Stress field near the fault edges of a medium-sized earthquake -Possibility of stress relaxation occurring before the earthquake

Abstract

By analyzing aftershock focal mechanisms of a magnitude 5.6 earthquake, we estimated the stress field around the fault in detail, and found that the direction of the σ1 axis had deviated from horizontal near the fault ends in the hanging wall of the fault. This rotation was not seen before the earthquake, and is consistent with the stress changes caused by the earthquake. It is inferred from these results that the differential stress was very small before the earthquake near the fault ends. On the other hand, there was no rotation of the σ1 axis near the fault except for the ends of the fault, and then it is estimated that the differential stress before the earthquake at the center of the fault was quite large, much larger than the stress changes at the end of the fault. These results suggest that stress relaxation may have occurred near the fault ends before the earthquake. This is consistent with the results of the 2017 Central Tottori earthquake (M6.8), in which the stress state near the fault ends was estimated for the first time from a large amount of high-precision aftershock data. The present study is considered important because a wealth of data is available even before the earthquake, and the stress field before the earthquake can be regarded as homogeneous even near the fault, showing no rotation of the σ1 axis. In the future, it will be important to investigate whether this result holds for other earthquakes of different sizes.