediments. It only outcrops in a few valleys where this post-Palaeozoic sedimentary cover has been eroded. To the South of the London-Brabant Massif, the Devono-Carboniferous formations, which constitute
the northern part of the Rhenohercynian belt, were deformed during the Variscan orogeny (from 330 Ma to 290 My ago). These calcareous and silico-clastic rocks rest unconformably on the Lower Palaeozoic massifs. A major fault, the Midi-Eifel-Aachen thrustcomplex, which approximately coincides with the limit of the North Variscan Front, crosses Belgium from East to West (Figure 2). Further North, DevonoCarboniferous rocks form the Campine basin which has not been deformed during the Variscan orogeny. Several faults with different orientations and variable dip cross the London-Brabant Massif (De Vos et al., 1993). To the NE of Belgium and in Southern Netherlands, several NW-SE trending normal faults delimit the Lower Rhine Graben, filled with up to 2000 m of predominantly Upper Oligocene to Quaternary sediments (Geluk et al., 1994). Over the London-Brabant Massif and the Campine Basin, the thickness of the Post Palaeozoic soft sediments (Cretaceous chalk, Tertiary sands and clays, Quaternary loess) increases continuously towards the North with a maximum of more than 900 m above the Campine Basin (Legrand, 1968).
The June 11th 1938 Earthquake On June 11, 1938 at 10:57 am (UT) a destructive earthquake occurred in the northern part of Belgium. With the data from 15 European seismic stations, Camelbeeck (1994) located the epicentre at 50◦ 44.1 of northern latitude and 3◦ 37.1 of eastern longitude (Figure 3). The focal depth was estimated at 19 ± 4 km. The epicentre is located at the southern limit of the Brabant Massif, with an estimated error of 4 km. Using data from 19 stations, a magnitude of MS = 5.0 ± 0.3 was calculated using the Prague formula (Karnik, 1969). Somville (1939) published a study of the earthquake effects, remarkable for this epoch. Main information concerning the damage, the effects on people and the numerous perturbations on the ground surface or underground water can be found in this publication. In his work, Somville (1939) discriminated the damages affecting houses from those observed on large buildings like churches and castles. The main damaging effects due to the earthquake are the falling of chimneys with its consequences on roofs and verandas, and the cracks in many walls. In Belgium, Somville evaluated to more than 17,500 the number of chimney falls. In 43 cities and villages, more than 10% of the
Figure 2. Schematic geological map of Belgium (redrawn from Legrand (1968), Colbeaux (1977) and Geluk et al. (1994)).
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Figure 3. Intensity distribution map for the 1938 earthquake.
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46 existing chimneys were down. Concerning large constructions (churches and castles), Somville provided numerous examples of falls of heavy decorative elements and many descriptions of strong oscillations of crosses and spires. Intensity was evaluated in each city using a statistical analysis of the falling of chimneys of houses. The archives, conserved at the Royal Observatory of Belgium, allowed us to complete Somville’s work and to establish a macroseismic map using the new EMS-98 (Grünthal, 1998). On Figure 3 are plotted the evaluated intensity values, from IV (or lower) to more than VI. A maximum intensity of VII has been assessed inside numerous isolated areas separated by large regions with a lower intensity. The decrease of intensity with the epicentral distance is not homogeneous and strongly depends on the azimuth at a regional scale. The area with intensity greater or equal to VI is elongated in an east-west direction with a length of about 160 km and a width of 50 km. The intensity attenuation is clearly stronger along the NS direction with an abrupt decrease to the South at the Variscan Front (Midi Fault). This highly fractured E-W zone could have played the role of a screen for the seismic waves propagating to the South. The elongation of the isoseismals during the 1938 earthquake was interpreted by Ahorner (1975) as an indication of a right-lateral strike slip movement along a WNW-ENE trending shear zone. Up to now, no reliable faultplane solution has been published for this earthquake, mainly due to the lack of P-wave first motion data. On the other hand, the comparison of the intensity repartition with the soft sediment map suggests that the regional geology could play a role in the damage distribution. In order to test this hypothesis of a large scale site effect, we studied the dynamic response of the Mesozoic and Cainozoic soft sediments covering the basement.
