Influence of fault plane heterogeneity on the seismic behavior in the Southern Kurile Islands Arc

Abstract

The seismic behavior in the southern Kurile Islands Arc is strongly influenced by spatial heterogeneity in the mechanical properties of the plate interface. Maps showing sites of maximum seismic moment release for the great underthrusting earthquakes of 1958, 1963, 1969, and 1973 provide a first‐order image of the major regions of enhanced strength, or asperities. Hypocenters of these and an additional 83 smaller earthquakes with mb ≥ 5.3 and known underthrusting mechanisms were recomputed using the method of joint hypocenter determination in order to assess better the influence that the strength distribution exerts on the interplate seismicity patterns. Nearly all of the smaller‐magnitude earthquakes locate outside of the principal asperities of the great earthquakes. This observation is consistent with substantial release of accumulated strain energy in asperity regions during the great earthquakes and a redistribution of stress in adjacent areas following their rupture. An along‐dip, depth dependent component of strength heterogeneity appears to influence the seismicity patterns. The asperities of the 1958 and 1973 great earthquakes and hypocenters of most of the Ms ≥ 7 earthquakes occur near the downdip edge of the 100‐km‐wide seismically active plate interface. This would be expected if shear resistance on the interface increases monotonically with lithostatic pressure up to the brittle‐ductile or stick slip‐stable slip transition that defines the base of the seismogenic zone. The asperities of the 1963 and 1969 earthquakes, however, appear to be located well above the downdip edge of the coupled plate interface. The occurrence of large asperities in middepth ranges of the seismogenic interface suggests the influence of along‐dip changes in other parameters, besides lithostatic pressure, that control shear resistance. Along‐strike segmentation of the Kurile Islands thrust zone may be a consequence of along‐strike alternation in the location of the largest asperities between the deep interface and the midinterface.