Intensity of Strong Ground Motion on Pre-Quaternary Stratum and Surface Soil Amplifications during the 1995 Hyogo-ken Nanbu Earthquake, Japan

Transcription

1 J. Phys. Earth, 44, , 1996 Intensity of Strong Ground Motion on Pre-Quaternary Stratum and Surface Soil Amplifications during the 1995 Hyogo-ken Nanbu Earthquake, Japan Susumu Ohno,1.* Masayuki Takemura2, Masanori Niwa,1 and Katsuya Takahashi 1 Kajima Technical Research Institute, Kajima Corporation, 1 Chofu, Tokyo 182, Japan 2 Kobori Research Complex, Kajima Corporation, Minato-ku, Tokyo 107, Japan We obtained an attenuation relation of response spectra to magnitude, source-to-site distance and soil classification by performing regression analysis of strong-motion data recorded in California. We applied the attenuation relation to the 1995 Hyogo-ken Nanbu earthquake and found that the estimates at pre-quaternary sites agree well with the observed values. Peak ground accelerations on pre-quaternary stratum in Kobe are consistent with the source models, and they are estimated to be about 350 cm/s2, while the estimates for Awaji Island vary with the source models from 600 to 1,200 cm/s2. We calculated the amplification factors of Quaternary stratum during the Hyogo-ken Nanbu earthquake by taking ratios between the observed records at Quaternary sites and estimates on pre-quaternary stratum. As the amplifications in the heavily damaged area are large near the natural period of wooden houses ( s), these short-period amplifications might be one of the reasons Kobe suffered extensive damage. The estimated amplification factors generally agree with those estimated from other earthquake records as well as with one-dimensional wave propagation theory, while the amplifications at alluvial sites were larger at periods of more than 1 s and might be affected by surface waves. 1. Introduction Many researchers have modeled strong-motion spectrum by attenuation formula as a function of magnitude (M), source-to-site distance (X), soil classification, etc., and have estimated the coefficients by performing regression analysis on observed data. (Campbell, 1985; Joyner and Boore, 1988, and references therein). This empirical method is known as empirical estimation. Major advantages of empirical estimation are: 1) the number of parameters is small; 2) the method can be applied to almost any location; and 3) the results are stable (the variance is small) despite the small number of parameters. On the other hand, major disadvantages of the method are: 1) it cannot be used to predict complicated site-dependent phenomena such as the effect of local topography; and 2) when it is applied beyond the data range (e.g., for near-source regions), the validity of the results decreases because the regression coefficients are determined empirically and are strongly dependent on the data distribution. Recently, to overcome the latter problem, a form of attenuation formula has been investigated from the physical point of view (Takemura et al., 1987; Fukushima and Tanaka, 1991), and the validity of the method for estimating regression coefficients has been discussed (Joyner and Boore, 1993). However, the problem of evaluating the effect of fault size in near-source regions has not been solved. Although the shortest distance from a fault to observation site (Xsh, often termed fault distance) has been most commonly used as source-to-site distance (X) for the near-source region (e.g., Joyner and Boore, 1981; Campbell, 1981; Fukushima and Tanaka, 1990), we think this is unreasonable because strong motion in the near-source region are affected from not only the nearest point but also other points on the fault plane, as pointed out by Shakal and Bernreuter (1981). To solve this problem, we proposed a new distance measure: equivalent hypo- Received August 8, 1995; Accepted April 4, 1996 * To whom correspondence should be addressed. 623

