[S-SS10] Seismicity

[SSS10-P06] Focal mechanisms of the double seismic zone in the Pacific plate, off Boso: Temporal change after the 2011 Tohoku EQ

*Guanyu Chen¹, Toshinori Sato², Tomoharu Sato¹, Asami Terada, Masanao Shinohara³, Kimihiro Mochizuki³, Tomoaki Yamada³, Kenji Uehira⁴, Takashi Shimbo⁵, Shuichi Kodaira⁵, Yuya Machida⁵, Ryota Hino⁶, Ryosuke Azuma⁶, Yoshio Murai⁷, Yoshihiro Ito⁸, Hiroshi Yakiwara⁹, Kenji Hirata⁴, Akinori Hashima³, Andrew M. Freed¹⁰ (1.Faculty of Science, Chiba University, 2.Graduate School of Science, Chiba University, 3.Earthquake Research Institute, University of Tokyo, 4.National Research Institute for Earth Science and Disaster Resilience, 5.Japan Agency for Marine-Earth Science and Technology, 6.Graduate School of Science, Tohoku University, 7.Institute of Seismology and Volcanology, Faculty of Science, Hokkaido University, 8.Disaster Prevention Research Institute, Kyoto University, 9.Nansei-toko Observatory for Earthquakes and Volcanoes, Kagoshima University, 10.Purdue University)

Keywords:Double seismic zone, temporal change of focal mechanism, the Pacific plate, off the Boso Peninsula, the 2011 Tohoku Earthquake

1. Introduction
The Pacific plate and the Philippine Sea plate subduct off the Boso Peninsula, and the Pacific plate has a double seismic zone at this place (e.g. Terada et. al., JpGU, 2016). Sato et. al. (JpGU, 2018) revealed that most focal mechanisms for the lower surface events have down-dip extension before the 2011 Tohoku EQ, and down-dip compression during 3 months after the EQ using land and ocean bottom seismic observations. This presentation shows 1 year temporal change of focal mechanisms for the double seismic zone’s events after the 2011 Tohoku EQ.

2. Data and analysis
The used data are waveform data from Hi-net by NIED, and the aftershock survey of the 2011 Tohoku EQ using ocean bottom seismometers (2011/3/15-2012/3/31). We select events in the double seismic zone, and pick polarities of the P wave first motions. We calculate emergent azimuth and dip of each event-receiver pair using a 3D structure determined by Terada et al. (JpGU, 2016), and search optimal focal mechanisms using a grid search method.

3. Results
During 1 year after the 2011 Tohoku EQ, most focal mechanisms have down-dip compression for the upper surface events. For the lower surface events, on the other hand, most focal mechanisms have down-dip compression by July, 2011, and many mechanisms have down-dip extension after September, 2011. This may indicate that a compressional stress change occurred at the depth of the Pacific plate just after the 2011 Tohoku EQ, because the EQ moved the Pacific plate downward at the shallower part, and compressed the deeper part of the plate. After that, the compressional stress may had relaxed about a half a year.


Acknowledgements

We thank captains and crew of aftershock observation cruises of the 2011 Tohoku Earthquake. We used waveform data from Hi-net by NIED.