Site effect evaluation The fundamental resonance frequency at one site is a major parameter for engineering studies in dynamics (earthquake engineering, traffic impact, vibration studies), which depends on geological layers’ geometry and dynamic characteristics (mainly Vs values). Classical geophysical methods for evaluating geometrical and dynamic properties of soil layers are the hole tests and, more recently, the inversion of surface waves generated by an artificial source (Jongmans, 1992). In
1989, Nakamura proposed to use microtremor measurements from a 3-D sensor to obtain the fundamental frequency at one site. The principle of the method (called hereafter the H/V method) is to compute at one single station the spectral ratio between the horizontal and the vertical components of the motion. In most cases the spectral ratio curve exhibits a peak at a frequency which is claimed to be the fundamental frequency of the site. Since 1989, this method has been extensively used in earthquake engineering (for a recent review, see Bard, 1999) and most studies have shown that the H/V method is actually able to directly retrieve the fundamental frequency of soft soils. Although the theoretical background of the method is still unclear and debated (Bard, 1999), this latter has quickly emerged as a quick and easy technique for providing the fundamental frequency at a given site (Bard, 1999). To this regard, this technique appears to be a new geophysical method able to bring information for characterising soft soil layers overlying a bedrock (Ibs-von Seht and Wohlenberg, 1999; Delgado et al., 2000) and usable for geological or geotechnical mapping purposes in urban areas (Teves-Costa et al., 1995; Gueguen et al., 1998; Giampiccolo et al., 2001). On the other hand, the use of the H/V curve for assessing the site response (and particularly the maximum amplification) is still strongly debated but no general correlation has been shown at the present time (Bard, 1999). Whereas some authors related H/V curves to damage distribution for a specific event (Mucciarelli et al., 1998; Duval et al., 1998), other ones did not find such interaction (Guegen et al., 1998; Trifunac et al., 2000). In this study, the analysis of microtremors is strictly limited to the determination of the fundamental resonance frequency. The northern part of Belgium is characterised by the presence of flat lying soft sediments with a regularly varying thickness, which are likely to generate site effects and could have influenced damage distribution during the June 11, 1938 earthquake. The H/V method has been applied in order to map the variation of the fundamental frequency over this area. H/V ratio measurements Forty-seven microtremor measurements were performed throughout the Brabant Massif and the Campine basin. We used LE-3D/5-sec seismometers connected to the classical PCM-MARS 5800 station build by Lennartz Electronic. These acquisition systems contain a 12 bits A/D converter, which has been
47 Table 1. Table presenting site names and numbers, site latitude and longitude and the measured frequency on site Station number
Town
Latitude north
Longitude east
Measured frequency (Hertz)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47
De Panne Nieuwpoort Hoogveld Knokke Brugge Eeklo Assenede St.-Niklaas Doel Mortsel (Antwerpen) Westmalle Mol Lommel Kinrooi Opoeteren Hechteren Huishout Mechelen Gent Ruiselede Tielt Ieper Harelbeke Dendermonde Vilvorde Wijgmaal Diest Maasmechelen Hasselt Oplinter Tourines Uccle Ninove Oudenaarde Celles Ath Quenast La Hulpe Limelette Orp Broekom Eben Haneffe Liernu Villers-la-Ville Mellet Tihange
51◦ 51◦ 51◦ 51◦ 51◦ 51◦ 51◦ 51◦ 51◦ 51◦ 51◦ 51◦ 51◦ 51◦ 51◦ 51◦ 51◦ 51◦ 51◦ 51◦ 51◦ 50◦ 50◦ 51◦ 50◦ 50◦ 50◦ 50◦ 50◦ 50◦ 50◦ 50◦ 50◦ 50◦ 50◦ 50◦ 50◦ 50◦ 50◦ 50◦ 50◦ 50◦ 50◦ 50◦ 50◦ 50◦ 50◦
