Tomotaka Iwata, l,* Ken Hatayama,1 Hiroshi Kawase,2 and Kojiro Irikura1

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1 J. Phys. Earth, 44, , 1996 Site Amplification of Ground Motions during Aftershocks of the 1995 Hyogo-ken Nanbu Earthquake in Severely Damaged Zone Array Observation of Ground - Motions in Higashinada Ward, Kobe City, Japan- Tomotaka Iwata, l,* Ken Hatayama,1 Hiroshi Kawase,2 and Kojiro Irikura1 1 Disaster Prevention Research Institute, Kyoto University, Uji 611, Japan 2Izumi Res. Inst., Shimizu Corp., Tokyo 100, Japan Site amplifications during aftershocks of the 1995 Hyogo-ken Nanbu earthquake were examined in and around the disaster area of Higashinada ward, Kobe City, Japan. Five temporary stations together with the KOB station (CEORKA) were installed just after the main shock to arrange an array for strong ground motion observations across a severe damage band about 1.5 km wide. Several aftershock records were obtained from these stations. The peak values of the observed ground accelerations and velocities at the soil sites were 3-20 times larger than those at the rock site. Spectral ratios between the soil sites and the rock site have peak frequencies characteristic to each station pair. We also estimated a thickness of about 1 km for the sedimentary basin just under the soil sites using SP-converted waves recorded at all soil sites. Waveform simulations using a dislocation point source in a horizontally stratified medium were performed to explain the observed records at both the rock and soil stations. We found that large amplifications at the soil sites were generated by a thick sedimentary layer including low-velocity surface layers. The heavy damage caused by the main shock is therefore strongly connected with the sedimentary structure. 1. Introduction On January 17, 1995, the 1995 Hyogo-ken Nanbu earthquake struck Kobe City and Awaji Island, Hyogo Prefecture, in the Kansai area of Japan. Large damage such as highway collapse, building and house destruction, and widespread fires occurred mainly in the downtown area of Kobe. An isoseismal map (Japan Meteorological Agency (JMA), 1995) shows that a belt zone of heavy damage runs east-northeast to west-southwest across the city. A similar concentrated earthquake disaster happened in the Sherman Oaks and the Santa Monica regions during the 1994 Northridge Earthquake, California, U.S.A. Graves (1995) tried to simulate the large amplitude and long-period later phases appearing in the strong ground motion records during the main shock observed at the Santa Monica City Hall using two-dimensional basin structure modeling. He concluded that a combination of both large-scale (deep basin) and small-scale (shallow micro-basin) structures are needed to explain the observed site responses. Gao et al. (1996) found localized large-site amplification in the damage area from an array observation of aftershocks. They interpreted the localized amplification as a focusing effect by the edge of the sedimentary region. In this study, to investigate the cause of spatial damage distribution, we made an array observation of aftershocks in downtown Kobe. The purposes are 1) to compare site effects between severely damaged and slightly damaged or no-damaged areas, and 2) to study the effects of surface geology on seismic motions. Preliminary results of the observation have already reported by Iwata et al. (1995). Received July 7, 1995., Accepted February 7, 1996 * To whom correspondence should be addressed. 553

2 554 T. Iwata et al. 2. Array Observation From January 18, 1995, the cooperative strongmotion observation group started to deploy strongmotion observation stations in the source region. By the beginning of February, the group installed more than 30 observation stations (Cooperative Strong Motion Observation Group, 1995). Temporary observations continued to the end of March, We installed stations in Higashinada Ward; the easternmost part of Kobe City, where severely damaged wooden houses were concentrated in a narrow band about 1.5 km wide. Totally, five temporal observation stations were installed close to the permanent station (KOB) of the Committee on Earthquake Observation and Research in the Kansai Area (CEORKA) (Toki et al., 1995). In Fig. 1, we show the location map of the stations used in this study together with a seismic intensity map (JMA, 1995). The largest aperture of this array is about 3 km. According to geological mapping (Hujita and Kazama, 1982), the KMC station is a rock site, whereas KOB, FKI, NOM, and ASY stations are installed on alluvium, and the FKE station is on a sand bank (Table 1). The FKI, NOM, Fig. 1. Location of array observation stations in Higashinada ward, Kobe City. Enclosed area shows the seismic intensity of seven as measured by JMA (1995). Table 1. List of array observation stations in Higashinada Ward, Kobe City. J. Phys. Earth