2 624 S. Ohno et al. central distance (Xeq), which is derived from shortperiod approximation of the strong motion energy spectrum based on the fault model theory (Ohno et al., 1993). In this paper, first, we evaluate an attenuation relation of response spectra by regression analysis of strong-motion data recorded in California, which include many near-source data. Secondly, we apply the evaluated attenuation relation to the 1995 Hyogo-ken Nanbu earthquake in order to estimate the regional distribution of strong motion on pre-quaternary stratum. Thirdly, the amplifications of ground motion due to Quaternary stratum are calculated through comparison between the observed records on Quaternary stratum and estimates on pre-quaternary stratum. The relation between the amplification of ground motion on Quaternary stratum and the severe damage in and around Kobe are also discussed. 2. Attenuation Relation of Acceleration Response Spectra 2.1 Equivalent hypocentral distance Equivalent hypocentral distance (Xeq) is derived from short-period approximation of the strongmotion energy spectrum based on the fault model theory by Ohno et al. (1993). Xeq is calculated using the following formula: where n is the number of segments on the earthquake fault plane, Moi(f) is the seismic moment density on the i-th segment and Xi is the distance between the i-th segment and the site. Xeq can take into account the effects of fault size, fault geometry, and inhomogeneous slip distribution on the fault plane. As Xeq means hypocentral distance from a virtual point source that provides the same energy to the site as does a finite-size fault, we can use Xeq in evaluating near-source strongmotion spectrum without changing the form of attenuation formula based on the point source theory. 2.2 Data The records used in estimating the attenuation relation comprise 496 horizontal components of 17 California earthquakes which occurred from 1940 to Table 1 shows a list of the earthquakes, the (1) Fig. 1. Locations of the earthquake faults used for estimating the attenuation relation. Rectangles denote projections of the fault models to ground surface and stars denote the epicenters. number of records for each earthquake, and the fault models used for calculating Xeq. The records are listed in Appendix A. Figure 1 shows the locations of the fault planes. We generally used moment magnitude (Mw) for earthquake magnitude (M), while we used local magnitude (ML) if Mw was unavailable. The portion of records after the initial S-wave arrival was used for calculating response spectrum. The Xeq for each record was calculated using the slip distribution of the fault model listed in Table 1, on the assumption that Moi(f) is proportional to slip on the i-th segment (Ohno et al., 1993). When the slip distribution was not available, uniform distribution was assumed. For the small earthquakes (Nos. 3, 6, 8, and 11), we used hypocentral distance because the event is small enough to be treated as a point source. Magnitude (M) ranges from 5.0 to 7.5, and Xeq ranges from 7 to 100 km (fault distance (XSh) from 0.4 to 90 km). Figure 2 shows M-XSh and M-Xeq distributions of the data. We classified the sites as pre-quaternary or Quaternary stratum by the classification scheme shown in Table 2, based on the descriptions of site geology in CDMG and USGS reports. There are 160 pre-quaternary and 336 Quaternary sites. Sites with very soft soil such as bay mud or artificial fill were excluded because the number of records was small and ground motions at such sites may be J. Phys. Earth

3 Strong Motions at Hyogo-ken Nanbu 625 Vol. 44, No. 5, 1996

4 626 S. Ohno et al. Fig. 2. Relationships between magnitude and distance for the data used to estimate the attenuation relation. Table 2. Site classification scheme for the observation stations in California. Fig. 3. Distribution of S-wave velocities at California observation sites used to estimate the attenuation relation. Only the sites where velocities of the ground are available (38 of a total of 80 pre-quaternary sites, 94 of a total of 168 Quaternary sites) are used here. strongly affected by the soil nonlinearity. Figure 3 is a histogram of S-wave velocities of the sites where the seismic surveys were performed. The velocity of underlying rock is used for pre- Quaternary sites with a shallow surface Quaternary layer. S-wave velocities at pre-quaternary sites are distributed in the range above 600 m/s. 2.3 Regression analysis Following the random-effects model of Joyner and Boore (1993), we modeled the 5%-damping acceleration response spectrum (S(T)) as: after compensating the effect of attenuation in distance. The coefficient b(t) is related to the Q-value (Qs) along the source-to-site propagation path of S-waves (Takemura et al., 1987) as described by the following equation: (3) q is also a dummy variable to stratify the sites (0 for pre-quaternary and 1 for Quaternary sites) and s(t) represents amplification due to Quaternary stratum versus pre-quaternary stratum. R(T) represents a rest part of S(T) which cannot ƒã be where pj is a dummy variable to stratify the earthquakes (1 for j-th earthquake, 0 otherwise), and Ej(T) represents spectrum amplitude radiated from the j-th earthquake fault on pre-quaternary stratum, (2) expressed by pjej(t)-logxeq-b(t)xeq+q s(t), and is assumed to take an independent value by each record and belong to the Gaussian distribution of which the average is zero and the variance is ƒðr2. Ej(T) is modeled as a linear equation of magnitude (Mj) as J. Phys. Earth

5 Strong Motions at Hyogo-ken Nanbu 627 where Mj is the magnitude of the j-th earthquake, E(T) represents a rest part of Ej(T) which ƒã cannot be expressed by a(t)mj+c(t), and is assumed to take an independent value by each earthquake and belong to the Gaussian distribution of which the average is zero and the variance is ƒðe2. Combining Eqs. (2) and (4), the attenuation formula as a function of magnitude (M), equivalent hypocentral distance (Xeq), and site geology (q) is obtained as (4) 4 and 5. Peak ground acceleration (Amax) on pre-quaternary stratum is estimated as from the attenuation relation of a 5% damping (7) response spectrum at a period of 0.02 s as an approximation of Amax. Hanks and McGuire (1981) and Boore (1983) derived the moment magnitude dependency of peak ground acceleration based on the random vibration theory and the ƒö-square source model as The variance (qs2) of Eq. (5) is calculated by The coefficients Ej(T), a(t), b(t), c(t), s(t), R2, and ƒðe2 were estimated by the maximum ƒð likelihood two-stage regression procedure (Joyner and Boore, 1993) at periods from 0.02 to 2.0 s. The estimated regression coefficients are shown in Figs. (5) (6) Equation (7) is consistent with the theoreticallyderived Eq. (8). Also, Fig. 5 shows that Ej(T) is well represented by the linear equation of Mw. The broken line in the chart of b(t) in Fig. 4 is the value of b(t) calculated from Qs in California obtained by Chin and Aki (1991), where Vs=3.5 km/s in Eq. (3). The regression coefficient of b(t) (the solid line in the chart of b(t) in Fig. 4) (8) Fig. 4. Estimated regression coefficients. The magnitude-dependency of maximum accelerations derived by Boore (1983), b(t) calculated from Qs in California by Chin and Aki (1991), and the estimates of R(T), ƒðe(t), and ƒðs(t) in Boore et al. (1993) are superimposed. See the text for an explanation of each ƒð coefficient. Vol. 44, No. 5, 1996

