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On the use of microtremor recordings in seismic microzonation
Soil Dynamics and Earthquake Engineering 17 (1998) 465–474 䉷 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain PII: S 0 2 6 7 - 7 2 6 1 ( 9 8 ) 0 0 0 1 4 - 1 0267-7261/98/$ - see front matter
On the use of microtremor recordings in seismic microzonation M. Bour*, D. Fouissac, P. Dominique & C. Martin BRGM, 117 avenue de Luminy, BP 167, 13276 Marseille Cedex 09, France (Received 6 March 1998; accepted 9 March 1998)
Experimental methods involving microtremor recordings are useful for determining site effects in regions of moderate seismic activity where ground motion records are few, and in urban or industrial contexts where the noise level is high. The aim of this study is to establish a microzonation by using the Nogoshi–Nakamura method,1,2 a simple experimental technique based on microtremor recordings. Since the physical phenomena underlying the method are only partially understood, the spectral responses obtained cannot be used alone. We, therefore, complete our experimental results by comparing them with the solutions of a one-dimensional numerical simulation (SHAKE91).3,4 The experimental programme was carried out on a plain near the Rhone Delta (South of France). H/V spectral ratios were calculated at 137 noise measurement points. In addition, we were able to compute the numerical transfer functions from soil columns defined by geotechnical characteristics inside the studied region. A comparison of the results obtained by the experimental and numerical methods showed that the fundamental frequencies are in good agreement, but that the amplitudes obtained by the two techniques are sometimes different. The analysis of H/V spectral ratios enabled us to establish maps to characterize the region: a resonance frequency map and maps of amplification levels as a function of frequency range, leading to a seismic microzonation for the whole of the region. 䉷 1998 Elsevier Science Ltd. All rights reserved Key words: site effects, microtremor measurements, H/V spectral ratio, microzonation, Nogoshi–Nakamura method.
component of any analysis of local seismic hazard. In practice, this requires numerical modelling of the dynamic behaviour of the soil(s), which always implies good knowledge of the geometry and of the physical and mechanical characteristics of the formations underlying the site. The reliability of the modelling, therefore, depends on the number and quality of the investigations carried out: drilling, geophysical studies, in-situ and laboratory tests, etc., enabling a representative model of the site to be built. In practice, it is often difficult to have access to this type of data for a small area. Experimental determination of lithological site effects has the advantage of avoiding the need for rigorous knowledge of the mechanical parameters of the soils and development of more-or-less reliable propagation models. Among the various instrumental approaches, the so-called Nogoshi–Nakamura1,2 method was chosen for its ease of application. The signals used are simply
1 INTRODUCTION Seismic hazard enables us to characterize potential seismic aggressions that need to be taken into account when designing new structures or upgrading existing ones. At local scale, this assessment requires: • • •
evaluation of the effects that could be induced by seismic tremors on the stability of the soils (e.g. landslides, liquefaction, settlement); allowance for the presence of seismogenic faults close to the sites, when their characteristics suggest possible coseismic deformations at the surface; definition of reference seismic motions for each site, taking account of the specific soil conditions.
Estimation of the local response of a site is a key *Author to whom correspondence should be addressed 465
466 M. Bour et al. Fig. 1. Location of microtremor recording points. Blue or green—1995 measurements; yellow—1996 measurements; red—permanently installed accelerometers (from 1995 to 1996).
Microtremor recordings in seismic microzonation
467
recordings of ambient noise, so this technique does not require either local or regional seismic activity, nor adequate reference sites. The elements of the Nogoshi– Nakamura method are presented in the first part of this article. Compared with other regions of France, the western part of Provence is characterized by a relatively high seismic hazard, notably owing to seismic activity associated with several faults that are known to have caused destructive earthquakes, such as the one in Lambesc in 1909 (I 0 ¼ VIII-IX; M ¼ 5.5). An experimental study was, therefore, made on a sector of the Rhone Delta, where a special effort is justified for several reasons: the presence of numerous critical installations, the proximity of an active fault, and the presence of soils whose geometric and dynamic properties can strongly amplify seismic motions. Our analysis led to a definition of local transfer functions at each of the sites where the geotechnical conditions justify their study, followed by identification and mapping of the zones presenting a homogeneous seismic response. This then enabled the zonation of the local seismic hazard to be refined in the studied area. Using available geotechnical data, the transfer functions obtained by numerical modelling and instrumental estimation were systematically compared.
2 METHOD BASED ON RECORDINGS OF AMBIENT NOISE Site effects due to surface geology are generally expressed as the spectral ratio (S 1) between the horizontal component of earthquake recordings at the surface of the soft layer (H S) and the ones at the ideally horizontal outcropping bedrock (H B): S1 ¼
HS HB
The instrumental method chosen here has been used for many years in Japan1 and was described by Nakamura in 1989.2 It consists in recording the ambient background noise caused by industrial activities and urban traffic, then calculating the spectral ratio of the horizontal and vertical components of the recorded signal. The term ‘microtremor’ is used to cover all ambient noise, i.e. both man-made noise, generally high frequency, generated by local surface sources such as industry and traffic, and natural low-frequency noise generated by tides, winds, teleseisms, etc. The Nogoshi–Nakamura technique is based on the assumption that: • •
microtremors are composed of several waves, but essentially Rayleigh waves propagating in a soft surface layer overlying a stiff substratum; the effect of the Rayleigh waves (E RW) on the noise motion is included in the vertical spectrum at the
Fig. 2. Stability of the H/V ratio with time. Thin lines—1995 measurements; thick lines—1996 measurements.
surface (V S), but not at the base of the layer (V B): ERW ¼ • • •
VS VB
the vertical component of microtremor motion is not amplified by the soft soil layer; the effect of the Rayleigh waves on microtremor motion is equivalent for the vertical and horizontal components; for a wide frequency range (0.2–20 Hz), the spectral ratio of the horizontal and vertical components of motion at the bottom of the layer is close to unity:
HB ¼1 VB In these conditions, the spectral ratio between the horizontal and vertical components of the background noise recorded at the surface of a soft layer enables the effects of the Rayleigh waves (E RW) to be eliminated, conserving only the effects resulting from the geological structure of the
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M. Bour et al.