2◦ 2◦ 2◦ 3◦ 3◦ 3◦ 3◦ 4◦ 4◦ 4◦ 4◦ 4◦ 5◦ 5◦ 4◦ 5◦ 4◦ 4◦ 3◦ 3◦ 3◦ 3◦ 3◦ 4◦ 4◦ 4◦ 5◦ 5◦ 5◦ 4◦ 4◦ 4◦ 4◦ 3◦ 3◦ 3◦ 4◦ 4◦ 4◦ 4◦ 5◦ 5◦ 5◦ 4◦ 4◦ 4◦ 5◦
0.465 0.49 0.48 0.32 0.37 0.35 0.297 0.31 0.22 0.29 0.22 0.27 0.19 0.17 0.28 0.28 0.36 0.45 0.46 0.44 0.48 0.61 0.61 0.51 0.65 0.63 0.45 0.42 0.74 0.99 1.18 0.91 0.79 0.69 1.74 1.92 9.9 1.25 1.3 0.58 0.58 0.52 0.45 0.91 3.45 10 0.59
05.384 07.285 03.704 19.140 11.875 11.908 14.454 12.454 18 39 09 58 16 08.6 11 2 14 59.8 09 09.1 04 10 04 31.9 04 54 01 41 03 20 01.638 00.055 50.505 52.515 00.215 55 32 55 30.9 59 01.3 59 05 54 33.3 50 32.6 47 07.1 48 12 49.332 49.703 42.727 37.939 39.778 43 51 40 48 42 45.6 46 51.7 47 15.5 37 39 35 30.8 34 39 30 19 31 51
34.955 44.174 54.957 17.439 18.557 36.060 43.961 04.817 15 36 27 44 41 11.9 39 15 22 37.2 45 50.6 39 15 23 46.8 47 07.2 28 49 43 42 24.362 17.181 01.056 18.394 03.083 25 15 41 34.1 00 43.7 43 36.1 21 44.2 58 27.7 45 26.1 124 54 06.062 37.331 28.496 44.221 08.980 29 15 34 22 59 29.8 19 30.6 39 19 50 48 23.9 31 50 28 44 15 53
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Figure 4. Location of the 47 stations used for noise measurements and comparison between the soft sediment thickness and the site fundamental period over the northern part of Belgium. The twelve stations of Figure 5 are shown by a circle.
proved to be very stable with a very low electronic noise level. The Least Significant Bit correspond to 1 µV and the sensitivity of the seismometer is 400 V.s/m. To be confident in the reliability of our measurements, tests have been conducted on the electronic noise caused by the electronic devices of the seismometer. Without amplification of the input signal, the electronic noise appears important in the frequency band 0.1–0.4 Hz. Tests were satisfactory with an amplification of 24 . All the measurements have been done with such an amplification. In this way, we benefited of the maximal possible digital resolution. The measurements have been conducted in order to avoid different possible sources of perturbation. The seismometer was generally protected against the wind although its influence can hardly be removed completely. The location of the measure sites is given in Figure 4. Table 1 contains the site name and number, the site latitude and longitude as well as the measured frequency. The microtremors were recorded during 20 minutes at each site with a frequency sampling of
250 Hz. H/V ratios were computed on twenty 120-sec windows with an overlap of 60 seconds. The mean curve and the mean ± one standard deviation curves were calculated at each site, except for Villers-la-ville where only one data set was available. Figure 5 shows the H/V ratio curves at twelve stations whose locations are shown by circles in Figure 4. Each column on Figure 5 represents an approximate SN line. All curves exhibit a peak at a frequency which is usually very stable in time, contrary to the peak amplitude which varies. Such variation is shown on Figure 9 which displays H/V curves measured at Uccle with a one year interval (during daytime). The same observation has already been made by several authors and could result from the effect of local sources (Bard, 1999). Also, curve shape variations from a sharp peak to a broad amplitude range are observed. This modification could result from different measurement conditions, including the weather (Mucciarelli, 1998). However, the peak frequency remains clear and stable for all stations. The main feature on Figure 5 is that, except
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Figure 5. H/V ratios measured at twelve stations whose location is shown by a circle in Figure 4. The figure layout corresponds to the SN and EW directions.
