JSPS Research Fellow (PD) at Disaster Prevention Research Institute, Kyoto University (京都大学防災研究所 地震災害研究センター 特定研究員(学振PD) )
【Title】
Spatial correlation of the shear strain energy change and the number of declustered crustal earthquakes in Japan
【Abstract】
The spatial correlation of geodetically estimated strain rate and the background seismicity has been discussed in several areas worldwide. In Japan, however, there are azimuthal differences between the geodetically estimated maximum contraction strain rate and the seismologically estimated maximum compression stress. Thus, the increase in strain may not necessarily represent stress accumulation. In this study, we compare the number of background events for the crustal seismicity in Japan with the shear strain energy change which considers the background stress field and the current crustal deformation.
We estimated the strain rate field using geodetically-estimated secular velocities according to the method using the basis function expansion. We calculated the stress change using the estimated strain tensor assuming the state of plane stress and evaluated the shear strain energy change considering the background stress field estimated by Uchide et al. (2022) at  intervals. We used  shallow ( km depth) crustal earthquakes and applied the nearest neighbor distance approach to decluster the catalog during 2002—2010.
The shear strain energy change is weakly positively correlated with the number of background earthquakes, and especially in SW Japan, this correlation is significantly better compared with the correlation between the strain rate and seismicity. In some areas, the seismicity rate is remarkably high compared with that expected from linear regression with shear strain energy change. These regions may be affected by crustal fluids that possibly weaken the fault strength.
