EVALUATION OF SITE AMPLIFICATION AND PHASE EFFECTS IN KUSHIMOTO TOWN, WAKAYAMA PREFECTURE, JAPAN

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EVALUATION OF SITE AMPLIFICATION AND PHASE EFFECTS IN KUSHIMOTO TOWN, WAKAYAMA PREFECTURE, JAPAN Yoshiya HATA 1 and Masayuki YAMADA 2 INTRODUCTION The occurrence of the 211 Tohoku earthquake (M W 9.

1 EVALUATION OF SITE AMPLIFICATION AND PHASE EFFECTS IN KUSHIMOTO TOWN, WAKAYAMA PREFECTURE, JAPAN Yoshiya HATA 1 and Masayuki YAMADA 2 INTRODUCTION The occurrence of the 211 Tohoku earthquake (M W 9.) along the Japan Trench motivated us to hypothesize an equally gigantic earthquake along the Nankai Trough, which is another plate boundary close to the coast of Japan. The evaluation of strong ground motions for such an earthquake should be based on a source model whose applicability for gigantic earthquakes has been demonstrated using existing strong motion records including those from the 211 Tohoku earthquake (Hata et al., 213a). As a preliminary study, we evaluated the site amplification and phase effects at the strong motion observation station sites in Kushimoto Town, Wakayama Prefecture. Since Kushimoto Town is located in the southernmost part of the main island of Japan, it is very close to the epicentral area of the future gigantic earthquake with M W 9. along the Nankai Trough (see Figs. 1 and 2). STRONG MOTION OBSERVATION STATIONS Distribution of the existing ground motion observation stations in Kushimoto Town is shown in Fig. 3 with a topographical map (Denshikokudo Web System) by Geospatial Information Authority of Japan. Wakayama Prefecture Kushimoto Town A site of interest Tokyo Wakayama Prefecture Figure 1 Location information. 1 Graduate School of Engineering, Osaka University, Suita, Japan, hata@civil.eng.osaka-u.ac.jp 2 Technological Development Division, Newjec Inc., Osaka, Japan, yamadams@newjec.co.jp 1

2 Kushimoto Town Moderate EQ 24/9/ 19:7 Mj7.1 SMGA Model Reference Case Cabinet office (212) Figure 2 A future gigantic earthquake along the Nankai Trough. In Fig. 3, the management organizations of the observation stations in Kushimoto Town differ (Aoi et al., 24; Uehara and Kusakabe, 24; Nishimae, 24; Shiba and Yajima, 24), and their installation condition by the topographical condition is also various. It has suggested that the site amplification and phase effects changes with each observation station sites. Distribution of the existing ground motion observation stations in Kushimoto Town with a bird s-eye view photograph is shown in Fig. 4. In Fig. 4, the conditions of the earthquake motion observation by geographical characteristics are various for every sites. For instance, the installation altitude is 49.2 m at site, 3.1 m at site, 3.1 m at K-NET Kushimoto site and 73.3 m at site. ANALYSIS OF INSTALLATION CONDITION Based on the findings acquired for the previous chapter, we conducted more detailed analysis using the existing Japanese national land information in this chapter. First, Fig. is deep underground geological map by National Institute of Advanced Industrial Science and Technology (disclosure map by AIST) with the existing ground motion observation stations in Kushimoto Town. In Fig., MLIT site, CRIEPI site and K-NET site are located in the same area of early Miocene to Middle Miocene marine and non-marine sediments. On the other hand, JMA site is located in Middle to Late Miocene non-alkaline felsic volcanic intrusive rocks area. Fig. 6 is superficial geologic map by National Land Information Division, National and Regional Policy Bureau, Ministry of Land, Infrastructure, Transport and Tourism (disclosure map by NLID-MLIT) with 4 station sites of interest. In Fig. 6, CRIEPI site and JMA site are same area which consists of Sandstone & Mudstone. However, MLIT site is located in Mudstone area, and K-NET site is located in Sand area. Next, Figs. 7 and 8 are land use maps with the station sites of interest. One of the maps (see Fig. 7) is based on NLID-MLIT (disclosure map). Another map (see Fig. 8) is published from Geospatial Information Authority of Japan, Ministry of Land, Infrastructure, Transport and Tourism (disclosure map by GIAJ-MLIT). In Fig. 7, the area where 4 sites of interest correspond is various respectively. In particular, MLIT site is located in Coniferous forest area, CRIEPI site is located in Fruit farm area, K-NET site is located in Local small city area and JMA site is located in Vegetable farm area. In Fig. 2