6 628 S. Ohno et al. Fig. 5. Estimated regression coefficients of Ej(T) with regression line of a(t)mw,+c(t) at some periods. approximately agrees with the value calculated from Qs in California. The estimate of s(t) indicates that only waves of periods longer than 0.2 s are amplified at Quaternary sites. This agrees with the other estimations of attenuation relation from California records based on geological classification (e.g., Joyner and Boore, 1981), and have been interpreted by the effect that attenuation of surface soil for short periods cancels amplification due to the impedance ratio between surface soil and underlying rock (Joyner and Boore, 1988). The ƒðr, ƒðe, and ƒðs estimated in the regression analysis by Boore et al. (1993) on recent California records are superimposed in Fig. 4, and they are similar to the estimates in this study. 2.4 Comparison with the other attenuation relations Boore et al. (1993) conducted regression analysis on peak ground accelerations and response spectra of strong-motion records mainly in California. Also, Fukushima (1994) obtained attenuation relations on peak ground accelerations around the world including recent California data. Although we do not compare their regression coefficients with those in this study because the functional forms of regression model and/or the site classification schemes are different, we compare peak ground accelerations and response spectra estimated by the attenuation relation of each study. Boore et al. (1993) classified the sites into 4 classes A, B, C, and D, based on shear-wave velocity averaged over the upper 30 m at a site. Shearwave velocities for classes A to D are 750 < Vs, 360 < Vs <750, 180 < Vs <360, and Vs < 180 m/s, respectively. When Vs at a site was unknown, an appropriate class was estimated by analogy with borehole measurements of geologic materials similar to the site. They did not use records at Class D sites in the regression analysis because the number of records was small. By comparing the lists of strong-motion records, we found that Class A and a part of Class B in Boore et al. (1993) correspond to pre-quaternary stratum in this study, and the rest of Class B and Class C correspond to Quaternary stratum. This is consistent with the distribution of shear-wave velocities in Fig. 3. On the other hand, Fukushima (1994) did not classifly the sites but stratified data in Japan and other areas. For comparison, we use his estimate for other areas where California data were classified. The definitions of source-to-site distance are different among the three studies. Boore et al. (1993) used the shortest distance from the projection of fault rupture to ground surface (dsh), Fukushima (1994) used the shortest distance from fault rupture (XSh) and this study uses equivalent hypocentral distance (Xeq). As this difference cannot be ignored at near-source distances, we compare the estimates only at far-source distances. J. Phys. Earth

7 Strong Motions at Hyogo-ken Nanbu 629 Figure 6 compares estimated peak ground accelerations versus MW under the assumption that dsh = Xsh = Xeq =100 km. The range of standard deviation is also plotted around the estimate for this study. The estimates for Boore et al. (1993) and Fukushima (1994) are approximately located within the estimate }standard deviation of this study. distances where the difference of definition of distance is not so significant. We apply this attenuation relation to the Hyogo-ken Nanbu earthquake in order to investigate the strong-motion characteristics during this earthquake because both the California and Hyogo-ken Nanbu earthquakes were Comparing the estimates in detail, the estimate (median value) of this study is located between Class A and Class B of Boore et cal. (1993), and is relatively larger than the estimate for Fukushima (1994). Figure 7 compares the response spectra estimated by the attenuation relation of Boore et al. (1993) with those of this study at Mw= 5.5, 6.5, and 7.5, under the assumption that dsh = Xeq =100 km. The estimates for pre-quaternary stratum in this study are generally located between those for Class A and Class B of Boore et al. (1993), and the estimates for Quaternary stratum in this study are approximately located between those for Class B and Class C of Boore et al. (1993). This is consistent with the distributions of shear-wave velocity in the site classifications of this study and those of Boore et al. (1993). The attenuation relation of this study, therefore, is in agreement with other attenuation relations at Fig. 6. Peak ground accelerations versus Mw estimated by the attenuation relations of Boore et al. (1993), Fukushima (1994), and this study, under the condition that dsh = Xsh = Xeq = 100 km. Fig. 7. Response spectra at MW=5.5, 6.5, and 7.5 estimated by the attenuation relations of Boore et al. (1993) and this study under the condition that dsh=xeq=100 km. Vol. 44, No. 5, 1996