3 Site Amplification of Aftershocks in Severely Damaged Zone 555 and ASY stations are located in the severely damaged band, and the KOB and FKE stations are in the vicinity. All seismometers were fixed either on the free surface or in the basements of one-story buildings. The observation system consisted of velocity or acceleration seismometers with a wide-frequency band ( Hz), a high dynamic range (> 80 db, up to 20 cm/s or 1 g ), and an event-trigger recorder with pre-trigger memory. Recording media was either IC-card or MO-disk. Each station was equipped with a clock calibrated by radio time signals. From January 19 to February 18, more than 80 measurements were recorded at six stations including the KOB station. We analyzed nine of the aftershock records obtained at almost all stations. 3. Data Analysis We show the epicenter distribution of the aftershocks in Fig. 2, and the hypocenter parameters determined by JMA in Table 2. In Fig. 3, we show examples of observed ground accelerations at five stations during the event at 2316 (JST) on January 25 and those from six stations during the event at 1619 (JST) on February 2. Data were recorded by either velocity seismometers or accelerometers. Data by the velocity seismometers are differentiated to accelerations in the frequency domain. The acceleration data was low-pass filtered with a cut-off frequency of 15 Hz. The horizontal peak values of ground acceleration at the soil sites are 3-20 times larger than those at the rock site (KMC) as shown in Fig. 3. In particular, Fig. 2. Location map of the aftershocks used in this study together with the KMC station. Table 2. Hypocenter parameters and trigger score of the Higashinada Ward array during the observation period January 19-February 18, Vol. 44, No. 5, 1996

4 556 T. Iwata et al. Fig. 3. Observed ground accelerations at array stations. Left: the event of 2316 (JST) on January 25, Right: the event at 1619 (JST) on February 2, From top to bottom: the NS, EW, and UD components of each station. Pasted seismic traces are arranged by S-wave onset. Triangular ticks shown in the vertical motion traces are the SP-converted waves of the bed rock and sediment interface. there are very large peak accelerations at the FKI, ASY, NOM, and FKE stations. Additionally, there is a distinct phase, especially in the vertical component between the P- and S-wave arrivals at all the soil site stations. This phase appears on all records analyzed. It is an SP phase converted at the interface of the sediment and basement rock because it is not seen at the rock site. We will estimate the thickness of sediments using this phase in Subsec The spectral amplitude ratios of the soil sites to the rock site, KMC, are given in Fig. 4. We made no correction of geometrical spreading for calculation of the spectral ratio. The time window length is s, including the S-wave part, and spectral amplitude ratios in the frequency range 1-10 Hz are shown in the figure. The ratios are calculated from the smoothed spectra of the vectorial summation of the 10% running mean of the two horizontal components. Large amplitude ratio of 3-20 times are obtained over this frequency range. Spectral ratios scatter widely among events, which may be the result of source radiation effects of the events and the complex underground structure in this array area. However, each spectral ratio of specific station pairs has an average tendency. The predominant frequencies of the ratios vary with the sites; 5 Hz for KOB/KMC, 3-5 Hz for NOM/KMC, 2 Hz for FKI/KMC, 2-4 Hz for ASY/KMC, and 2 Hz for FKE/KMC. The variation of predominant frequencies reflects the thickness variation of soft surface deposits beneath the soil sites, and the large amplifications around 2-4 Hz at the FKI, ASY, and FKE stations are strongly connected with the heavy damage in the region. 4. Discussion 4.1 Estimation of sediment thickness using SPconverted waves Here, we will estimate the thickness of sediments beneath the soil stations using SP-converted waves. We read the time difference between S and SP phases on each record by eye and show the time difference at each station in Fig. 5. The S-SP time differences take characteristic values at each station. Referring J. Phys. Earth

5 Site Amplification of Aftershocks in Severely Damaged Zone 557 Fig. 4. S-wave amplitude spectral ratios of the vectorial summation of the two horizontal components of soil sites with the KMC station (rock site) as the reference point. Spectral ratios are calculated from the smoothed amplitude spectra of the 10% running mean. Fig. 5. Time differences between S and SP phases for each station. Vol. 44, No. 5, 1996