Table 1. Mechanical properties of soil columns whose transfer functions are presented in Fig. 3. The mean location of the columns within the extended Lambert II kilometre system is given in brackets Material
Thickness (m)
Column 1 (809; 1829.5) Sandy hydraulic backfill Peaty formations Bedrock Column 2 (803.7; 1831.5) Gravelly sand backfill Fine sand, moderately dense Peaty silt Bedrock Column 3 (802.5; 1828.8) Sandy backfill Sea sand and/or silty sand Plastic loam Bedrock Column 4 (803.4; 1828) Hydraulic sand and muddy silt Sand Silt Bedrock Column 5 (804; 1831.4) Gravelly sand backfill Fine silty sand, very loose Fine sand, moderately dense Clayey sand, very loose Bedrock Column 6 (808.7; 1830.6) Black peat with clay Bedrock
Density (kg/m 3)
G max (MPa)
Damping (%)
2 4 —
1700 1700 2100
250 150 800
106 40 1340
8 10 2
2 3 2 —
1700 1900 1700 2100
250 250 150 800
106 120 40 1340
8 8 10 2
1.5 10.5 6 —
1700 1900 1800 2100
200 350 200 800
68 233 72 1340
8 5 8 2
3 7 6 —
1700 1900 1800 2100
150 400 200 800
40 304 72 1340
10 5 8 2
2 3 4 2 —
1700 1700 1900 1700 2100
250 150 400 150 800
106 40 304 40 1340
8 10 5 10 2
2 —
1900 2100
250 800
120 1340
8 2
site: S2 ¼
V s (m/s)
S1 H ¼ S ERW VS
In the following sections, this spectral ratio will be called the H/V spectral ratio. Many theoretical5,6 and experimental7–10 studies have shown that the spectral ratio obtained in this manner enables an adequate determination of the site fundamental frequency. However, the Nogoshi–Nakamura method does not seem to be able to provide all the information required for a reliable estimation of the amplification of surface ground motion. One should not forget that this technique is based on various assumptions, as yet not totally verified, concerning the nature of the incident background noise. Our present lack of physical understanding of the exact composition of the noise imposes a rather qualitative view of the results obtained by this approach.
well known compared with other regions of France and has been the subject of recent studies.11 The seismotectonic schema underlines in particular the proximity of the seismogenic Salon–Cavaillon Fault, whose presumed trace at the surface cuts through some of the industrial facilities at Fos-sur-Mer. All the measurement points (137 in all) in the experiment are shown on the map in Fig. 1. The equipment used comprises an autonomous Lennartz Mars88 recorder connected to a three-directional Gu¨ralp CMG-5T accelerometer. In this experiment, the recording system operated continuously for about 10 min. The recorder includes a GPS system so that the geographic position of each measurement point is precisely known (Fig. 1). For each measurement point, the spectral ratios were calculated as follows: • • •
3 EXPERIMENTAL MEASUREMENTS The experimental study was undertaken over a region of about 60 km 2 in the South of France during 1995 and 1996. Many critical installations are located in this sector of the Rhone Delta, near the town of Fos-sur-Mer. The seismotectonic context of western Provence is relatively
• • •
offset correction; selection of 8192 point windows (between 10 and 20 windows); application of a symmetric Hanning taper on 20% of the signal; computation of Fourier spectra in the three directions (E–W, N–S, UP); smoothing of the Fourier spectra by a window whose width is 10% of the frequency; calculation of the H/V spectral ratios, defined by: p (EW2 þ NS2 )=(2:UP2 );
Microtremor recordings in seismic microzonation
469
Fig. 3. H/V spectral ratios versus SHAKE transfer functions. Thin lines—H/V spectral ratios; thick lines—SHAKE transfer functions.
•
calculation of the mean spectral ratio and its standard deviation.
Measurements recorded at yearly intervals at three overlapping points (nos. 39, 40 and 41 in Fig. 1) revealed the stability of the H/V spectral ratio (Fig. 2). The form of the obtained spectral ratios is very close, the fundamental
frequency is identical; only the value of the amplification is slightly modified at one of the three sites. Other studies12,13 have yielded similar results on the stability over time of the general form of the H/V spectral ratios. Consequently, we were able to complement the initial data recorded in 1995 with new measurements recorded 1 year later.
Fig. 4. Map of fundamental frequencies.
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M. Bour et al.
Fig. 5. Map of amplifications at fundamental frequencies.
