J. Phys. Earth, 44, 577-590, 1996 Analysis of Ground-Motion Amplification Characteristics in Kobe City Considering a Deep Irregular Underground Structure Interpretation of Heavily Damaged Belt Zone - during the 1995. Hyogo-ken Nanbu Earthquake-— Masato Motosaka 1,* and Masayuki Nagano 2 1 Disaster Control Research Center, Faculty of Engineering, Tohoku University, Aramaki-Aoba, Aoba-ku, Sendai 980-77, Japan 2 Kobori Research Complex , Kajima Corporation, KI-Building, Minato-ku, Tokyo 107, Japan To estimate the amplification characteristics of ground motion in the heavily damaged belt zone in Kobe City during the 1995 Hyogo-ken Nanbu earthquake, three-dimensional wave propagation analyses of a two-dimensional, deep irregular underground structure model with vertical discontinuity were performed using the hyperelement method for incident planar waves expected from the wavefields due to the source mechanism. The observation records from Kobe University, a rock site, are used as control data. The ground motion at the surface of the Osaka group layers and at ground surface are calculated, The effects of the deep irregular underground structure and shallow surface layers on ground-motion amplification are discussed. The analytical results show that ground motions in the heavily damaged belt zone was amplified due to a focusing effect in the deep irregular underground structure as well as the shallow surface layers, and that the calculated maximum acceleration distributions coincide closely with the distribution of structural damage. unknown seismic fault beneath the belt zone (e.g. 1, Introduction Watanabe et al., 1995). Some reports suggest the The characteristic damage distribution around "Nagisa phenomena" (e.g. Suzuki et al., 1995), Kobe City during the 1995 Hyogo-ken Nanbu which means the amplification of ground motion earthquake is reported for a "heavily damaged belt due to shallow surface geology near the basin edge. zone" (e.g. Takemura and Tsuji, 1995) 1-2 km wide Is the shallow surface geology the only reason for (shown in Fig. 1), where the highest death toll and the belt zone? At a very high amplitude level, most of the injured were concentrated and where maximum acceleration exceeded 800 cm/s2 and seismic intensity, as measured by JMA, was VII. maximum velocity 100 cm/s, basically causing the Seismic observations recorded very large velocity soil to become a high damping material. Therefore, and acceleration amplitudes perpendicular to the the "Nagisa phenomena" is unlikely to occur, al- Rokko faults. The maximum recorded acceleration though one-dimensional amplification could be at Fukiai (FKI), in the belt zone, exceeded 800 cm/s2 expected. Interested in the deep irregular under- and the maximum acceleration and velocity at ground structure of the northwest edge of the Osaka Takatori (TKT) exceeded 600 cm/s2 and 130 cm/s, basin, the authors performed an analytical inves- respectively, in two horizontal directions, although tigation (Motosaka and Nagano, 1995) from the the maximum acceleration observed at Kobe Uni- standpoint that the damage in the belt zone is versity (KBU), a rock-site observation station, was attributed to the amplification of ground motion about 300 cm/s2 or less and the maximum velocity due to focusing effects in the deep irregular under- was 55 cm/s (BRI, 1995; Toki et al., 1995). ground structure as well as in the shallow surface Various interpretations of the damaged belt zone layers. In this study, the focusing effect is used in a have been reported. Some reports suggest an wide meaning, including amplification due not only Received July 15, 1995; Accepted November 20, 1995 * To whom correspondence should be addressed . 577 578 M. Motosaka and M. Nagano Fig. 1. Heavily damaged zone in Kobe City during the 1995 Hyogo-ken Nanbu earthquake and schematic figure diagram of an underground structure near the Rokko fault zone. The JMA seismic intensity was VII in the hatched heavily damaged zone. Line a-a' corresponds to the analyzed section. The ratios of structural damage along line b-b' are referred to in Fig. 19. The locations of the Hankyu line (HK), JR line (JR), and Hanshin line (HS) are indicated. Route 2 (R2) is located between JR and HS. Locations of major strong-motion observation sites: Kobe University (KBU), Motoyama (KOB), Fukiai (FKI), Takatori (TKT), Shin Kobe (SKB), and the Kobe Harbor Office (KBH) are indicated. to focusing of diffracted body waves at the irregular (R43) is analytically investigated. Section 2 describes boundary and vertically propagated body waves but the underground structure model investigated. The also to the superposition of basin-induced surface analytical method is briefly described in Sec. 3. In wave and vertically propagated body waves. The Sec. 4, the ground motion at the surface of the Osaka focusing effect is also suggested by such authors as