[S-SS06] Statistical seismology and underlying physical processes

[SSS06-05] Hypocenter determination assuming a 1-D velocity model with station corrections and its result for the Tohoku region

*Youichi Asano¹ (1.National Research Institute for Earth Science and Disaster Resilience)

Keywords:hypocenter determination, station correction, Tohoku earthquake, interplate earthquake, intraplate earthquake

We have evaluated station corrections for our routinely hypocenter determination assuming a 1-D velocity model. At first, we estimated centroid moment tensors (CMTs) for 653 earthquakes in the period of 2016-2020 from the waveform data of the F-net broadband seismometers and Hi-net tiltmeters. Next, we estimated hypocenters of these earthquakes from picked P- and S-wave arrival times at the S-net, Hi-net, and other stations operated by the JMA, ADEP, Hokkaido University, Hirosaki University, Tohoku University or University of Tokyo. Station corrections were evaluated from the residuals of this hypocenter determination under a constraint of hypocentral depths fixed to centroid depths of CMTs. Considering epicentral distance dependency of the residuals for shallow earthquakes, we used the residuals for deeper earthquakes than 20 km and longer epicentral distance than 75 km. We applied these station corrections to our hypocenter determination and relocated earthquakes used for evaluating these station corrections. The estimated depths of the earthquakes mainly located within 10 km from their centroid depths showed effectiveness of our station corrections. Finally, we analyzed 3019 earthquakes which occurred off Fukushima and Miyagi in the period from June, 2016 to February 14, 2021. In the obtained results, we can see an ENE dipping earthquake cluster probably just along the boundary between the subducting Pacific Plate and the overriding plate. Some earthquakes probably along the plate boundary are also distributed in focal area of the 2011 Tohoku earthquake. Furthermore, the 2021 off Fukushima earthquake with M_JMA 7.3 and its aftershocks are distributed below the dipping earthquake cluster. Therefore, these earthquakes are considered to be intraplate events in the subducting Pacific Plate.