4 EXPERIMENTAL SPECTRAL RATIOS VERSUS NUMERICAL TRANSFER FUNCTIONS To validate the experimental technique, H/V spectral ratios were compared with the transfer functions obtained from a one-dimensional numerical simulation (SHAKE91),3,4 which consists of the response analysis of horizontally layered soils under seismic solicitation, with a linear equivalent soil behaviour. Use of geotechnical data for each of the industrial sites and a synthesis of drilling data extracted from the BRGM’s Subsurface Database enabled us to determine soil columns representative of each industrial site and to define the Crau Miocene gravels as the seismic substratum of the Fos-surMer industrial zone.14 The soil columns therefore stop at this formation whose depth is well identified at all the sites. The formations overlying the substratum, intersected by drilling at all the sites, are relatively homogeneous; they are mainly peat, sand, loam, and occasionally clay. Their characteristics are generally poor; they are presented in Table 1 for six industrial sites. On the other hand, since the quantity and nature of the geotechnical data available for each of the sites is highly heterogeneous, there are uncertainties as to depths and geomechanical and seismic characteristics of these formations. The shear-wave velocities have, for example, been extrapolated from geotechnical tests, since no cross-hole test was available for the zone. Using the soil configurations in Table 1, a transfer function was calculated for each industrial site using the SHAKE913,4 numerical code. Several rock accelerograms were used: both artificially generated accelerograms and real accelerograms taken from Italian and Californian earthquakes, whose characteristics are believed to be similar to those associated with seismic sources in the South of France. The transfer function obtained for each site is the average of the transfer functions calculated for 14 rock accelerograms. In addition, recordings of background noise for each site were used to calculate H/V spectral ratios. The mean spectral ratio obtained at an industrial site is the average of
several spectral ratios (between 5 and 18) measured within the boundaries of the facility. Fig. 3 compares the transfer functions specific to each site and obtained by the two methods. This figure reveals good overall concordance between the transfer functions obtained by the two methodologies. Although the amplification obtained by the H/V ratio is generally higher than that obtained by the numerical modelling, the fundamental frequency of each site is very similar with the two methods. Column 6 is assimilated with rock by both methods: fundamental frequency greater than 20 Hz for the numerical calculation, and amplitude less than 3 for the experimental approach. Only the spectral ratios evaluated for column 5 show a deviation of about 1 Hz from the fundamental frequencies. Geographically, since the site of column 5 is alongside that of column 2, this does not justify a difference of 4 m in the total thickness of the sediments, as deduced from the geotechnical data. It is obvious that in reducing the thickness of the surficial formation for column 5, the transfer function calculated by SHAKE is significantly closer to the H/V spectral ratio. For surficial formations less than 10 m thick, the amplitudes obtained by the two methods are equivalent, whereas for thicknesses exceeding 10 m, the amplifications obtained by the Nogoshi–Nakamura method are about 50% greater than those of the numerical method. We also note the presence of a harmonic resonance frequency on the transfer functions calculated by SHAKE, which is absent in the H/V spectral ratios. On the basis of these results, it would appear that it is possible to use the H/V spectral ratio to determine the fundamental frequency and establish the seismic microzonation in terms of a predominant frequency map of the studied region.
5 SEISMIC MICROZONATION Each measurement point provides a spectral ratio and enables an estimation of the fundamental frequency and
Microtremor recordings in seismic microzonation
471
Fig. 6. Map of amplifications at: (a) 2–4 Hz; (b) 4–6 Hz; (c) 6–8 Hz.
the maximum value of the amplification at the site studied. By spatial interpolation between these points, we can deduce a map of resonance frequencies over the Fos-surMer zone (Fig. 4), and a map of the maximum amplifications observed at these fundamental frequencies (Fig. 5). It is important to mention the qualitative character of the maximum amplification values. The Nogoshi–Nakamura method does not presently enable the level reached by the peak of the H/V spectral ratio to be related to the amplification of a signal at the surface relative to that in the bedrock
during a strong tremor. Only the relative amplifications between two measurement points are assumed to be significant. On the map in Fig. 4, the fundamental frequencies decrease from north-east to south-west, which is interpreted as a thickening of the alluvial deposits in this direction. Similarly, we observe on the map in Fig. 5 that the zone in the north-east amplifies the ground motion less than that in the south-west by a factor ranging from 2 to 4. This confirms the thickening of the sediments covering the
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Fig. 7. Map of surficial deposit thicknesses.
Fig. 8. Grouping of H/V spectral ratios.
Microtremor recordings in seismic microzonation seismic substratum which appears to plunge from north-east to south-west. Unlike in Fig. 5 where the amplitudes could correspond to very different fundamental frequencies, Fig. 6 presents three maps of mean amplifications calculated in the frequency ranges 2–4 Hz, 4–6 Hz and 6–8 Hz. It can be seen that at 2–4 Hz, only the south-west sector shows significant amplification of the ground motion; at 4–6 Hz one notes a central band of amplification and above 6 Hz, apart from a few isolated points, the spectral amplification is insignificant. The trends of the spatial variation of the seismic response of the soils at Fos-sur-Mer are therefore confirmed: the zone of maximum amplification of the ground motion is displaced from south-west to north-east when the considered frequencies increases. This reflects a thinning of the sedimentary filling in this same direction. Apart from the thinning or disappearance of the surficial formations in the north-eastern sector of the zone studied, we note the presence of a highly localized rock outcrop in the south-west (co-ordinates 804.5, 1828.5 expressed in kilometres—extended Lambert II). Use of geotechnical investigation data for each of the industrial sites and the synthesis of drilling data extracted from the BRGM’s Subsurface Database enabled us to determine the depth of the top of the Crau gravels in the form of an isopach map of the surficial formations (Fig. 7). This map underlines well the deepening of the top of the Crau gravels from the north-east, where it crops out, to the south-west where it reaches a depth of about 20 m. It also shows a rock outcrop in the south-west. As shown in Fig. 4, the fundamental frequency map agrees well with our geotechnical model: the smaller fundamental frequency values are in the south-west part of the studied region which corresponds to the deeper position of the bedrock. In a second step, we classified each of the H/V spectral ratios into different families with the same shape, i.e. having the same amplitude and fundamental frequency: Fig. 8 presents the grouping of these spectral ratios. This classification defines four main groups, represented by four numbered zones in Fig. 9, which constitutes the
473
microzonation of the studied region. Zones 1 and 2 have respective resonance frequencies of 2.5 and 3 Hz. Zone 3 corresponds to spectral ratios having an amplification in a band of frequencies between 3 and about 6 Hz. Zone 4 corresponds to the seismic substratum (Crau alluvium with good geomechanical properties): the maximum amplification remains weak and less than 3, except at very high frequencies. Four secondary groups were also determined; these are constituted by only a few measurement points with very specific spectral ratios, which could not be assigned to one of the four main groups. As might be anticipated from the previous results, the different zones range from southwest to north-east, apart from zone 2 which occurs on both sides of zone 3. The last step (not presented here) was to complete the microzonation by the horizontal elastic response spectra determined in the areas identified as being seismically homogeneous. These response spectra were first calculated using the one-dimensional numerical technique and then modified from experimental H/V spectral ratios.