3 1 km N Figure 3 Distribution of strong motion observation stations with topographical map. 1 km N Figure 4 Distribution of strong motion observation stations with a bird's-eye view photograph. 3

4 Early Miocene to Middle Miocene marine and non-marine sediments Early Miocene to Middle Miocene non-alkaline felsic volcanic rocks Early Miocene to Middle Miocene non-alkaline mafic volcanic rocks Middle to Late Miocene non-alkaline felsic volcanic intrusive rocks Early to Middle Miocene mafic plutonic rocks Late Pleistocene to Holocene marine and non-marine sediments Figure Deep underground geological map with distribution of the observation stations. Sandstone Rhyolite Mudstone Porphyry Sandstone and Mudstone -1 Sand Sandstone and Mudstone -2 Gravel and Sand Figure 6 Superficial geologic map with distribution of the observation stations. 4

5 Fruit farm Local small city Coniferous forest Vegetable farm Figure 7 Land use map by National Land Information Division, National and Regional Policy Bureau, Figure 7 MLIT with distribution of the observation stations in Kushimoto Town. Plateau, Terrace Sand, Gravelbar, Gravelbank Coastal plain, Delta, Embankment Reclaimed land Mountain, Forest Figure 8 Land use map by Geospatial Information Authority of Japan, MLIT with distribution of the Figure 8 observation stations in Kushimoto Town.

6 Mountains and Hill lands Upper terrace Valley plain Cliff Delta and coastal plain Dry river-bed Middle terrace Man-made surface Figure 9 Geomorphological land classification map with distribution of the observation stations. Reddish dry brown forest soils Residnal immature soils Dry brown forest soils Sand and Gravel Yellowish dry brown forest soils Figure 1 Soil map with distribution of the observation stations in Kushimoto Town. 6

7 Site Amplification Factors 1 1 Seismic Bedrock ~ Surface Frequency (Hz) Figure 11 Comparison of site amplification factors from seismic bedrock to ground surface at the Figure 11 existing observation stations in Kushimoto Town. 8, sites of MLIT and CRIEPI are located in Mountain & Forest area. On the other hand, K-NET site is located in Reclaimed land, and JMA site is located in Plateau & Terrace area. Moreover, Distribution of 4 station sites of interest based on geomorphological land classification map by NLID-MLIT (disclosure map) is shown in Fig. 9. In Fig. 9, sites of MLIT and CRIEPI are same area which consists of Mountains & Hill lands. However, K-NET site is located in Man-made surface area. Then, JMA site is located between Cliff area and Upper terrace area. Finally, Fig. 1 is soil map focused on the surface subsoil by NLID-MLIT (disclosure map) with 4 station sites of interest. In Fig. 1, CRIEPI site and K-NET site are same area which consists of Sand & Gravel. However, MLIT site is located in Yellowish dry brown forest soils area. Furthermore, K-NET site is located in Residnal immature soils area. Therefore, based on the above-mentioned consideration result by Figs., 6, 7, 8, 9 and 1, depending on the legend of the adopted contour map, the classification of 4 observation stations of interest is various. It is suggested that a possibility that the site effects have a significant difference in 4 observation stations of interest. EVALUATION OF SITE EFFECTS IN PERMANENT STATION SITES The site amplification factor for has already been evaluated in the study by Nozu et al. (27) based on spectral inversion. However, the site amplification factors at MLIT, JMA and CRIEPI stations have not been reported yet. In this study, the spectral ratio method is applied to evaluate the site amplification factors at MLIT, JMA and CRIEPI stations. The method is based on the moderate earthquake records obtained at the reference station and the target sites simultaneously. The target sites are MLIT, JMA and CRIEPI stations. The reference station is the. For each combination of the target and the reference, the spectral ratio of the Fourier amplitude of the records at the reference station and the target site is calculated. The effects of geometrical spreading and anelastic attenuation are considered as the path effect (Boore, 1983, Petukhin et al., 23) to correct the Fourier spectra. The mean of the corrected spectral ratio (the target site / the reference station) is calculated. Finally, the site amplification factor at the target site can be obtained as the product of the site amplification factor of the reference station and the spectral ratio. Here, the frequency range for the evaluation of the site amplification factor is from.2hz to 1Hz, because the site amplification factor at the reference station is reliable in this frequency range. Fig. 11 shows the site amplification factors for the ground motion observation stations located in Kushimoto Town. As shown in Fig. 11, the characteristics of the site amplification factors are not similar between these observation station sites. 7

