Asperity distribution and earthquake occurrence in the southern Kurile Islands arc

The seismicity in the southern Kurile Islands arc is characterized by a complex overlapping of aftershock zones of a few great and many large earthquakes (Ms [greater, double equals] 7.0). The spatial relationships of these aftershock zones can be explained in terms of a simple pattern of fault plane heterogeneity. The four largest earthquakes in this region are, from north to south, the 1963 Kurile Islands (Mw = 8.5), the 1958 Etorofu (Mw = 8.3), the 1969 Kurile Islands (Mw = 8.2) and the 1973 Nemuro-Oki (Mw = 7.8) events. Detailed body wave studies indicate that the moment release associated with each event is concentrated in a small portion of the aftershock area, with scale lengths Ms [greater, double equals] 7.0) earthquakes both before and after the great events. The second mode involves either rupture of relatively weak regions which results in low-stress-drop events, or rupture of smaller asperities. The earthquakes produced by failure of small asperities tend to repeatedly rupture the same regions, and they have impulsive source time functions indicating rupture of highly stressed regions of the fault surface. By analysis of all Ms [greater, double equals] 7.0 events since 1946 in this region we find a segmentation of the southern Kurile Islands thrust zone into strongly coupled regions with two scale lengths (dominant and secondary asperities) and weakly coupled regions (the surrounding areas generating the low-stress-drop events). The largest earthquakes and the smaller asperity earthquakes occur at the down-dip portion of the lithospheric interface, while the low-stress-drop events occur much closer to the trench axis. This implies a difference in the degree of mechanical coupling between the subducting and overriding plates, with the deeper portions having greater strength than the shallower parts.