6 CONCLUSION As France is a country of moderate seismicity, we were drawn to experimental methods using microtremor recordings to establish a seismic microzonation. First, we showed that by comparison with the one-dimensional numerical simulation (SHAKE91), the H/V spectral ratio approach provides a simple means of determining the predominant frequency of a soil site. In a second stage, the H/V spectral ratio technique enabled us to establish a map of predominant frequency: this turned out to be a useful tool for establishing a seismic microzonation of the whole studied region. Beside this, for each seismically homogenous area defined by the microzonation, elastic response spectra were determined from one-dimensional numerical calculations; these response spectra are then taken into consideration by each critical facility in the industrial zone at Fos-sur-Mer. This study has been shown that the method of H/V
Fig. 9. Microzonation.
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spectral ratios, based on the recording of background noise, can provide precious, reliable data on the seismic behaviour of thin, gently dipping surficial layers. The use of recordings of background noise appears to provide a very useful complement to the numerical approaches traditionally used in seismic microzonation studies for determining the fundamental frequency of the surficial formations in the linear domain, when they are weakly constrained by lack of geotechnical data. It is clear that the numerical and experimental approaches should be combined so as to better constrain the microzonation of a given region, in particular those of weak seismic activity and/or high levels of ambient noise. Reservations that can be formulated concerning the Nogoshi–Nakamura method concern two points. Firstly, the current impossibility of using the values of maximum measured amplifications; the reasons are both physical, because the ellipticity of Rayleigh waves is theoretically infinite at the resonance frequency and numerical because the type of signal processing can significantly influence the amplitudes of the calculated spectral ratios. Secondly, we are not able to confirm that the results obtained with the H/V method for low deformation levels are transposable to strong motions. Non-linear effects generally lead to a diminution of ground acceleration amplification factors and a slight shift of the dominant frequency towards the lower frequencies. At low acceleration levels, as in the present study (pga ⬍ 0.3g), the differences between the fundamental frequencies determined with a linear or a non-linear assumption are probably not significant. This problem could possibly be resolved, in part, by combining the background noise method with numerical models that are truly capable of taking into account the non-linear behaviour of soils.15
4. 5. 6. 7. 8.
9.
10.
11.
12.
13.
REFERENCES 1. Nogoshi, M. On fundamental nature of microtremors and its application. J. Min. Coll. Akita Univ., Jpn, Ser. A, 1978, 5(3), 1–51. 2. Nakamura, Y. A method for dynamic characteristics estimation of subsurface using microtremor on the ground surface. Rep. Railway Tech. Res. Inst., Jpn, 1989, 30(1), 25–33. 3. Schnabel, P. B., Lysmer, J. and Seed, H. B., SHAKE: a computer program for earthquake response analysis of
14.
15.
horizontally layered sites, Report no. EERC 72-12, Earthquake Engineering Research Center, Univ. California, Berkeley, 1972. Idriss, I. M. and Sun, J. I., User’s Manual for SHAKE91, Department of Civil and Environmental Engineering, Univ. California, Davis, 1992. Lachet, C. and Bard, P. Y. Numerical and theoretical investigations on the possibilities and limitations of Nakamura’s technique. J. Phys. Earth, 1994, 42, 377–397. Dravinski, M., Ding, G. and Wen, K. L. Analysis of spectral ratios for estimating ground motion in deep basins. Bull. Seism. Soc. Am., 1996, 86, 646–654. Lermo, J. and Chavez-Garcia, F. J. Are microtremors useful in site response evaluation?. Bull. Seism. Soc. Am., 1994, 84, 1350–1364. Teves-Costa, P., Matias, L. and Bard, P. Y. Seismic behaviour estimation of thin alluvium layers using microtremor recordings. Soil Dynam. Earthq. Engng, 1996, 15, 201–209. Malagnini, L., Tricarico, P., Rovelli, A., Herrmann, R. B., Opice, S., Biella, G. and de Franco, R. Explosion, earthquake and ambient noise recordings in a Pliocene sediment-filled valley: inferences on seismic response properties by reference- and non-reference-site techniques. Bull. Seism. Soc. Am., 1996, 86, 670–682. Seekins, L.C., Wennerberg, L., Margheriti, L. and Liu, H. P. Site amplification at five locations in San Fransisco, California: a comparison of S waves, Codas and microtremors. Bull. Seism. Soc. Am., 1996, 86, 627–635. Combes, P., Godefroy, P., Goula, X., Levret, A., Sauret, B. and Terrier, M., Contribution a` l’e´tude des dangers d’installations industrielles a` ‘risque spe´cial’: prise en compte de l’ale´a sismique en Provence occidentale—de´finition des se´ismes de re´fe´rence selon une approche de´terministe de l’ale´a sismique re´gional, Rapport BRGM, R 39092, 1990. Duval, A. M., Bard, P. Y., Me´neroud, J. P. and Vidal, S. Mapping site effect with microtremors. In Proc. 5th International Conference on Seismic Zonation, Vol. 3, Nice, France. Ouest Editions Presses Acade´miques, Nantes, pp. 1522–1529. Suzuki, T., Adachi, Y. and Tanaka, M. Application of microtremor measurements to the estimation of earthquake ground motions in Kushiro city during the Kushiro-Oki earthquake of 15 January 1993. Earthq. Engng Struct. Dynam., 1995, 24, 595–613. Dominique, P., Fouissac, D., Bour, M. and Martin, C., Etude pilote de surveillance sismique d’une zone a` risque des Bouches-du-Rhoˆne. Microzonage sismique des sites industriels de Fos-sur-Mer, phase 1996, Rapport BRGM, R 31383 ENV 4S 90, 1996. Modaressi, H., Evaluation of seismic response spectra using a unified numerical approach. In 11th European Conference on Earthquake Engineering, Paris, September 6–11, 1998.