8 630 S. Ohno et al. shallow inland earthquakes. 3. Strong-Motion Characteristics of the Hyogo-ken Nanbu Earthquake 3.1 Source process The Hyogo-ken Nanbu earthquake occurred on January 17, The location of the epicenter was estimated by the Japan Meteorological Agency (JMA) to be at the northern edge of Awaji Island (34.6 N, E), and the focal depth to be 14 km. The JMA magnitude (MJ) was 7.2 and Kikuchi (1995) estimated the moment magnitude (Mw) to be 6.9. The aftershocks were distributed along the Nojima fault of Awaji Island and the Suma- Suwayama-Gosukebashi faults (parts of the Rokko fault system) in the northern part of Kobe. Surface fault trace was found along the Nojima fault (Nakata et al., 1995). The length and depth of the aftershock distribution were approximately km and up to 20 km, respectively (DPRI, Kyoto Univ.). Many researchers have estimated the source process of the 1995 Hyogo-ken Nanbu earthquake by the inversion method using seismic and/or geodetic data. Many of these studies used regional (epicentral distance less than 150 km) and longperiod (more than 2 s) earthquake records (Ide et al., 1995; Sekiguchi et al., 1995). Wald (1995) and Yoshida et al. (1996) added long-period teleseismic records to regional records for joint inversion. Yoshida et al. (1996) also used geodetic data. On the other hand, Kakehi et al. (1996) used shortperiod (2-10 Hz) seismic waves for the acceleration envelope inversion (Kakehi and Irikura, 1996). The common features of the source process estimated in these studies are summarized as follows. 1) The rupture started from the hypocenter and propagated towards NE and SW directions bilaterally. The duration of the main rupture was within 10 s. 2) There were two large-slip regions, one in a shallow position (0-5 km) just below the Nojima fault on the Awaji side and the other in a deep position (10-15 km) near the hypocenter on the Kobe side. 3) Slip vector over the fault plane shows that right-lateral strike-slip was predominant, which is coincident with the source mechanism estimated from initial P-wave motions. There were some dip-slip components on the Kobe side near the hypocenter. There were some differences in detail among the source models. In the models of Kakehi et al. (1996) and Sekiguchi et al. (1995), the large-slip area has a shallow portion as well as a deep portion beneath Kobe. Such shallow slip beneath Kobe is not estimated in the other models. Also, Ide et al. (1995) estimated that the slip on the Awaji side was much larger than that on the Kobe side, while such concentration of slip is not estimated in the other models. 3.2 Topography and geology The topography and geology of the Kobe area are consequences of activity of the Rokko fault system. The area can be broadly divided into two regions: the Rokko Mountains and the lowlands south of the mountains. Granite is widely distributed in the Rokko Mountains. The south area of the mountains can be topographically classified into alluvial fans, seaside lowlands, and reclaimed land. These areas correspond geologically to fluvial sediments, alluvial plains (sandy soil near the surface), and soft, thick clay layers, respectively (Kobe City, 1985). The region which suffered severe damage is approximately coincident with the area of alluvial fans and seaside lowlands (Takemura et al., 1995). Damage was relatively light in the Rokko Mountains and on the reclaimed land. 3.3 Strong-motion records Many organizations recorded earthquake data during the Hyogo-ken Nanbu earthquake. For this study, we use observation records (some of which are only peak ground accelerations) from the Japan Meteorological Agency (JMA), the Committee of Earthquake Observation and Research in the Kansai Area (CEORKA), the Port and Harbor Research Institute (PHRI), the Railway Technical Research Institute (JR: Japan Railway), the Kansai Electric Power Co., Ltd. (KEPCO), and the Matsumura- Gumi Construction Company. Large accelerations were recorded during the earthquake, such as 818 cm/s2 at the Kobe JMA Observatory near the causative fault. We classified the observation sites as pre- Quaternary, diluvium or alluvium according to the classification scheme shown in Table 3. For site classification, we used geological descriptions in reports from CEORKA, PHRI, KEPCO, and Matsumura-Gumi, and geological maps for the other sites. The records are listed in Appendix B together with site classifications. Figure 8 shows the locations of observation sites near the epicenter. J. Phys. Earth

