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Spatial distribution of earthquakes off the east coast of the Kanto region along the Japan Trench deduced from ocean bottom seismographic observations and their relations with the aftershock sequence of the 2011 off the Pacific coast of Tohoku Earthquake
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2011
Received: 22 April 2011
Accepted: 23 June 2011
Published: 27 September 2011
The 2011 Tohoku earthquake hit the forearc region of the Japan Trench on March 11, 2011. The rupture zone seemed to reach off the coast of the Kanto region. We had conducted ocean bottom seismographic observations off the coast of the Kanto in 2008 and estimated 851 hypocenter locations around the south part of the 2011 off the Pacific coast of Tohoku Earthquake by using over 50 ocean bottom seismometers (OBSs) and routine data jointly. The hypocenters distributed some clusters, and we found a few seismic gaps at the boundary zones of the clusters. The most remarkable seismic gap was positioned at the edge of intervened Philippine Sea plate (PHS) between the North American plate (NA) and Pacific plate (PAC). We compare the epicenter distributions with the 2011 aftershock distribution determined by routine data. The aftershocks are segmented spatially and there are some seismic gaps among the segments. The remarkable low places are consistent with the boundary zones of each cluster we estimated from the 2008 data. We infer that those regions have strong heterogeneity resulting from strong deformations caused by various subduction processes, such as intervening PHS between NA and PAC, seamount chains and changes in physical properties.
The 2011 off the Pacific coast of Tohoku Earthquake whose magnitude Mj was 9.0, determined by the Japan Meteorological Agency (JMA), occurred near the east coast of Honshu, Japan on March 11, 2011 in the subduction zone of the Pacific plate (PAC). The rupture spread away both north and south and a large aftershock (Mj 7.7) hit at the south end about 30 minutes after the mainshock. Seismic information such as the source mechanisms and the foci show that both events result from thrust faulting between the North American plate (NA) and the subducted PAC.
In 2008, we conducted passive and active seismic experiments in which we used 55 OBSs (Fig. 1(b)). In this paper, we show the previous ocean bottom passive seismographic observations in 2008, their results, in particular hypocenter distribution and compare them with the recent massive seismic activity.
2. Data and Method
We combined data from 55 OBSs (Kanazawa et al., 2005; Shinohara et al., 2009) and 63 routine land-based seismic stations operated by the National Research Institute for Earth Science and Disaster Prevention (NIED), JMA, Tohoku University and the Earthquake Research Institute, University of Tokyo (Fig. 1(b)), picked P and S arrival times of 1100 earthquakes manually on a computer display (Urabe and Tsukada, 1991), located foci using a nonlinear global search algorithm (Lomax et al., 2009). We assumed P-wave velocity structure based on active seismic refraction surveys (Miura et al., 2003; Nakahigashi et al., in preparation) and V p /V s ratio of 1.73 for travel time calculations. Lateral heterogeneity along the subducting direction was considered, although structural variation was neglected along the parallel trench direction. In general, structural undulation along the trench is less than along the subducting direction. In this study region, we have to pay attention to the contact zone between PAC and PHS. However, active source data show that the lateral velocity variation is smooth although the acoustic impedance contrast is clear at the boundary zone (Nakahigashi et al., in preparation). We applied time corrections for each station and each phase, to eliminating the effects of velocity uncertainly and heterogeneity, in particular just beneath the OBSs. These were derived from averaged differences between observed and calculated times in all hypocenters, and fixed after repeating the procedure five times.
3. Results and Discussion
Mechanism solutions by NIED (Fukuyama et al., 1998) with the focal depths deduced from this study (Fig. 2(b)) imply that low-angle thrust type earthquakes on the plate boundary have predominance in regions A–D. These are typical seismic patterns in subduction zones. In contrast, you can see another type of activity in region E near the coast. Vertical cross-sections (Fig. 2(c)) clearly show the discrepancy. Many hypocenters form a plane which represents the subducting PAC in regions A–D, although there are few events in the upper plate and the slab. Otherwise, the slab shape is unclear in region E, where PHS is estimated to be interposed. Focal depths are broad (6–52 km) and there are mechanism variations in the cluster near the coast and few events are in the forearc region except near the coast. The spatial differences of both hypocenter distribution patterns and mechanism solutions between regions A–D and region E seem to result from whether NA and PAC are in direct contact or PHS exists in between NA and PAC.
Mochizuki et al. (2008) proposed that subducted seamounts caused low friction in the plate boundary and created seismically quiet areas. One of these corresponds to the other seismic gap in region B in Fig. 3 and it is the boundary zone between region X and Z. Moreover, focal depths by JMA near the Japan Trench in regions B and C in Fig. 2 have remarkable variations. The fact that our results do not show the large variation in depth in the region implies that there is great heterogeneous velocity structure.
The other boundary between region S near the trench and regions X and Z seems to match the so-called up-dip limit for the seismogenic zone (e.g., Tsuru et al., 2000). Regions near trenches such as region S, are known as strong tsunami generation areas, and background seismicity is relatively low, although the aftershocks were located in them and the notion of up-dip limit should be reconsidered.
These characteristics in the boundary zones strongly suggest that structural heterogeneities are key to understanding basic information such as where, when and how earthquakes occur. We anticipate that there is some heterogeneity on the plate boundary between regions Y and Z.
We conducted ocean bottom seismographic observations in 2008 and estimated 851 hypocenter locations around the south part of the 2011 Tohoku earthquake. We found several seismic gaps and the most remarkable one is positioned near the edge of the PHS. The aftershocks are segmented spatially and there are some seismic gaps among segmentations. We infer that the seismic gaps have strong heterogeneity resulting from strong deformations. Various subduction processes such as intervened PHS between NA and PAC, seamount chains and variations in physical properties have the ability to cause the deformations.
The authors are grateful to Messrs T. Yagi, S. Hashimoto, N. Takeda, S. Suzuki, K. Suzuki, R. Amamiya and Drs. T. Shinbo, Y. Yamamoto, R. Azuma, and R. Miura for helping OBS experiments. Anonymous reviewers gave us very helpful comments. We are grateful to NIED and JMA for allowing us to use the waveform data collected at on-line seismic stations and from earthquake catalogs. This research was partially supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, and by the cooperative research program of the Earthquake Research Institute, University of Tokyo.
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