between stations Haneffe and Broekom, a systematic decrease of the resonance frequency is observed from South to North, from a few Hz in the southern part to about 0.3 Hz in the North. Curves of fundamental period values have been plotted in Figure 4, which show a regular increase of the fundamental period from South to North, in good agreement with the augmentation of the soil thickness. The only discrepancy appears in the eastern part of the area (in the vicinity of Haneffe) where lower period values have been measured. These results, concerning three sites where the bedrock is shallow, could be due to the presence of soft superficial layers in the area but this hypothesis has to be verified. Figure 6 presents the fundamental period as a function of the soil thickness for all the measurements and shows a gross correlation between the two variables. Supposing that the post Palaeozoic sediments are homogeneous, the natural period T of the layer is given by T = 4H/Vs where H is the soil
thickness and Vs the shear wave velocity. The nonlinear shape of the curve fitted to the data (Figure 6) however indicates that Vs increases with depth (from about 120 m/s to 950 m/s) and that the sediments can not be considered as one homogeneous layer, which is consistent with the geological data and the evolution of compactness with depth. In our study, H/V spectral ratio measurements then appear to be a very quick and reliable way for quickly mapping the sediment thickness overlying the bedrock at a regional scale and to point out anomalies resulting from variations in the sedimentary pile. Site response evaluation Seismic prospecting was performed at two sites (Uccle and Gent) in order to investigate the soil properties as a function of depth. Seismic waves were generated by explosions and recorded by twenty-four 4.5 Hz geophones with a 5 m spacing. The method of surface
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Figure 6. Relation between the soil thickness and the fundamental period values derived from the H/V method.
wave inversion (Herrmann, 1987) was used to derive the depth-evolution of the S-wave velocity Vs while the picking of the first arrivals gave the P-wave velocity values. The models used during the inversion process were made of 13 layers, with a 5 m thickness near the surface and a 10 m thickness at depth as the resolution becomes weaker. The results of the seismic measurements at Uccle are given in Figure 7. Vs values continuously increase from 200 m/s to 375 m/s down to a maximum resolved depth of 32 m. The inversion of artificially generated surface waves is unable to give Vs values below 32 m due to the lack of energy over 0.2 seconds (Figure 7). Another method using microtremors measured by small-aperture arrays, was then applied in order to get the dispersion curve part in the low frequency range. In this approach, microtremors are assumed to be surface waves and are supposed to be stochastic and stationary in time and space. Initially proposed by Aki (1957) and recently extended by Bettig et al. (2001), the method computes correlation coefficients as a function of frequency and derives the phase velocity of the waves by matching a Bessel function J0 to an azimuthal average of correlation coefficients. A triangular array of four 1 Hz seismometers with various apertures (25 m, 50 m and 100 m, see Figure 8) was deployed in Uccle and microtremors were recorded on the vertical component during 20 minute time periods. The obtained dispersion curve, plotted in Figure 8, shows that phase velocity values were derived between 0.2 and 0.9 seconds. Microtremor measurements then allowed to extend in the high period range the curve retrieved from the surface waves generated by a shot. Although the two curves do not connect perfectly, the phase velocity values are of the same order and the differences around 0.2 seconds are far below the uncertainty derived from the energy plot of Figure
7. Inversion of the global dispersion curve allowed to obtain the evolution of Vs with a good resolution down to about 60 m (Figure 8). On the same figure are shown the P-wave velocity values derived from the analysis of refracted waves, which increase from 400 m/s at the surface to 2300 m/s at about 32 m depth . The strong Vp contrast observed at 32 m is probably due to the presence of the water table which does not influence Vs values. A similar profile has been derived in Gent from shallow seismic prospecting (surface wave and refraction results) and deep borehole data which gave the layer thickness variations (Legrand, 1968). Theoretical 1D transfer functions (Kennett, 1983) were computed at Uccle and Gent where geophysical prospecting was carried out. The model parameters were chosen based on the information provided by the geotechnical maps and the seismic measurements. The computations are compared to the H/V curves obtained at the two sites in Figure 9. The peak frequency values are very similar on both curves, indicating the reliability of the H/V method to obtain the natural frequency of the sites. In Uccle, measurements made at two different times (one year of difference) lead to the same frequency value. On the other hand, the H/V curve shapes and amplitudes are very variable with time and do not fit the theoretical transfer functions. These results agree with the main conclusions of Bard (1999). In both sites, the theoretical amplification at the resonance frequency reaches about 6.
