Keywords:Stress drop, Frictional properties on the Pacific plate, Spatial heterogeneity
We analyzed stress drops of 721 earthquakes with the magnitude of 4.0 to 5.0 off south-east of Hokkaido, Japan and investigated the spatial heterogeneity of the difference of shear strength and dynamic stress level on the Pacific plate. We deconvolved observed P and S waves with those of collocated small earthquakes and derived the source effect of the earthquakes. We then estimated corner frequencies of the earthquakes and calculated stress drops by using a circular fault model. The values of stress drop showed a spatial pattern consistent with slip distributions of historical large earthquakes. This suggests that frictional properties on the plate interface show little temporal change and their spatial pattern can be monitored by stress drops of moderate-sized earthquakes. The spatial heterogeneity would give clues for estimating the slip pattern of a future large earthquake and discussing a policy for the disaster mitigation, especially for regions where slip patterns of historical large earthquakes are unclear.