On the use of microtremor recordings in seismic microzonation M. Bour*, D. Fouissac, P. Dominique & C. Martin BRGM, 117 avenue de Luminy, BP 167, 13276 Marseille Cedex 09, France (Received 6 March 1998; accepted 9 March 1998)
Experimental methods involving microtremor recordings are useful for determining site effects in regions of moderate seismic activity where ground motion records are few, and in urban or industrial contexts where the noise level is high. The aim of this study is to establish a microzonation by using the Nogoshi–Nakamura method,1,2 a simple experimental technique based on microtremor recordings. Since the physical phenomena underlying the method are only partially understood, the spectral responses obtained cannot be used alone. We, therefore, complete our experimental results by comparing them with the solutions of a one-dimensional numerical simulation (SHAKE91).3,4 The experimental programme was carried out on a plain near the Rhone Delta (South of France). H/V spectral ratios were calculated at 137 noise measurement points. In addition, we were able to compute the numerical transfer functions from soil columns defined by geotechnical characteristics inside the studied region. A comparison of the results obtained by the experimental and numerical methods showed that the fundamental frequencies are in good agreement, but that the amplitudes obtained by the two techniques are sometimes different. The analysis of H/V spectral ratios enabled us to establish maps to characterize the region: a resonance frequency map and maps of amplification levels as a function of frequency range, leading to a seismic microzonation for the whole of the region. 䉷 1998 Elsevier Science Ltd. All rights reserved Key words: site effects, microtremor measurements, H/V spectral ratio, microzonation, Nogoshi–Nakamura method.
component of any analysis of local seismic hazard. In practice, this requires numerical modelling of the dynamic behaviour of the soil(s), which always implies good knowledge of the geometry and of the physical and mechanical characteristics of the formations underlying the site. The reliability of the modelling, therefore, depends on the number and quality of the investigations carried out: drilling, geophysical studies, in-situ and laboratory tests, etc., enabling a representative model of the site to be built. In practice, it is often difficult to have access to this type of data for a small area. Experimental determination of lithological site effects has the advantage of avoiding the need for rigorous knowledge of the mechanical parameters of the soils and development of more-or-less reliable propagation models. Among the various instrumental approaches, the so-called Nogoshi–Nakamura1,2 method was chosen for its ease of application. The signals used are simply
1 INTRODUCTION Seismic hazard enables us to characterize potential seismic aggressions that need to be taken into account when designing new structures or upgrading existing ones. At local scale, this assessment requires: • • •
evaluation of the effects that could be induced by seismic tremors on the stability of the soils (e.g. landslides, liquefaction, settlement); allowance for the presence of seismogenic faults close to the sites, when their characteristics suggest possible coseismic deformations at the surface; definition of reference seismic motions for each site, taking account of the specific soil conditions.
Estimation of the local response of a site is a key *Author to whom correspondence should be addressed 465
466 M. Bour et al. Fig. 1. Location of microtremor recording points. Blue or green—1995 measurements; yellow—1996 measurements; red—permanently installed accelerometers (from 1995 to 1996).
Microtremor recordings in seismic microzonation
467
recordings of ambient noise, so this technique does not require either local or regional seismic activity, nor adequate reference sites. The elements of the Nogoshi– Nakamura method are presented in the first part of this article. Compared with other regions of France, the western part of Provence is characterized by a relatively high seismic hazard, notably owing to seismic activity associated with several faults that are known to have caused destructive earthquakes, such as the one in Lambesc in 1909 (I 0 ¼ VIII-IX; M ¼ 5.5). An experimental study was, therefore, made on a sector of the Rhone Delta, where a special effort is justified for several reasons: the presence of numerous critical installations, the proximity of an active fault, and the presence of soils whose geometric and dynamic properties can strongly amplify seismic motions. Our analysis led to a definition of local transfer functions at each of the sites where the geotechnical conditions justify their study, followed by identification and mapping of the zones presenting a homogeneous seismic response. This then enabled the zonation of the local seismic hazard to be refined in the studied area. Using available geotechnical data, the transfer functions obtained by numerical modelling and instrumental estimation were systematically compared.
2 METHOD BASED ON RECORDINGS OF AMBIENT NOISE Site effects due to surface geology are generally expressed as the spectral ratio (S 1) between the horizontal component of earthquake recordings at the surface of the soft layer (H S) and the ones at the ideally horizontal outcropping bedrock (H B): S1 ¼
HS HB
The instrumental method chosen here has been used for many years in Japan1 and was described by Nakamura in 1989.2 It consists in recording the ambient background noise caused by industrial activities and urban traffic, then calculating the spectral ratio of the horizontal and vertical components of the recorded signal. The term ‘microtremor’ is used to cover all ambient noise, i.e. both man-made noise, generally high frequency, generated by local surface sources such as industry and traffic, and natural low-frequency noise generated by tides, winds, teleseisms, etc. The Nogoshi–Nakamura technique is based on the assumption that: • •
microtremors are composed of several waves, but essentially Rayleigh waves propagating in a soft surface layer overlying a stiff substratum; the effect of the Rayleigh waves (E RW) on the noise motion is included in the vertical spectrum at the
Fig. 2. Stability of the H/V ratio with time. Thin lines—1995 measurements; thick lines—1996 measurements.
