The Delayed Decrease of Seismicity In The Eastern Margin of The Japan Sea Due To The Megathrust Event In 2011 Along The Japan Trench

In the eastern margin of the Japan Sea, off the west coast of Tohoku district, the seismicity increased right after the M9 megathrust event off the east coast of the Tohoku district on March 11, 2011. Four months later, the seismicity decreased to the half level of that before the M9 event. Such quantitative study was done by the point-process model selection with AIC. The decrease lasted for eight years until an M6.7 event occurred within the area in 2019. When we compare the seismicity change between before and after the M9 event, with the post seismic change of the maximum shear stress obtained by the viscoelastic simulation for a thousand years after the M9 event, we can estimate a loading rate of the shear stress in the area before the M9 as 24 kPa/y. For the term after the M9 event, the rate is a half of it; 12 kPa/y. When we assume the whole dilatation change due to the M9 event had been canceled by the time of the M6.7, the increasing rate of the mean stress after the M9 event is 21 kPa/y at most. When we will be able to use JMA catalog for 2020 or later years, we can obtain the seismicity level after the M6.7 quantitatively, and we will be able to narrow down this estimation.