Discussion This study has shown that the resonance frequency over the northern part of Belgium (Brabant massif and Campine basin) consistently decreases northward, in parallel with the thickness increase of the Mesozoic and Cainozoic sedimentary deposits (Figure 4). This frequency ranges from a few Hz at the southern limit of the Brabant Massif, where the basement is shallow, to 0.25 Hz (4s) in the northern part of Belgium where the sediment thickness reaches more than 900 m. The intensity map of the June 11, 1938 earthquake clearly shows an elongation of the isoseismals in the EW direction, parallel to the curves of equal thickness. Intensity values and resonance period curves are plotted on the same map (Figure 10). To the North, intensity values of VI or more are observed at a distance less than 50 km from the epicentre, while this distance reaches more than 120 km to the East. The damage pattern then seems to be linked to the variation
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Figure 7. Surface wave inversion at Uccle. Top left: seismograms recorded with an artificial explosive source. Top right: Phase velocity dispersion curve. Bottom left: Phase velocity dispersion curves, measured (dots) and computed (line). Bottom right: Vs profile and resolving kernels.
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Figure 8. Top: A: Dispersion curve obtained from an artificial source and the microtremors. B: geometry of the surface array (25, 50 and 100 m aperture) used in Uccle. Red triangles: 1 Hz seismometers. Bottom: shear wave profile and resolving kernels obtained after inversion of both data set.
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Figure 9. Comparison between the 1D theoretical transfer function and the H/V ratio curves at Gent and Uccle.
of resonance periods, as most of the strong damage (I ≥ VI) concentrates in a zone characterized by a soil resonance frequency between 2.0 and 0.5 Hz, approximately. The relationship between intensity and resonance period at the sites is highlighted in Figure 10B which presents the mean intensity values (with error bars) computed at each station within a 10 km radius circle and plotted versus resonance periods. The highest intensity values (around VI) were obtained for low resonance periods (< 2 sec.) and the intensity clearly decreases with the site resonance period. Damage during an earthquake results from the interaction between the ground motion and the dynamic response of the structure, the seismic motion being controlled by the convolution of source, propagation and site effects. In earthquake engineering, the impact of ground motion on structures is usually estimated by the response spectrum. In order to evaluate the effect of the 1938 event on structures at Uccle and Gent,
a synthetic seismogram was computed for a similar earthquake at an epicentral distance of 50 km, using the method of Boore (1996). This signal which takes into account source and propagation effects was then convolved with the two transfer functions (Figure 9) at Gent and Uccle and the corresponding acceleration response spectra are presented in Figure 11 as well as the spectrum on the bedrock at the same distance. At low period (high frequency), the spectral acceleration at both sites is amplified two times, in regard to the bedrock. Between 5 Hz (0.2 sec.) and 1 Hz (1 sec.), the acceleration spectrum at Uccle is systematically higher than at Gent, with a maximum ratio of almost 3 at the resonance frequency of Uccle (1Hz). This amplification range at Uccle more or less includes the resonance frequency of both masonry houses (a few Hz and more) and large dimension structures like castles and churches (1–2 Hz). For example, the fundamental mode of the church tower of Boussu near Mons was evaluated at a frequency of 1.41 Hz (Defaut and Deneyer, 1999). The peculiar shape of the area damaged during the 1938 earthquake can then be explained by the correspondence of the frequency ranges of the source energy, of the site effects and of the resonance of the buildings existing at that time. Below 0.5 Hz, very little energy was probably radiated by the earthquake