8 Time (s) Time (s) Figure 12 Comparison of the observed acceleration time histories during the 24 off Tokaido Earthquake at the existing observation stations which means the difference of the site phase effects due to the future gigantic earthquake along the Nankai Trough. 8

9 Time (s) Time (s) Figure 13 Comparison of the observed velocity time histories during the 24 off Tokaido Earthquake at the existing observation stations which means the difference of the site phase effects due to the future gigantic earthquake along the Nankai Trough. 9

1 km TSS-2 TSS-3 TSS- TSS-1 TSS-4 Figure 14 Distribution of the created temporary station sites. N Site Amplification Factors 1 1 TSS-1 TSS-2 TSS-3 TSS-4 TSS- Seismic Bedrock ~ Surface 1.

10 1 km TSS-2 TSS-3 TSS- TSS-1 TSS-4 Figure 14 Distribution of the created temporary station sites. N Site Amplification Factors 1 1 TSS-1 TSS-2 TSS-3 TSS-4 TSS- Seismic Bedrock ~ Surface Frequency (Hz) Figure 1 Comparison of site amplification factors at the created temporary station sites in Kushimoto Town. The acceleration and velocity time histories recorded at the stations of interest in Kushimoto Town by the 24 off Tokaido Earthquake which occurred in the epicentral area of the Gigantic Nankai Trough Earthquake (see Fig. 2) is shown in Figs. 12 and 13. Here, this approach is based on a recognization that the Fourier phase of a small event is often a good approximation of the Fourier phase of the large event at the same site, as long as the location of the small event is close to the main rupture area of the large event (e.g., Nozu and Irikura, 28; Hata et al., 211). Thus, the 24 off Tokaido Earthquake which is closest to the main rupture area of the Gigantic Nankai Trough Earthquake is selected. In Figs. 12 and 13, since the general forms of the observed seismic waveforms are various, the characteristics of the site phase effects are not similar between these observation station sites. Therefore, in Figs. 11, 12 and 13, the ground shaking characteristics in Kushimoto Town differs not only in the site amplification factor but in the site phase effect. It suggests that the observation records obtained in Kushimoto Town due to the future Nankai Trough Earthquake are greatly different. EVALUATION OF SITE EFFECTS IN TEMPORARY STATION SITES In the above mentioned ANALYSIS OF INSTALLATION CONDITION section, the permanent observation station located in the Kushimoto Town area with constitution of flatlands which huge tsunami attacks during the future gigantic earthquake along the Nankai Trough is only K-NET Kushimoto. Moreover, in the Kushimoto Town area, the classifications of geology and topography were almost same. We examined the difference in the ground shaking characteristics in the Kushimoto City area, in order to confirm the validity of the classification based on the existing maps (see Figs., 6, 7, 8, 9 and 1). In particular, as shown in Fig. 14, Temporary Station Sites (TSS) with very high density (e.g., Hata et al., 214) were created at TSS-1, TSS-2, TSS-3, TSS-4 and TSS- for 8 days considering the limitation of power supply from battery. The details of the instruments of the observation system can be found in Hata et al. (213b). As an observation result, we succeeded in observation of a moderate earthquake which occurred in Akinada (214/3/14 2:7 8km Mj6.1). The moderate earthquake was used for the evaluation of the site amplification factor at TSS-1, TSS-2, TSS-3, TSS-4 and TSS- because of its observation records at not only but also the created temporary station sites (TSS-1, TSS-2, TSS-3, TSS-4 and TSS-). In this study, then, the spectral ratio method is applied to evaluate the site amplification factors at TSS-1, TSS-2, TSS-3, TSS-4 and TSS-. The method is almost the same as the evaluation at MLIT, JMA and CRIEPI stations (see EVALUATION OF SITE EFFECTS IN PERMANENT STATION 1