surface (V S), but not at the base of the layer (V B): ERW ¼ • • •
VS VB
the vertical component of microtremor motion is not amplified by the soft soil layer; the effect of the Rayleigh waves on microtremor motion is equivalent for the vertical and horizontal components; for a wide frequency range (0.2–20 Hz), the spectral ratio of the horizontal and vertical components of motion at the bottom of the layer is close to unity:
HB ¼1 VB In these conditions, the spectral ratio between the horizontal and vertical components of the background noise recorded at the surface of a soft layer enables the effects of the Rayleigh waves (E RW) to be eliminated, conserving only the effects resulting from the geological structure of the
468
M. Bour et al.
Table 1. Mechanical properties of soil columns whose transfer functions are presented in Fig. 3. The mean location of the columns within the extended Lambert II kilometre system is given in brackets Material
Thickness (m)
Column 1 (809; 1829.5) Sandy hydraulic backfill Peaty formations Bedrock Column 2 (803.7; 1831.5) Gravelly sand backfill Fine sand, moderately dense Peaty silt Bedrock Column 3 (802.5; 1828.8) Sandy backfill Sea sand and/or silty sand Plastic loam Bedrock Column 4 (803.4; 1828) Hydraulic sand and muddy silt Sand Silt Bedrock Column 5 (804; 1831.4) Gravelly sand backfill Fine silty sand, very loose Fine sand, moderately dense Clayey sand, very loose Bedrock Column 6 (808.7; 1830.6) Black peat with clay Bedrock
Density (kg/m 3)
G max (MPa)
Damping (%)
2 4 —
1700 1700 2100
250 150 800
106 40 1340
8 10 2
2 3 2 —
1700 1900 1700 2100
250 250 150 800
106 120 40 1340
8 8 10 2
1.5 10.5 6 —
1700 1900 1800 2100
200 350 200 800
68 233 72 1340
8 5 8 2
3 7 6 —
1700 1900 1800 2100
150 400 200 800
40 304 72 1340
10 5 8 2
2 3 4 2 —
1700 1700 1900 1700 2100
250 150 400 150 800
106 40 304 40 1340
8 10 5 10 2
2 —
1900 2100
250 800
120 1340
8 2
site: S2 ¼
V s (m/s)
S1 H ¼ S ERW VS
In the following sections, this spectral ratio will be called the H/V spectral ratio. Many theoretical5,6 and experimental7–10 studies have shown that the spectral ratio obtained in this manner enables an adequate determination of the site fundamental frequency. However, the Nogoshi–Nakamura method does not seem to be able to provide all the information required for a reliable estimation of the amplification of surface ground motion. One should not forget that this technique is based on various assumptions, as yet not totally verified, concerning the nature of the incident background noise. Our present lack of physical understanding of the exact composition of the noise imposes a rather qualitative view of the results obtained by this approach.
well known compared with other regions of France and has been the subject of recent studies.11 The seismotectonic schema underlines in particular the proximity of the seismogenic Salon–Cavaillon Fault, whose presumed trace at the surface cuts through some of the industrial facilities at Fos-sur-Mer. All the measurement points (137 in all) in the experiment are shown on the map in Fig. 1. The equipment used comprises an autonomous Lennartz Mars88 recorder connected to a three-directional Gu¨ralp CMG-5T accelerometer. In this experiment, the recording system operated continuously for about 10 min. The recorder includes a GPS system so that the geographic position of each measurement point is precisely known (Fig. 1). For each measurement point, the spectral ratios were calculated as follows: • • •
3 EXPERIMENTAL MEASUREMENTS The experimental study was undertaken over a region of about 60 km 2 in the South of France during 1995 and 1996. Many critical installations are located in this sector of the Rhone Delta, near the town of Fos-sur-Mer. The seismotectonic context of western Provence is relatively
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offset correction; selection of 8192 point windows (between 10 and 20 windows); application of a symmetric Hanning taper on 20% of the signal; computation of Fourier spectra in the three directions (E–W, N–S, UP); smoothing of the Fourier spectra by a window whose width is 10% of the frequency; calculation of the H/V spectral ratios, defined by: p (EW2 þ NS2 )=(2:UP2 );
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Fig. 3. H/V spectral ratios versus SHAKE transfer functions. Thin lines—H/V spectral ratios; thick lines—SHAKE transfer functions.
•
calculation of the mean spectral ratio and its standard deviation.
Measurements recorded at yearly intervals at three overlapping points (nos. 39, 40 and 41 in Fig. 1) revealed the stability of the H/V spectral ratio (Fig. 2). The form of the obtained spectral ratios is very close, the fundamental
frequency is identical; only the value of the amplification is slightly modified at one of the three sites. Other studies12,13 have yielded similar results on the stability over time of the general form of the H/V spectral ratios. Consequently, we were able to complement the initial data recorded in 1995 with new measurements recorded 1 year later.
Fig. 4. Map of fundamental frequencies.
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Fig. 5. Map of amplifications at fundamental frequencies.
