Spatial pattern in stress drops of moderate-sized earthquakes on the Pacific Plate off the south-east of Hokkaido, Japan: implications for the heterogeneity of frictional properties

We show that the spatial heterogeneity in the coseismic displacement of large earthquakes likely reflects the spatial characteristics of the frictional properties and that it can be inferred from the stress drop of moderate-sized earthquakes. We analyzed stress drops of 686 earthquakes with magnitudes of 4.0 to 5.0 off the south-east of Hokkaido, Japan, and investigated the spatial heterogeneity between 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 the corner frequencies of the earthquakes and calculated stress drops using a circular fault model. The values of stress drops showed a spatial pattern consistent with slip distributions of historical large earthquakes. Earthquakes that occurred in the area with a large coseismic slip during the 1968 Tokachi-oki (MW 8.2) and the 2003 Tokachi-oki (MW 8.0) earthquakes had large values of stress drop, whereas earthquakes in the afterslip area of the 2003 Tokachi-oki earthquake showed smaller values. In addition, an area between coseismic ruptures of the 1973 Nemuro-oki (MW 7.8) and the 2003 Tokachi-oki earthquakes had a large value of stress drop. Ruptures occurred in this area during the 1952 Tokachi-oki earthquake (MW 8.1), and the area acted as a barrier during the 2003 Tokachi-oki earthquake. These facts suggest that the frictional properties of the plate interface show little temporal change, and their spatial pattern can be monitored by stress drops of moderatesized earthquakes. The spatial heterogeneity provides important information for estimating the slip pattern of a future large earthquake and discussing a policy for disaster mitigation, especially for regions in which slip patterns of historical large earthquakes are unclear.