9 Strong Motions at Hyogo-ken Nanbu 631 Table 3. Site classification scheme for the Hyogo-ken Nanbu earthquake observation stations. The CEORKA Sakai site (CSK) and the JR Kakogawa site (KKG) are located on alluvium according to geological maps, but we classified these two sites as diluvium because: 1) we think the alluvium layer is thin since the locations are close to "basement" in the geological maps; 2) CEORKA describes the soil at Sakai as "thin alluvium"; and 3) the estimated amplification factors for these sites have characteristics more similar to those of diluvial sites than those of other alluvial sites, as described later. Comparing the classification scheme of Table 2 with Table 3, pre-quaternary site is considered almost equivalent for both California and Hyogoken Nanbu earthquake data. Almost all diluvial and alluvial sites in the Hyogo-ken Nanbu earthquake records correspond to Quaternary sites in the California data, except sites on reclaimed land. As described before, the California data excluded the records on reclaimed land, while the Hyogo-ken Nanbu earthquake data included the records on reclaimed land as alluvium (PI, KN8, AM3, AMG, and MK in Fig. 8). Fig. 8. Epicenter of the 1995 Hyogo-ken Nanbu earthquake and locations of near-source observation sites used in this analysis. Triangles, crosses, and circles indicate observation sites on pre-quaternary, diluvium, and alluvium, respectively. The * mark shows the location where the subsurface structure is used for the theoretical calculation of amplification factors. Vol. 44, No. 5, 1996

10 632 S. Ohno et al. Fig. 9. Contour of the estimated peak ground accelerations together with the surface projection of the fault model. The peak ground accelerations are estimated by Eq. (7) for MW= 6.9 and Xeq calculated using the source model by Kakehi et al. (1996). 3.4 Strong ground motions on pre-quaternary stratum Kakehi et al. (1996) obtained a distribution of acceleration radiation intensity (wi) (Kakehi and Irikura, 1996), which means the relative contribution of the i-th segment of the fault plane to the acceleration envelope amplitude. As the period range analyzed in Kakehi et al. (1996) approximately corresponds to that in this paper, we calculated Xeq by substituting wi for Moi(f) in Eq. (1) and estimated peak ground accelerations on pre-quaternary stratum using Eq. (7) with MW = 6.9. Figure 9 shows contours of the estimated peak ground accelerations together with the surface projection of the fault model by Kakehi et al. (1996). Due to inhomogeneous energy radiation on the fault plane, there are two large acceleration areas: one in Awaji Island and the other in Kobe. The maximum estimate in Awaji Island is about 750 cm/s2, which is much larger than the maximum estimate of about 350 cm/s2 in Kobe. Figure 10 shows the relation between observed peak ground accelerations and Xeq, together with curves indicating the estimates on pre-quaternary Fig. 10. Relation between peak ground acceleration and Xeq calculated using the source model by Kakehi et al. (1996). Triangles, crosses and circles indicate observed values of horizontal components. The solid and broken curves denote the estimate and the estimate }the standard deviation, respectively. The estimate is obtained by Eq. (7) with Mw= 6.9. stratum and the estimates }the standard deviation (ƒðs). The observed values at the pre-quaternary sites are distributed within the range of the standard deviation. The observed values at the diluvial and alluvial sites are 1.4 and 1.8 times larger, respectively on the average, than the estimates on pre- Quaternary stratum, although the observed values at the alluvial sites are widespread. Figure 11 shows response spectra of the near-fault observation records at Kobe University together with the estimate on pre-quaternary stratum. As the records of CEORKA were measured velocity, we obtained acceleration waves by passing the data through a 20 Hz high-cut filter after differentiating. The estimates agree well with the observed values within the range of the standard deviation, except for the 1.5-s peak in NS component. This large peak may be due to the "forward rupture directivity effect," which causes a large long-period pulse in the component normal to the J. Phys. Earth