4 EXPERIMENTAL SPECTRAL RATIOS VERSUS NUMERICAL TRANSFER FUNCTIONS To validate the experimental technique, H/V spectral ratios were compared with the transfer functions obtained from a one-dimensional numerical simulation (SHAKE91),3,4 which consists of the response analysis of horizontally layered soils under seismic solicitation, with a linear equivalent soil behaviour. Use of geotechnical data for each of the industrial sites and a synthesis of drilling data extracted from the BRGM’s Subsurface Database enabled us to determine soil columns representative of each industrial site and to define the Crau Miocene gravels as the seismic substratum of the Fos-surMer industrial zone.14 The soil columns therefore stop at this formation whose depth is well identified at all the sites. The formations overlying the substratum, intersected by drilling at all the sites, are relatively homogeneous; they are mainly peat, sand, loam, and occasionally clay. Their characteristics are generally poor; they are presented in Table 1 for six industrial sites. On the other hand, since the quantity and nature of the geotechnical data available for each of the sites is highly heterogeneous, there are uncertainties as to depths and geomechanical and seismic characteristics of these formations. The shear-wave velocities have, for example, been extrapolated from geotechnical tests, since no cross-hole test was available for the zone. Using the soil configurations in Table 1, a transfer function was calculated for each industrial site using the SHAKE913,4 numerical code. Several rock accelerograms were used: both artificially generated accelerograms and real accelerograms taken from Italian and Californian earthquakes, whose characteristics are believed to be similar to those associated with seismic sources in the South of France. The transfer function obtained for each site is the average of the transfer functions calculated for 14 rock accelerograms. In addition, recordings of background noise for each site were used to calculate H/V spectral ratios. The mean spectral ratio obtained at an industrial site is the average of
several spectral ratios (between 5 and 18) measured within the boundaries of the facility. Fig. 3 compares the transfer functions specific to each site and obtained by the two methods. This figure reveals good overall concordance between the transfer functions obtained by the two methodologies. Although the amplification obtained by the H/V ratio is generally higher than that obtained by the numerical modelling, the fundamental frequency of each site is very similar with the two methods. Column 6 is assimilated with rock by both methods: fundamental frequency greater than 20 Hz for the numerical calculation, and amplitude less than 3 for the experimental approach. Only the spectral ratios evaluated for column 5 show a deviation of about 1 Hz from the fundamental frequencies. Geographically, since the site of column 5 is alongside that of column 2, this does not justify a difference of 4 m in the total thickness of the sediments, as deduced from the geotechnical data. It is obvious that in reducing the thickness of the surficial formation for column 5, the transfer function calculated by SHAKE is significantly closer to the H/V spectral ratio. For surficial formations less than 10 m thick, the amplitudes obtained by the two methods are equivalent, whereas for thicknesses exceeding 10 m, the amplifications obtained by the Nogoshi–Nakamura method are about 50% greater than those of the numerical method. We also note the presence of a harmonic resonance frequency on the transfer functions calculated by SHAKE, which is absent in the H/V spectral ratios. On the basis of these results, it would appear that it is possible to use the H/V spectral ratio to determine the fundamental frequency and establish the seismic microzonation in terms of a predominant frequency map of the studied region.
5 SEISMIC MICROZONATION Each measurement point provides a spectral ratio and enables an estimation of the fundamental frequency and
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Fig. 6. Map of amplifications at: (a) 2–4 Hz; (b) 4–6 Hz; (c) 6–8 Hz.
the maximum value of the amplification at the site studied. By spatial interpolation between these points, we can deduce a map of resonance frequencies over the Fos-surMer zone (Fig. 4), and a map of the maximum amplifications observed at these fundamental frequencies (Fig. 5). It is important to mention the qualitative character of the maximum amplification values. The Nogoshi–Nakamura method does not presently enable the level reached by the peak of the H/V spectral ratio to be related to the amplification of a signal at the surface relative to that in the bedrock
during a strong tremor. Only the relative amplifications between two measurement points are assumed to be significant. On the map in Fig. 4, the fundamental frequencies decrease from north-east to south-west, which is interpreted as a thickening of the alluvial deposits in this direction. Similarly, we observe on the map in Fig. 5 that the zone in the north-east amplifies the ground motion less than that in the south-west by a factor ranging from 2 to 4. This confirms the thickening of the sediments covering the
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Fig. 7. Map of surficial deposit thicknesses.
Fig. 8. Grouping of H/V spectral ratios.
Microtremor recordings in seismic microzonation seismic substratum which appears to plunge from north-east to south-west. Unlike in Fig. 5 where the amplitudes could correspond to very different fundamental frequencies, Fig. 6 presents three maps of mean amplifications calculated in the frequency ranges 2–4 Hz, 4–6 Hz and 6–8 Hz. It can be seen that at 2–4 Hz, only the south-west sector shows significant amplification of the ground motion; at 4–6 Hz one notes a central band of amplification and above 6 Hz, apart from a few isolated points, the spectral amplification is insignificant. The trends of the spatial variation of the seismic response of the soils at Fos-sur-Mer are therefore confirmed: the zone of maximum amplification of the ground motion is displaced from south-west to north-east when the considered frequencies increases. This reflects a thinning of the sedimentary filling in this same direction. Apart from the thinning or disappearance of the surficial formations in the north-eastern sector of the zone studied, we note the presence of a highly localized rock outcrop in the south-west (co-ordinates 804.5, 1828.5 expressed in kilometres—extended Lambert II). Use of geotechnical investigation data for each of the industrial sites and the synthesis of drilling data extracted from the BRGM’s Subsurface Database enabled us to determine the depth of the top of the Crau gravels in the form of an isopach map of the surficial formations (Fig. 7). This map underlines well the deepening of the top of the Crau gravels from the north-east, where it crops out, to the south-west where it reaches a depth of about 20 m. It also shows a rock outcrop in the south-west. As shown in Fig. 4, the fundamental frequency map agrees well with our geotechnical model: the smaller fundamental frequency values are in the south-west part of the studied region which corresponds to the deeper position of the bedrock. In a second step, we classified each of the H/V spectral ratios into different families with the same shape, i.e. having the same amplitude and fundamental frequency: Fig. 8 presents the grouping of these spectral ratios. This classification defines four main groups, represented by four numbered zones in Fig. 9, which constitutes the
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microzonation of the studied region. Zones 1 and 2 have respective resonance frequencies of 2.5 and 3 Hz. Zone 3 corresponds to spectral ratios having an amplification in a band of frequencies between 3 and about 6 Hz. Zone 4 corresponds to the seismic substratum (Crau alluvium with good geomechanical properties): the maximum amplification remains weak and less than 3, except at very high frequencies. Four secondary groups were also determined; these are constituted by only a few measurement points with very specific spectral ratios, which could not be assigned to one of the four main groups. As might be anticipated from the previous results, the different zones range from southwest to north-east, apart from zone 2 which occurs on both sides of zone 3. The last step (not presented here) was to complete the microzonation by the horizontal elastic response spectra determined in the areas identified as being seismically homogeneous. These response spectra were first calculated using the one-dimensional numerical technique and then modified from experimental H/V spectral ratios.