group layers for incident planar waves assumed from Akamatsu et al. (1995) and Nakagawa (1995), but the wavefield due to the source mechanism are ad- analytical investigations have not been performed. dressed. A planar 2-D analysis for vertical incident In the meantime, for investigating the structural S-waves is performed, as well as a 3-D (commonly damage to various structures and for reconstruction called 2.5-D) analysis for the oblique incident of the damaged area, it is important to evaluate the SH-wave propagating along the fault. The latter features of ground motion in the source area with incident wavefield is assumed for Kobe in the strong geological variety through simulation anal- direction of SH-wave dominance in the source ysis. In this case, it is necessary to estimate the mechanism of the earthquake; namely, the strike- strength of ground motion at the earth's surface as slip of the vertical fault. The observation records well as at the upper surface of the Osaka group obtained at Kobe University are used as control layers. data in these analyses. Section 5 discusses ground This paper describes the wave propagation in two- motion at the soil surface affected by amplification dimensional (2-D) irregular underground structure resulting from the shallow surface layers, indicating models of an orthogonal cross section of the fault strong nonlinearity. Section 6 discusses the sensitiv- through Kobe University in Nada ward (refer to ity of the adopted Q-values to the maximum ground line a-a' in Fig. 1). The amplification characteristic acceleration. The maximum values obtained analyt- of ground motion in the heavily damaged zone from ically are compared to the observation records and the Hankyu line (HK) through the JR line (JR), the estimated distribution of the maximum accelera- Route 2 (R2), and Hanshin line (HS) to Route 43 tions are compared to the distribution of structural J. Phys. Earth Effects of Deep Irregular Underground Structures on Ground Motion in Kobe City 579 damage. Section 7 makes concluding remarks. clear geological discontinuity is seen due to the fault between the mountain area and the sediment. Figure 2. Underground Structure and Analytical Model 2 shows the bedrock depth contours of the Osaka With regard to the deep underground structure basin based on a geological survey and seismic of the Hanshin area, it is suggested that the bedrock refraction data (Iwasaki et al., 1995). In particular, comprised of Rokko granite has a very steep gra- the boring data indicating 724 m at Ninomiya-cho, dient at the foot of the Rokko mountain area, Chuo-ku, Kobe (Iwami, 1987) is added in the figure. corresponding to the edge of the Osaka basin. A The gravity anomaly around the Osaka basin (e.g. Fig. 2. Contours of depth to the bedrock in the Osaka basin based on refraction and boring data (after Iwasaki et al., 1995). The bedrock depth of-0.72 kin at Ninomiya-cho in Kobe, based on boring data, is added (after Iwami, 1987). Fig. 3. Seismic reflection profile along the Ishiyagawa line by the Committee of Earthquake Observation and Research in the Kansai Area (after Iwasaki et al., 1995). Vol. 44, No. 5, 1996 580 M. Motosaka and M. Nagano Kobayashi et al., 1995) also suggests a vertically reflection survey (refer to Fig. 3) made by the discontinuous deep underground structure along the Committee of Earthquake Observation and Re- south foot of the Rokko mountain range. This search in the Kansai Area (CEORKA) suggest that suggests that there are thick sediment layers con- there is a major vertical discontinuity almost sisting of: from the top, alluvium, diluvium, Osaka orthogonal to the Rokko fault, beneath the HK and Kobe group layers and weathered granite on along the Ishiyagawa measuring line including the the granite bedrock south of the vertical discon- Kobe University site (Iwasaki et al., 1995). The tinuity. velocity structure, up to a depth of GL-70 in below In numerical modeling of a 2-D section orthog- the strong-motion site at Kobe University, has also onal to the Rokko fault, the following geological been investigated by CEORKA (Iwasaki et al., information is taken into account. The results of a 1995). Fig. 4. Two-dimensional models of an underground structure orthogonal t M o the Rokko fault plane. (a) odel-1 and (b) Model-2. HK, JR , R2, HS, and R43 indicate Hankyu line, JR line line , Route 2, Hanshin , and Route 43 (Harbor Expressway), respectively . The 2.5-D wave propagation characteristics are investigated for the incident SH-wave propagating along the unde i rground structure with a vertical ncident angle of ƒÆ. The incident wave is assumed from the wavefield from th e source mechanism of nearly strike-slip on a vertical fault . J. Phys. Earth Effects of Deep Irregular Underground Structures on Ground Motion in Kobe City 581 Table 1. Soil profiles of rock site in Model-1 and Model-2.