6 558 T. Iwata et al. Fig. 6. Thickness of sediment under each observation station. Horizontal axis shows the distance from the edge of the basin. to the P- and S-wave velocity values given for the sediments of the Osaka basin modeled by Kagawa et al. (1993), we assume the S-wave velocity of 0.65 km/s and the P-wave velocity of 2.5 km/s for the sediment layer, and 3.2 and 5.7 km/s for the bedrock. We calculated the theoretical S-SP time of a one-dimensional velocity structure model using the given hypocenter for each station-event pair. For each station, we estimated the thickness of sediments so that the sum of residual time between the observed and theoretical S-SP times has a minimum value. Figure 6 shows the estimated thickness of sediment under each observation station. Thicknesses of the sediments are km. Even under the KOB station, which is the closest observation site from the edge of the basin (horizontal distance is less than 1 km), the thickness of sediment is about 0.8 km. These assumptions, such as the uniform velocity structure and the vertical incidence, are oversimplified. However, a quite important result is that the slope of the edge of the basin seems to be almost vertical as judged by the existence of SPconverted wave records at the KOB station. A reflection survey using an artificial source was done near our array site to determine the edge shape of sediments after the main shock (CEORKA, 1995, personal communication). In that cross section, there exist several thrusting-type faults around the edge of the basin and the thickness of sediments is believed to be more than 1 km. Our estimation of sediment thickness using SP-converted waves coincide with the results of the reflection survey. 4.2 Modeling of underground structures through waveform simulations We simulate the waveforms observed during the January 25 event at five stations, KMC (rock site), and KOB, FKE, ASY, and FKI (soil sites), in order to quantitatively estimate the effects of the underground structures around the soil sites. This event occurred 16 km deep and 8 km away from the KMC station as located by JMA. First, using the data at the KMC station (rock site) and an appropriate underground structure model for the rock site, the moment tensor of the event is inverted to match the waveforms. Next, underground structures for the soil sites are determined to match the synthetic waveforms to the observed records with a fixed source model obtained previously. The simulation was made using the discrete wavenumber method for a point dislocation source in a horizontally stratified medium (Bouchon, 1981). The source time function was assumed to be the derivative of 1/2 tan(t/to) with to =0.4 s. The moment tensor inversion procedure utilized is the same as that of Iwata and Kamae (1995). In Fig. 7, we show the moment tensor inversion results and comparisons between synthetic and observed waveforms. Traces for the KMC station are the inversion results. The parameters for the underground structure model are shown in Table 3. The obtained moment tensor solution is similar to the focal mechanism determined by the push-pull distribution of P-wave onsets read by high-gain seismographs (Katao, 1995). Synthetic waveforms at the soil sites are calculated using the underground structure model in Table 3 and the obtained source model. Synthetic waveforms for the ASY, FKE, and FKI stations agree well with observed records in the portions of not only the main S-wave but also the SP-converted phase. For the KOB station, we first calculated synthetic waveforms using the same underground structure model as that for the ASY, FKE, and FKI stations. However, the amplitudes of the synthetic waveforms are larger than those of the observed waveforms at the KOB station. We assumed other underground structure models only for the KOB station in Table 3. Synthetic waveforms for the KOB station explain that the SP-converted phases, however, do not match well to the observed main S-wave records. These results of forward modeling of the soil site data with a point source model suggest that large amplifications at the ASY, J. Phys. Earth

7 Site Amplification of Aftershocks in Severely Damaged Zone 559 Fig. 7. Comparison of the synthetic and observed displacements at the KMC, ASY, KOB, FKE, and FKI stations for the event at 2316 (JST) on January 25, Upper traces: synthetic. Lower traces: observed. Tick marks in UD-component indicate the phase of SP-converted waves. Synthetic waveforms at the KMC station are the result of moment tensor inversion. The obtained moment tensor solution is shown at the bottom. Synthetic waveforms for the other stations are from forward modeling with the stratified underground structure model for soil sites (Table 3) and the point source obtained from the moment tensor inversion. FKE, and FKI stations could be explained by a simple horizontally stratified underground structure consisting of deep sediments and low-velocity surface layer (one-dimensional). However, the edge shape strongly affects waveforms at the KOB station. Pitarka et al. (1996) discussed relations between waveforms and basin shape using the twodimensional finite difference method (FDM). Their simulations explain well the shape of observed waveforms, and they conclude that the low-velocity surface layers are needed to coincide waveforms. 5. Conclusions Array aftershock observations were made in Higashinada ward, Kobe City, to estimate the site responses in severely and slightly damaged areas. Five temporary stations for strong ground motion observations were installed just after the main shock Vol. 44, No. 5, 1996