11 Strong Motions at Hyogo-ken Nanbu 633 Fig. 11. Observed and estimated 5% damping acceleration response spectra for the Kobe University observation site (KU). The broken curves denote observed values of two horizontal components. The solid curves denote the estimate and the estimate±the standard deviation. The estimate is obtained by the attenuation relation on pre-quaternary stratum with Mw= 6.9 and Xeq=16 km, which is calculated using the source model by Kakehi et al. (1996). fault when both the rupture slip and the rupture propagation are towards the site (e.g., Sommerville et al., 1995). Analytically, Motosaka and Nagano (1996) simulated ground motion at Kobe University using a right-lateral strike-slip moving dislocation model, and showed that the normal component becomes larger than the parallel component to the fault. Xeq does not take this effect into account, but the effect does not appear at short-periods (less than 1 s in this earthquake) and only appears under the restricted conditions described above. The results in Figs. 10 and 11 indicate that the amplitudes of incident wave on pre-quaternary stratum during the Hyogo-ken Nanbu earthquake were similar to those in California earthquakes of Mw= Amplifications on Quaternary stratum We calculated the amplification factor at each observation site shown in Fig. 8 which belongs to JMA, CEORKA, PHRI, and JR by taking a ratio between the observed response spectrum and the estimate for pre-quaternary stratum. To analyze JR records, we digitized the acceleration response spectra using figures from Nakamura (1995). Figure 12 shows the estimated amplification factor at each site. Similar to the peak ground accelerations in Fig. 10, the observed values at the pre-quaternary sites agree well with the estimates, while the observed values at the diluvial and alluvial sites are systematically larger than the estimates. The estimated amplification factors at the diluvial sites are not strongly dependent on frequency and the values are roughly Amplification factors for the alluvial sites are almost the same as those for the diluvial sites at periods less than 0.5 s, but are larger at the longer periods, and the values are approximately 5 at periods of 1-2 s. We compare the amplification factor estimated above with the theoretical amplification of soil for the sites where the site response has been already investigated, or the soil profile at or near the site is available. The estimated amplification at JMA Kobe Observatory (KBE) on diluvium is larger than that at the other diluvial sites at periods longer than 0.2 s. As this site is located on the top of a hill that the height is 20 m and the northern edge is cliff, local topography might have affected the large amplification. Tohdo (1995) calculated the amplification of KBE due to the subsurface layers shallower than the Upper Osaka Group (diluvium) with the effect of local topography. This theoretical amplification is superimposed in the chart of KBE in Fig. 12, and roughly agrees with the estimate in this study. Although the theoretical amplification is smaller than that of the record, this is probably because Tohdo (1995) did not take the amplification by the Upper Osaka Group into account, but which might be included in our estimates. Table 4 shows subsurface structure models for the sites where amplifications are calculated theoretically. The subsurface structures at S1 and S2 in Fig. 8, which were estimated by Aoki et al. (1990) using inversion of long-period microtremors, are utilized for ABN and FKS. The PS-logging result up to 83 m deep is used for N. The subsurface structure up to 60m deep by Murai (1993), at M (Mukogawa) in Fig. 8, is adopted for AMG. As shown in Table 4, deep structures for PI and AMG are assumed based on the report by the Japanese Society of Soil Vol. 44, No. 5, 1996

12 J 634 S. Ohno et al. Mechanics and Foundation Engineering (1992). As these models are not obtained at the same location of the strong-motion stations except for PI, we compare amplification characteristics roughly and do not discuss the detailed peaks and dips. As for damping, both frequency-independent and dependent cases are investigated. h=7.5/v5 (Q= Vs/15) is used for the former case. For the latter case, damping factors are assumed as h = 0.075f-0.5 for the shallow layers and h = f-0.68 for the deep layers, based on the summary of Q-1 from Fukushima and Midorikawa (1994). Theoretical amplification is calculated by taking a ratio between the surface motion and the motion when the bottom layer is outcropped using 1-D linear SHwave propagation theory, and the calculated result is superimposed in Fig. 12. It can be seen that the effect of damping frequency-dependency was not so significant in these cases. At the diluvial site ABN, the theoretical amplifications have similar characteristics to the estimated ones. At the alluvial sites PI, FKS, and AMG, the theoretical amplifications are similar to the estimated ones at short periods except for PI, but become smaller at periods longer than 1 s. As for PI, the estimated amplification is not only less. Phys. Earth

13 Strong Motions at Hyogo-ken Nanbu 635 Fig. 12. Site amplification factors estimated by taking ratios between observed and estimated response spectra. The estimated response spectra are obtained for pre-quaternary stratum with MW = 6.9 and Xeq calculated using the source model by Kakehi et al. (1996). The theoretical amplification factors are superimposed for KBE, ABN, FKS, PI, and AMG. See the text for details. than the theoretical value but also less than unity at short periods. This is probably due to the large attenuation at short periods by the effects of soil nonlineality, as observed in the vertical array records at this site (Kawase et al., 1995). Such nonlinear site response can also be found among the near-source records of the 1989 Loma Prieta earthquake, California (Chin and Aki, 1991). Consequently, except for the long-period components at the alluvial sites, our estimates of amplifications by Quaternary stratum generally agree with the theoretical amplifications. It is considered to be valid that the estimates of amplification due to Quaternary stratum as well as those of strongmotion spectrum on Pre-Quaternary stratum. The discrepancy between the estimates and the theoreti- Vol. 44, No. 5, 1996