11 SITES section). That is, the method is based on the moderate Akinada Earthquake record obtained at the reference station and the target sites simultaneously. The target sites are TSS-1, TSS-2, TSS-3, TSS-4 and TSS-. The reference station is the. Therefore, the evaluated site amplification factors at TSS-1, TSS-2, TSS-3, TSS-4 and TSS- were based on one spectral ratio instead of the mean of two or more spectral ratios. Fig. 1 shows the site amplification factors for the created TSS-1, TSS-2, TSS-3, TSS-4 and TSS- located in Kushimoto Town area with flatlands. As shown in Fig. 1, the characteristics of the site amplification factors are not similar in Kushimoto Town area. In Figs. 11 and 1, furthermore, the ground shaking characteristics in Kushimoto Town differs not only in the mountain area (see Fig. 11) but also in the flatland area (see Fig. 1). It suggests that the detailed investigation about the ground shaking characteristics in Kushimoto Town is required. SUMMARY AND CONCLUSIONS This manuscript mentioned the evaluation of site amplification and phase effects at the existing station sites in Kushimoto Town as a fundamental approach for calculation of strong motion estimation due to the future Gigantic Earthquake along the Nankai Trough. As a result, we indicated that the possibility that the difference in the characteristics of the strong motion in Kushimoto Town was comparatively large during the Nankai Trough Earthquake. In future study, we will approach calculation of a strong motion in Kushimoto Town using the site effects evaluated in this paper. REFERENCES Aoi, S., Kunugi, T. and Fujiwara, H. (24) Strong-motion seismograph network operated by NIED: K-NET and KiK-net, Jour. of Japan Association for Earthquake Engineering, 4(3): Boore, D. M. (1983) Stochastic simulation of high-frequency ground motions based on seismological models of the radiated spectra, Bulletin of the Seismological Society of America, 73(6A): Hata, Y., Ichii, K. and Nozu, A. (211) A practical method to estimate strong ground motions after an earthquake based on site amplification and phase characteristics, Bulletin of the Seismological Society of America, 11(2): Hata, Y., Yamada, M. and Nozu, A. (213a) Estimation of strong motion a Kushimoto Town, Higashimuro County, Wakayama Prefecture during a scenario earthquake with M W 9. along the Nankai Trough based on the SPGA model, Proc. of the Kansai Geo-Symposium 213, Hata, Y., Tokida, K. and Hayashi, K. (213b) A preliminary approach on evaluation of site amplification factors using microtremor H/V spectra, Ground Engineering, 31(1): Hata, Y., Komai, S., Kamai, T., Wang, G. and Nozu, A. (214) Strong motion estimation at residential area in Nankodai, Izumi Ward, Sendai City for the 1978 off Miyagi Prefecture Earthquake and the 211 off the Pacific coast of Tohoku Earthquake, Jour. of Japan Society of Civil Engineers A1, (4). Nishimae, Y. (24) Observation of seismic intensity and strong ground motion by Japan Meteorological Agency and local governments in Japan, Jour. of Japan Association for Earthquake Engineering, 4(3): Nozu, A., Nagao, T. and Yamada, M. (27) Site amplification factors for strong-motion sites in Japan based on spectral inversion technique and their use for strong-motion evaluation, Jour. of Japan Association for Earthquake Engineering, 7(3): Nozu, A. and Irikura, K. (28) Strong-motion generation areas of a great subduction-zone earthquake: waveform inversion with empirical Green's functions for the 23 Tokachi-oki earthquake, Bulletin of the Seismological Society of America, 98(1): Petukhin, A., Irikura, K., Ohmi, S. and Kagawa, T. (23) Estimation of Q-values in the seismogenic and aseismic layers in the Kinki Region, Japan, by elimination of the geometrical spreading effect using ray approximation, Bulletin of the Seismological Society of America, 93(4): Shiba, Y. and Yajima, H. (24) Observation network for strong motions operated by CRIEPI, Jour. of Japan Association for Earthquake Engineering, 4(3): Uehara, H. and Kusakabe, T. (24) Observation of strong earthquake motion by National Institute for Land and Infrastructure Management, Jour. of Japan Association for Earthquake Engineering, 4(3):

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