Ground Motion Characteristics During the 2011 Off the Pacific Coast Of
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Soils and Foundations 2012;52(5):769–779 The Japanese Geotechnical Society Soils and Foundations www.sciencedirect.com journal homepage: www.elsevier.com/locate/sandf Ground motion characteristics during the 2011 off the Pacific Coast of Tohoku Earthquake Hiroyuki Gotoa,n, Hitoshi Morikawab aDisaster Prevention Research Institute, Kyoto University, Japan bDepartment of Built Environment, Tokyo Institute of Technology, Japan Received 12 January 2012; received in revised form 9 May 2012; accepted 20 June 2012 Available online 20 December 2012 Abstract The 2011 off the Pacific Coast of Tohoku Earthquake ðMW 9:0Þ caused severe damage not only because of the great tsunami, but also because of the ground motions. In the present paper, we report some details of the source mechanism and the ground motions during the earthquake, and discuss the ground motion characteristics related to the damage in the Tsukidate and Furukawa areas. Damage grades in Tsukidate, where the largest peak ground accelerations were observed, are not severe in comparison to those in Furukawa. The pseudo-velocity response spectrum for the ground motions observed in the Furukawa area is similar to that in the JR Takatori records for the 1995 Kobe earthquake. & 2012 The Japanese Geotechnical Society. Production and hosting by Elsevier B.V. Open access under CC BY-NC-ND license. Keywords: The 2011 off the Pacific Coast of Tohoku Earthquake; Earthquake; Ground motion; Peak ground acceleration; Pseudo velocity response spectrum; Frequency contents (IGC: A-01, C-00) 1. Introduction accelerations of over 980 cm=s2, and two stations observed seismic intensities of over 6.5 on the Japan Meteorological On 11 March 2011, a large earthquake hit the eastern Agency (JMA) scale. It should be noted that the damaged part of mainland Japan. The earthquake, and particularly areas do not correspond to the locations for which the the great tsunami caused by it, resulted in the death of observing the large accelerations and intensities. more than ten thousand persons. Structures were also Residential damage, due to the ground motions, was damaged not only due to the tsunami, but also due to the concentrated in the Furukawa area of Osaki City and the ground motions, e.g., elevated bridges of the Tohoku Sanuma area of Tome City, which are both located in Shinkansen (Takahashi, 2011) and geotechnical structures inland areas of Miyagi Prefecture (Goto, 2011). The (Tohata et al., 2011). Strong ground motions during the seismic intensities observed at the seismic stations located earthquake were observed over almost the entire county of in these areas were in the range of 5.5–6.5. On the other Japan. At least 18 stations observed peak horizontal hand, no significant damage was reported in the Tsukidate area of Kurihara City, where a seismic intensity of over 6.5 nCorresponding author. was observed (Goto, 2011). This means that the damage E-mail address: goto@catfish.dpri.kyoto-u.ac.jp (H. Goto). grades are not consistent with the JMA intensities. Peer review under responsibility of The Japanese Geotechnical Society. In this paper, we review the source process and the ground motions during the earthquake, and then discuss the ground motion characteristics related to the damage in the Tsukidate and Furukawa areas. 0038-0806 & 2012 The Japanese Geotechnical Society. Production and hosting by Elsevier B.V. Open access under CC BY-NC-ND license. http://dx.doi.org/10.1016/j.sandf.2012.11.002 770 H. Goto, H. Morikawa / Soils and Foundations 52 (2012) 769–779 2. Characteristics of source process earthquake were not evaluated. The earthquake ruptured over several segments, which had been evaluated as The earthquake, officially named ‘‘The 2011 off the independent earthquakes. Pacific Coast of Tohoku Earthquake’’ by JMA, occurred Fig. 2 shows the distribution of seismic intensities on the at 14:46 (JST, GMTþ9) on 11 March 2011. The hypo- JMA scale. Intensities over 6.5 were recorded at K-NET center was located in the offshore region of the eastern MYG004, located in Kurihara City of Miyagi Prefecture, part of mainland Japan, and the depth was 24 km. The and at KiK-net TCGH16, located in Haga Town of moment magnitude ðMW) was 9.0 (Hirose et al., 2011), Tochigi Prefecture. Intensities over 6.0 were widely making it the largest earthquake observed in Japan since recorded in Miyagi, Fukushima, Ibaraki, and Tochigi 1900, and the fourth largest earthquake in the world since Prefectures (see Fig. 1)(Hoshiba et al., 2011). The 1900. Fig. 1 shows the distribution of aftershocks occur- observation of an intensity over 6.5 occurred for only the ring within 24 h after the main shock, which almost covers second time since the 1995 Kobe earthquake in Japan; the the fault zone of the main shock. This implies that the other was the record in