14 636 S. Ohno et al. Table 4. Subsurface structure models for calculation of site response. cal values at long periods will be discussed later. The Takatori (TKT) and Takarazuka (TKR) sites are located in the heavily damaged area, and the estimated amplifications at these sites are large at periods not only longer but also shorter than 1 s. As the natural period of a wooden house is about s (Tajime et al., 1977), the amplification at short periods might be one reason for the heavy damage. 4. Discussion 4.1 Dependency on source models As described in the former section, the estimated rupture processes are generally in agreement, but there are some differences. In order to investigate the dependency of source models in estimating the intensity of strong motions, we calculated Xeq using the slip distribution on the fault inferred by Wald (1995) and Ide et al. (1995) instead of Moi(f) in Eq. (1) and estimated peak ground accelerations on pre-quaternary stratum. Figures 13 and 14 show contours of the peak ground accelerations estimated using the two source models. The peak ground accelerations estimated based on the source model of Wald (1995) are relatively smaller than those estimated based on the source model of Kakehi et al. (1996), but the two are similar. Compared to the estimates from Wald (1995) and Kakehi et al. (1996), the estimates from Ide et al. (1995) are not so different at Kobe, but are very large and exceed 1,200 cm/s2 in the southwest (Awaji Island). This large discrepancy in northern Awaji Island is probably because there are no strong-motion records and a source model may have large variation J. Phys. Earth

15 Strong Motions at Hyogo-ken Nanbu 637 Fig. 13. Contour of the estimated peak ground accelerations together with the surface projection of the fault model. The peak ground accelerations are estimated by Eq. (7) for Mw = 6.9 and Xeq calculated using the source model by Wald (1995). in the estimation there. From the point of view of evaluating strong motions, it is worth noting that estimates can have large variations even for the same earthquake. Still, we investigated the characteristics of strong motion in Kobe, and this discrepancy in Awaji Island does not affect our results. Figure 15 shows the average amplification factors estimated using the above three source models for different site classifications and the averages are very similar. 4.2 Amplification properties according to site geology We discuss amplification characteristics of Quaternary stratum by comparing our estimates from the Hyogo-ken Nanbu earthquake records with amplifications obtained from other earthquake records. In order to investigate site amplification characteristics in relation to surface geology at periods less than 1 s, Takemura et al. (1991) analyzed strong motions in the southern part of the Tohoku region of Japan. Their results can be summarized as follows; 1) ground motions at Quaternary sites are amplified due to the impedance ratio between surface layer Fig. 14. Contour of the estimated peak ground accelerations together with the surface projection of the fault model. The peak ground accelerations are estimated by Eq. (7) for =6.9 and Xeq calculated using the source Mw model by Ide et al. (1995). and basement; 2) amplification of short-period waves is smaller than that of long-period waves because attenuation of surface layer cancels the amplification of short-period waves; and 3) the foregoing tendencies are stronger for alluvial sites than diluvial sites, Phillips and Aki (1986) analyzed coda waves of weak motion in central California and indicated similar results. Fukushima and Tanaka (1990) investigated the effects of soil type on peak ground accelerations observed in Japan and found that accelerations at rock sites are smaller than that at soil sites. The estimated amplification factors for the Hyogo-ken Nanbu earthquake, shown in Figs. 12 and 15, agree with these findings. Motosaka and Nagano (1996) investigated the amplification of surface layers for the Hyogo-ken Nanbu earthquake ground motions by 2-D wave propagation analysis giving consideration to the irregularity of subsurface structures. They estimated the amplification factors at Upper Part of Osaka Group (diluvium) versus rock site (Kobe University of CEORKA) to be about twice, and those at ground surface (alluvium) to be about three times. Although their definitions of diluvium and alluvium do not Vol. 44, No. 5, 1996

16 638 S. Ohno et al. Fig. 15. Amplification factors for different geological site classifications. Average amplification factors from the Hyogo-ken Nanbu earthquake data (this study; left three charts), from the California earthquake data (this study), and from the Chiba-ken-toho-oki earthquake data (Midorikawa and Sakukawa, 1993). Amplification factors for the Hyogo-ken Nanbu earthquake are calculated by taking ratios between response spectra of observed records versus the estimates on pre-quaternary stratum. The estimates are obtained using three source models: Kakehi et at. (1996), Wald (1996), and Ide et al. (1995). strictly agree with our definitions, their results are in general agreement with our estimates. The amplification factor of Quaternary versus pre-quaternary stratum estimated from the California data is shown in Fig.15(re-plot of 10 s(t) in Fig. 4 after adjusting the scale). The estimated amplification at periods less than 0.2 s is approximately unity, and is smaller than that estimated from the Hyogo-ken Nanbu earthquake records. At periods longer than 0.2 s, the estimated amplification factor from the California data approximately corresponds to that from the Hyogo-ken Nanbu earthquake records on diluvium. According to the site classification schemes shown in Tables 2 and 3, Quaternary sites in California include both diluvial and alluvial sites. The amplification factors estimated from California data may be smaller because the sites with very soft soils such as bay mud or reclaimed land were excluded, and the depths of Quaternary stratum at California sites are relatively shallow. Amplification factors (for diluvial and alluvial sites versus Tertiary sites) estimated from the 1987 Chiba-ken-toho-oki earthquake records are also plotted in Fig. 15 (Midorikawa and Sakukawa, 1993). Estimated amplification factors for the diluvial sites from the Hyogo-ken Nanbu and Chiba-ken-toho-oki earthquake records are almost the same. However, the estimated amplification factors for the alluvial sites from the Hyogo-ken Nanbu earthquake data are twice those from the Chiba-ken-toho-oki earthquake data at periods longer than 0.5 s, though the amplification factors are in agreement at periods shorter than 0.5 s. As described in the former section, this discrepancy for the alluvial sites at long periods is also seen in comparison with theoretical amplifications. Several studies have examined the effects of shallow and deep subsurface structures of the Osaka Plain on seismic waves (Ohba and Toriumi, 1992; Murai, 1993; Ohba et al., 1994). The results of these studies can be summarized as follows: 1) body waves J. Phys. Earth