8 560 T. Iwata et al. Table 3. Assumed underground structure models for the KMC station (rock site), the ASY, FKE, and FKI stations (soil sites), and the KOB station (soil site). surface conditions. We are grateful to CEORKA and its secretary, Takao Kagawa, for the assistance given. We also thank Koji Matsunami, Arben Pitarka, Haruko Sekiguchi, Shin'ichi Matsushima, and Toshimi Satoh for their help to deploy and/or maintain instruments. Kobe Medical College (KMC), Fukuike Elementary School (FKI), Seido Elementary School (ASY), Motoyama Daini Elementary School (NOM), and Kobe Merchant Marine College (FKE), kindly allowed us to install our instruments, though many refuges are still there. We are thankful to two anonymous reviewers for critically reading the original manuscript and for providing helpful suggestions. This study was partially supported by Grants-in-Aid for Scientific Research from the Japanese Ministry of Education, Science, Sports and Culture (No , , ). REFERENCES to arrange an array across the severe-damage band of about 1.5 km wide. Analyzing several aftershock data, we found that the peak values of the observed ground accelerations and velocities during aftershocks at the soil sites were 3-20 times larger than those at the rock site. The spectral ratios for the soil sites in the damaged area to the rock site are greater than 10 in the frequency range 2-4 Hz. Using SP-converted waves recorded at all soil sites, we estimated the thickness of the sedimentary basin just under the soil sites. Thicknesses of the sediments were estimated to be km under the soil sites. Waveform, simulations using a dislocation point source in a horizontally stratified medium were also performed to explain the observed records. Large amplifications at the soil sites were generated by the underground structure model with thick sedimentary layer and low-velocity surface layers. The heavy damage caused by the main shock is therefore strongly connected with the geological Bouchon, M., A simple method to calculate Green's function for elastic layered media, Bull. Seismol. Soc. Am., 71, , Cooperative Strong Motion Observation Group, Cooperative strong motion observations in the fault area of the 1995 Hyogoken-nambu earthquake, in Reports on the 1995 Hyogoken-Nambu Earthquake and Its Disaster, pp , Gao, S., H. Liu, P. M. Davis, and L. Knopoff, Localized amplification of seismic waves and correlation with damage due to the Northridge earthquake: Evidence for focusing in Santa Monica, Bull. Seismol. Soc. Am., 86B, S209-S230, Graves, R. W., Preliminary analysis of long-period basin response in the Los Angeles region from the 1994 Northridge earthquake, Geophys. Res. Let., 20, , Huzita, K. and T. Kasama, Geology of the Osakaseihokubu district, Quadrange series, scale 1 : 50,000, Kyoto 11 (50), Geol. Surv. Jpn., 112 pp., Iwata, T. and K. Kamae, Source parameter determination using regional strong motion network data, Proc. 5th Int. Conf. Microzonation, Nice, , Iwata, T., K. Hatayama, H. Kawase, K. Irikura, and K. Matsunami, Array observation of aftershocks of the 1995 Hyogoken-nambu earthquake at Higashinada ward, Kobe city, J. Natl. Disast. Sci., 16, 41-48, Osaka District Meteorological Observatory, Japan Meteorological Agency and Earthquake Prediction Information Division, Japan Meteorological Agency, The 1995 Hyogoken-Nanbu earthquake and its aftershocks, Report of the coordinating committee for earthquake J. Phys. Earth

9 Site Amplification of Aftershocks in Severely Damaged Zone 561 prediction, 54, , Geographical Survey Institute, Ministry of Construction Japan, Kagawa, T., S. Sawada, Y. Iwasaki, and J. Nanso, Modeling of the deep underground structure of the Osaka basin, Proc. 22th JSCE Earthq. Eng. Symp., , 1993 (in Japanese). Katao, H., Source mechanisms of aftershocks of the 1995 Hyogo-ken Nambu Earthquake, Report of the Coordinating Committee for Earthquake Prediction, the 115th Meeting, 1995 (in Japanese). Pitarka, A., K. Irikura, T. Iwata, and K. Kagawa, Basin structure effects in the Kobe area inferred from the modeling of ground motions from two aftershocks of the January 17, 1995 Hyogo-ken Nanbu earthquake, J. Phys. Earth, 44, , Toki, K., K. Irikura, and T. Kagawa, Strong motion records in the source area of the Hyogoken-nambu earthquake, January 17, 1995 Japan, J. Nat. Disast. Sri., 16, 23-30, Vol. 44, No. 5, 1996

STRONG GROUND MOTION PREDICTION FOR HUGE SUBDUCTION EARTHQUAKES USING A CHARACTERIZED SOURCE MODEL AND SEVERAL SIMULATION TECHNIQUES

13 th World Conference on Earthquake Engineering Vancouver, B.C., Canada August 1-6, 24 Paper No. 655 STRONG GROUND MOTION PREDICTION FOR HUGE SUBDUCTION EARTHQUAKES USING A CHARACTERIZED SOURCE MODEL