10:45 AM - 12:15 PM
[SSS10-P12] Temporal change in focal mechanisms of double seismic zone’s events in the Pacific plate after the 2011 Tohoku EQ
Keywords:double seismic zone, focal mechanism, the 2011 Tohoku Earthquake, Pacific plate, ΔCFF
1. Introduction
The slab of the Pacific plate subducting at the Japan trench has a double seismic zone. Sato et al. (JpGU, 2018) and Chen et al. (JpGU) reported focal mechanisms in the double seismic zone off the Boso Peninsula before and after the 2011 Tohoku Earthquake using land and ocean (ocean bottom seismometers) seismic stations. Temporal change of the focal mechanisms after the 2011 Tohoku EQ was as follows; Both in the upper and lower events, down-dip compression was dominant just after the Tohoku EQ, and then down-dip extension was dominant. This study investigates temporal change of the focal mechanisms in the double seismic zone beneath Tohoku area before and after the Tohoku EQ. We also study the effect of the Tohoku EQ and its afterslip on the double seismic zone.
2. Data and analysis methods
We used event data from Hi-net and CMT solutions of mechanisms from F-net. Selected events are located 37.5-39.5N, 139-142E, and more than 60 km depth. This area is west side of the maximum slip area of the Tohoku EQ. We used events occurred between March 2010 and March 2015 (5 years), and larger than M2.0. We used TSEIS (Tsuruoka, 1998) for this selection.
If CMT solution of events exists (commonly, events larger than M3.5 have CMT solution), we used this. For the events smaller than M3.5, we obtained event data (wave data) from Hi-net, and analyzed using WIN system (Urabe and Tsukada, 1992). From the data of the first motion, we estimated focal mechanisms using the FPFIT program (Reasenberg and Oppenheimer, 1985).
We also calculated ΔCFF on each event in the double seismic zone by the Tohoku EQ and its afterslip using Coulomb3.3 (Toda et al. 2011).
3. results
During 1 year before the Tohoku EQ, down-dip compression was dominant in the upper events, and down-dip extension was dominant in the lower events. These can be explained by unbending of the subducting slab.
After the Tohoku EQ, in the upper events, down-dip compression was dominant just after the EQ. But, 4 to 8 months after the EQ, down-dip extension was dominant, and 8 to 12 months, down-dip extension and compression events occurred evenly. 1 to 4 years after the EQ, down-dip compression was dominant, which is the same situation before the EQ. In the lower events, down-dip compression was dominant just after the EQ. Then, down-dip extension was dominant gradually. 1 to 4 years after the EQ, the situation was the same as that before the EQ.
The result of ΔCFF shows that the dominance of compression in the upper and lower events just after the EQ is the effect of the Tohoku EQ. On the other hand, events in 4 to 12 months after the EQ can not be explained by the ΔCFF caused from the Tohoku EQ and its afterslip. But, the ΔCFF caused by only the afterslip can explains the extension in the upper events in 4 to 8 months. This result suggests that the effects of the Tohoku EQ disappear in a short period of several months, and stress relaxation occurs rapidly.
Acknowledgment
We used the data of Hi-net and F-net by NIED, WIN system and TSEIS by ERI, Univ. Tokyo, and Coulomb 3.3 program by USGS.
The slab of the Pacific plate subducting at the Japan trench has a double seismic zone. Sato et al. (JpGU, 2018) and Chen et al. (JpGU) reported focal mechanisms in the double seismic zone off the Boso Peninsula before and after the 2011 Tohoku Earthquake using land and ocean (ocean bottom seismometers) seismic stations. Temporal change of the focal mechanisms after the 2011 Tohoku EQ was as follows; Both in the upper and lower events, down-dip compression was dominant just after the Tohoku EQ, and then down-dip extension was dominant. This study investigates temporal change of the focal mechanisms in the double seismic zone beneath Tohoku area before and after the Tohoku EQ. We also study the effect of the Tohoku EQ and its afterslip on the double seismic zone.
2. Data and analysis methods
We used event data from Hi-net and CMT solutions of mechanisms from F-net. Selected events are located 37.5-39.5N, 139-142E, and more than 60 km depth. This area is west side of the maximum slip area of the Tohoku EQ. We used events occurred between March 2010 and March 2015 (5 years), and larger than M2.0. We used TSEIS (Tsuruoka, 1998) for this selection.
If CMT solution of events exists (commonly, events larger than M3.5 have CMT solution), we used this. For the events smaller than M3.5, we obtained event data (wave data) from Hi-net, and analyzed using WIN system (Urabe and Tsukada, 1992). From the data of the first motion, we estimated focal mechanisms using the FPFIT program (Reasenberg and Oppenheimer, 1985).
We also calculated ΔCFF on each event in the double seismic zone by the Tohoku EQ and its afterslip using Coulomb3.3 (Toda et al. 2011).
3. results
During 1 year before the Tohoku EQ, down-dip compression was dominant in the upper events, and down-dip extension was dominant in the lower events. These can be explained by unbending of the subducting slab.
After the Tohoku EQ, in the upper events, down-dip compression was dominant just after the EQ. But, 4 to 8 months after the EQ, down-dip extension was dominant, and 8 to 12 months, down-dip extension and compression events occurred evenly. 1 to 4 years after the EQ, down-dip compression was dominant, which is the same situation before the EQ. In the lower events, down-dip compression was dominant just after the EQ. Then, down-dip extension was dominant gradually. 1 to 4 years after the EQ, the situation was the same as that before the EQ.
The result of ΔCFF shows that the dominance of compression in the upper and lower events just after the EQ is the effect of the Tohoku EQ. On the other hand, events in 4 to 12 months after the EQ can not be explained by the ΔCFF caused from the Tohoku EQ and its afterslip. But, the ΔCFF caused by only the afterslip can explains the extension in the upper events in 4 to 8 months. This result suggests that the effects of the Tohoku EQ disappear in a short period of several months, and stress relaxation occurs rapidly.
Acknowledgment
We used the data of Hi-net and F-net by NIED, WIN system and TSEIS by ERI, Univ. Tokyo, and Coulomb 3.3 program by USGS.