17 Strong Motions at Hyogo-ken Nanbu 639 are dominant at periods shorter than 1-1.5s; 2) the "delayed wave" (Ohba and Toriumi, 1992), a surface wave created at the mountain-plain boundary and propagating through the plain with an apparent velocity of about 0.6 km/s, has a predominant period of 1-2 s; and 3) surface waves are dominant at the longer periods. Also, a peak period of 1 s was obtained from the spectral ratio between microtremors at Rokko Island (east of Port Island) and in the Rokko Mountains (Kajima, 1995). Yamanaka and Aoi (1996) identified surface waves with an apparent velocity of about 0.5 km/s from aftershock records observed along the Sumiyoshi River, which runs through Kobe from north to south. Motosaka and Nagano (1996) analytically found that seismic waves are created at the Rokko fault due to the irregularity of subsurface structures, and that these waves propagate into the Osaka basin. The period range of 1-2 s, which are the predominant periods of estimated amplification factors for the alluvial sites, is therefore considered a transition range of dominant wave-type from body wave (shorter than s) to surface wave (longer than 1-2 s) in the Kobe-Osaka area. The contribution of surface waves is considered to be one of the factors why the amplification of the alluvial sites is so large at periods of 1-2 s. We therefore conclude that the amplification characteristics of Quaternary stratum estimated from the Hyogo-ken Nanbu earthquake records are generally in agreement with those from other earthquake records, although the effects of surface waves at periods of 1-2 s may need further investigation. 5. Conclusions We applied the attenuation relation evaluated from California records to the 1995 Hyogo-ken Nanbu earthquake. As the estimates at pre- Quaternary sites agreed well with the observed values, we conclude that amplitudes of incident wave on pre-quaternary stratum in the Hyogo-ken Nanbu earthquake were similar to those in Mw = 6.9 California earthquakes. Peak ground accelerations on pre-quaternary stratum in Kobe were consistent with the source models, and were estimated to be about 350 cm/s2, while the estimates for Awaji Island vary according to source model from 600 to 1,200 cm/s2. Strong motions at diluvial and alluvial sites were larger than those at pre-quaternary sites. The estimated amplification factors at the diluvial sites were not strongly dependent on frequency and the values are roughly The estimated amplification factors for the alluvial sites were almost the same as those for diluvial sites at periods less than 0.5 s, but were larger at the longer periods with values of approximately 5 at periods of 1-2 s. As the amplifications in the heavily damaged area was large around the natural period of wooden houses ( s) as well as at long periods, these short-period amplifications might be one of the reason why Kobe suffered extensive damage. The amplification characteristics of Quaternary stratum estimated from the Hyogo-ken Nanbu earthquake records are generally in agreement with those inferred from the 1-D SH-wave propagation theory as well as those from other earthquake records, except that the amplifications at alluvial sites were larger at periods of more than 1 s and might be affected by surface waves. The authors would like to thank the California Division of Mines and Geology and the United States Geological Survey for making available the strong-motion records for California. The authors would also like to thank the Japan Meteorological Agency, the Committee of Earthquake Observation and Research in the Kansai Area, and the Port and Harbor Research Institute for making available the 1995 Hyogo-ken Nanbu earthquake records. REFERENCES Aoki, Y., M. Horike, and Y. Takeuchi, Estimation of P- and S- wave velocity structure in the Osaka plain, 8th Japan Earthquake Engineering Symposium, , 1990 (in Japanese with English abstract). Archuleta, R. J., A faulting model for the 1979 Imperial Valley earthquake, J. Geophys. Res., 89, , Archuleta, R. J. and S. M. Day, Dynamic rupture in a layered medium: the 1966 Parkfield earthquake, Bull. Seismol. Soc. Am., 70, , Boore, D. M., Stochastic simulation of high-frequency ground motions based on seismological models of the radiated spectra, Bull. Seismol. Soc. Am., 73, , Boore, D. M., W. B. Joiner, and T. E. Fumal, Estimation of response spectra and peak accelerations from Western North American earthquakes: an interim report, U. S. Geological Survey Open-File Report , Campbell, K. W., Near-source attenuation of peak Vol. 44, No. 5, 1996

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