6 CONCLUSION As France is a country of moderate seismicity, we were drawn to experimental methods using microtremor recordings to establish a seismic microzonation. First, we showed that by comparison with the one-dimensional numerical simulation (SHAKE91), the H/V spectral ratio approach provides a simple means of determining the predominant frequency of a soil site. In a second stage, the H/V spectral ratio technique enabled us to establish a map of predominant frequency: this turned out to be a useful tool for establishing a seismic microzonation of the whole studied region. Beside this, for each seismically homogenous area defined by the microzonation, elastic response spectra were determined from one-dimensional numerical calculations; these response spectra are then taken into consideration by each critical facility in the industrial zone at Fos-sur-Mer. This study has been shown that the method of H/V
Fig. 9. Microzonation.
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spectral ratios, based on the recording of background noise, can provide precious, reliable data on the seismic behaviour of thin, gently dipping surficial layers. The use of recordings of background noise appears to provide a very useful complement to the numerical approaches traditionally used in seismic microzonation studies for determining the fundamental frequency of the surficial formations in the linear domain, when they are weakly constrained by lack of geotechnical data. It is clear that the numerical and experimental approaches should be combined so as to better constrain the microzonation of a given region, in particular those of weak seismic activity and/or high levels of ambient noise. Reservations that can be formulated concerning the Nogoshi–Nakamura method concern two points. Firstly, the current impossibility of using the values of maximum measured amplifications; the reasons are both physical, because the ellipticity of Rayleigh waves is theoretically infinite at the resonance frequency and numerical because the type of signal processing can significantly influence the amplitudes of the calculated spectral ratios. Secondly, we are not able to confirm that the results obtained with the H/V method for low deformation levels are transposable to strong motions. Non-linear effects generally lead to a diminution of ground acceleration amplification factors and a slight shift of the dominant frequency towards the lower frequencies. At low acceleration levels, as in the present study (pga ⬍ 0.3g), the differences between the fundamental frequencies determined with a linear or a non-linear assumption are probably not significant. This problem could possibly be resolved, in part, by combining the background noise method with numerical models that are truly capable of taking into account the non-linear behaviour of soils.15
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REFERENCES 1. Nogoshi, M. On fundamental nature of microtremors and its application. J. Min. Coll. Akita Univ., Jpn, Ser. A, 1978, 5(3), 1–51. 2. Nakamura, Y. A method for dynamic characteristics estimation of subsurface using microtremor on the ground surface. Rep. Railway Tech. Res. Inst., Jpn, 1989, 30(1), 25–33. 3. Schnabel, P. B., Lysmer, J. and Seed, H. B., SHAKE: a computer program for earthquake response analysis of
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horizontally layered sites, Report no. EERC 72-12, Earthquake Engineering Research Center, Univ. California, Berkeley, 1972. Idriss, I. M. and Sun, J. I., User’s Manual for SHAKE91, Department of Civil and Environmental Engineering, Univ. California, Davis, 1992. Lachet, C. and Bard, P. Y. Numerical and theoretical investigations on the possibilities and limitations of Nakamura’s technique. J. Phys. Earth, 1994, 42, 377–397. Dravinski, M., Ding, G. and Wen, K. L. Analysis of spectral ratios for estimating ground motion in deep basins. Bull. Seism. Soc. Am., 1996, 86, 646–654. Lermo, J. and Chavez-Garcia, F. J. Are microtremors useful in site response evaluation?. Bull. Seism. Soc. Am., 1994, 84, 1350–1364. Teves-Costa, P., Matias, L. and Bard, P. Y. Seismic behaviour estimation of thin alluvium layers using microtremor recordings. Soil Dynam. Earthq. Engng, 1996, 15, 201–209. Malagnini, L., Tricarico, P., Rovelli, A., Herrmann, R. B., Opice, S., Biella, G. and de Franco, R. Explosion, earthquake and ambient noise recordings in a Pliocene sediment-filled valley: inferences on seismic response properties by reference- and non-reference-site techniques. Bull. Seism. Soc. Am., 1996, 86, 670–682. Seekins, L.C., Wennerberg, L., Margheriti, L. and Liu, H. P. Site amplification at five locations in San Fransisco, California: a comparison of S waves, Codas and microtremors. Bull. Seism. Soc. Am., 1996, 86, 627–635. Combes, P., Godefroy, P., Goula, X., Levret, A., Sauret, B. and Terrier, M., Contribution a` l’e´tude des dangers d’installations industrielles a` ‘risque spe´cial’: prise en compte de l’ale´a sismique en Provence occidentale—de´finition des se´ismes de re´fe´rence selon une approche de´terministe de l’ale´a sismique re´gional, Rapport BRGM, R 39092, 1990. Duval, A. M., Bard, P. Y., Me´neroud, J. P. and Vidal, S. Mapping site effect with microtremors. In Proc. 5th International Conference on Seismic Zonation, Vol. 3, Nice, France. Ouest Editions Presses Acade´miques, Nantes, pp. 1522–1529. Suzuki, T., Adachi, Y. and Tanaka, M. Application of microtremor measurements to the estimation of earthquake ground motions in Kushiro city during the Kushiro-Oki earthquake of 15 January 1993. Earthq. Engng Struct. Dynam., 1995, 24, 595–613. Dominique, P., Fouissac, D., Bour, M. and Martin, C., Etude pilote de surveillance sismique d’une zone a` risque des Bouches-du-Rhoˆne. Microzonage sismique des sites industriels de Fos-sur-Mer, phase 1996, Rapport BRGM, R 31383 ENV 4S 90, 1996. Modaressi, H., Evaluation of seismic response spectra using a unified numerical approach. In 11th European Conference on Earthquake Engineering, Paris, September 6–11, 1998.