Detecting the stress anomaly before occurrence the 2016 Kumamoto Earthquake, Kyushu Island, Japan

To evaluate future seismic activity around fault zones, understanding the stress state and its buildup is crucial. There are two major factors of earthquake occurrence, the stress concentration and the fault strength weakening, but it is difficult to distinguish between the two. Therefore, when estimating the stress field around the fault zone, it is important to consider the stress concentration prior to the earthquake. In this study, we modeled the stress field incorporating pre-seismic stress concentration at the focal area of the 2016 Kumamoto earthquake (Mj 7.3), Kyushu, Japan, using focal mechanism data and slip distribution on the mainshock fault plane. The estimated stress field shows the presence of stress concentration around the earthquake fault, corresponding to approximately 29% of the co-seismic stress change before the earthquake sequence. This stress concentration may reflect the strength heterogeneity and crustal deformation in the upper and lower crust, as suggested in previous studies. In addition, we also found the stress concentration after the mainshock, and its stress concentration may contribute to aftershock and post-seismic deformation.