Kawaguchi during the Mid-Niigata dimensions of the fault zone were about 500 km in length Prefecture earthquake of 2004. The intensity distribution is along the coast line by about 200 km in width. Fig. 1 also not simply attenuated from the east coast, but also varies shows the centroid moment tensor (CMT) solutions strongly along the coast line. This implies that the source estimated by JMA (Hirose et al., 2011) and F-net orga- rupture process was not spatially uniform. nized by the National Research Institute for Earth Science Fig. 3 shows the envelopes of the borehole accelero- and Disaster Prevention (NIED). They indicate that the grams observed by KiK-net (NIED). In order to highlight mechanism was a reverse fault with a compressional axis in the high frequency components, the accelerograms are an almost east-to-west direction, which is estimated as filtered in the range of 0.5–10 Hz, and the envelopes are N 1031E by JMA and N1101E by NIED. The location of averaged with moving windows of 2.0 s. Stations were the seismic fault and the mechanism imply an inter-plate selected to be almost parallel to the strike direction of the earthquake on the plate boundary between the North seismic fault estimated by JMA (N1931E). First and American plate and the Pacific plates. second wave groups, with an interval of about 50 s, were The Headquarters for Earthquake Research Promotion mainly observed at the northern stations, e.g., wave groups had warned of the occurrence of an earthquake in offshore being observed at the earliest time at IWTH05. The wave Miyagi Prefecture with a probability of 99% and a scale of groups are not clear at the southern stations, but a third MJMA 7.5 within 30 years. The event was expected to be wave group was mainly observed at the southern stations, similar to the Miyagi-oki earthquake ðMW 7.6) of 1978. e.g., wave groups being observed at the earliest time at However, the events corresponding to the Tohoku TCGH13. This implies that the rupture process of the earthquake involved at least three significant events radiat- ing high-frequency energy. The first two pulses mainly CMT solution (NIED) 40° Iwate Prefecture 40° Miyagi Prefecture JMA intensity 38° 7.0 Fukushima Prefecture 38° 6.5 Tochigi Prefecture 6.0 CMT solution (JMA) Ibaraki Prefecture 5.5 36° depth [km] 02550 36° 5.0 100 km 100 km 4.5 138° 140° 142° 144° 138° 140° 142° 144° Fig. 1. Aftershock distribution within 24 h after the main shock, and CMT solutions estimated by JMA and F-net (NIED). Fig. 2. Distribution of seismic intensities on the JMA scale. H. Goto, H. Morikawa / Soils and Foundations 52 (2012) 769–779 771 0.5−10Hz envelope (smoothing in 2s window) 500 IWTH08 IWTH13 IWTH08 IWTH02 40° IWTH13 400 IWTH02 IWTH04 IWTH04 IWTH05 IWTH05 300 MYGH06 MYGH06 MYGH08 MYGH08 38° MYGH09 [km] MYGH09 200 FKSH17 FKSH18 FKSH17 FKSH09 FKSH18 FKSH11 FKSH09 IBRH12 FKSH11 TCGH13 100 TCGH16 IBRH12 IBRH11 TCGH13 IBRH19 TCGH16 36° IBRH11 0 IBRH19 100 km 0 50 100 150 200 140° 142° time [s] Fig. 3. Envelopes of borehole accelerograms in EW component observed by KiK-net (NIED). Stations are selected to be almost parallel to the strike direction of the seismic fault. affected the ground motions observed in Iwate and Miyagi close to each other (Asano and Iwata, in press; Kurahashi Prefectures, while the third pulse affected the ground and Irikura, 2011). This means that a time lag of about 50 s motions observed in Ibaraki and Tochigi Prefectures. was required between the events. Goto et al. (in press) Several research groups have reported on the source suggests that reflection waves from the free surface affected process during the earthquake. GPS data and low- the generation of the latter event. frequency ground motions imply that a large slip region is located on the shallower side of the seismic fault (e.g. Miyazaki et al., 2011; Suzuki et al., 2011; Yoshida et al., 3. Characteristics of ground motions 2011). They estimated a maximum slip of about 35–50 m, which is almost in the same order as the geodetic data Ground motions during the earthquake were observed observed just above the hypocenter (Sato et al., 2011). On by several seismic networks organized by NIED, JMA, the other hand, ground motions corresponding to the Port and Airport Research Institute (PARI), Building frequency components higher than 0.1 Hz imply that Research Institute (BRI), National Institute for Land several strong motion generation areas (SMGAs) are and Infrastructure Management (NILIM), and other located on the deeper side of the seismic fault (e.g. Asano organizations. Figs. 4 and 5 show the distributions of peak and Iwata, in press; Kurahashi and Irikura, 2011). A high- ground accelerations (PGA) and peak ground velocities frequency radiation area is also estimated by the back- (PGV), respectively, in the horizontal components calcu- projection method to be located on the deeper portion of lated from the available records of the ground motion.