but very few buildings were sensitive at this low frequency. The influence of the seismic source dimension and the corresponding frequency spectrum dependence can be indirectly pointed out from the macroseismic studies of the past earthquakes affecting the area. During the 1983 Liege earthquake (MS = 4.7), very few large constructions were affected while such damage was widely reported after the 1992 Roermond MS = 5.4) and 1938 earthquake (MS = 5.0). The large scale effect of the soft sedimentary layers covering the London-Brabant Massif appears to be the major factor controlling the damage pattern during the 1938 earthquake, and it has to be considered in any seismic hazard assessment in the region. To the South, the intensity values strongly decrease with distance and almost no damage has been observed to the South of the Midi Fault. This highly fractured E-W zone could have played the role of a screen for the seismic waves propagating to the South.
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Figure 10. A. Map comparing the intensity distribution during the 1938 earthquake and the isoperiod curves. MF: Midi Fault. B. Intensity versus H/V fundamental periods plot (The circles represent the average of the intensities calculated in a 9 km radius for each H/V locations and the dotted lines/squares the standart deviation).
Figure 11. Comparison between acceleration response spectra on the bedrock and at Uccle and Gent (with site effect).
Conclusions This study has shown that simple large-scale geological structures are likely to control the damage
distribution during earthquakes by modifying the frequency content of the motion. The Ms = 5.0 1938 earthquake occurring in the Brabant massif (Belgium) caused damage whose pattern is abnormally elongated in the EW direction. This orientation is perpendicular to the northward deepening of slightly tilted Mesozoic and Cainozoic soft sediments whose thickness varies from a few meters to about 900 m at the NetherlandsBelgium border. The evolution of the fundamental resonance frequency over the northern part of Belgium (an area of about 15,000 km2) was estimated using microtremor records. Forty-seven H/V measurements were performed with 5 second seismometers over the area. Most of the results show a northward regular increase of the fundamental period in agreement with the augmentation of the soil thickness. The measured resonance frequency values were consistent with theoretical computations performed at different sites on the basis of existing information and shallow seismic experiments. At one particular site (Uccle) where borehole data were available, microtremor measure-
55 ments using an array of four seismological stations with different apertures allowed to obtain the low frequency part of the Rayleigh wave dispersion curve, extending the range covered by the analysis of surface waves artificially generated. The Vs profile derived from the surface wave inversion corroborates the 1 Hz natural frequency of the site. Comparison between the intensity map of the 1938 earthquake and the resonance period of sediments shows a clear relation between the two parameters. During the 1938 earthquake, site effects play a prominent role due to the dimension of the source whose corner frequency was about 1 Hz. This study illustrates the need for taking into account such large-scale geological structures in seismic hazard studies and the interest of microtremor measurements for assessing ground amplification.
Acknowledgements This study was supported by the contracts NM/33/03 and NM/12/02 ‘Evaluation et réduction du risque sismique en Belgique’ with the ‘Services du Premier Ministre – Services fédéraux des affaires scientifiques, techniques et culturelles’ for the Belgian state. We thank B. Bettig and P.-Y. Bard for providing the software computing the dispersion curves from array measurements. We thank B. Bukasa for his constant help during the field works and Sarah Englert for his re-evaluation work of the 1938 earthquake. We thank the reviewers for their contribution to the paper throughout their comments.
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