The Kuril Trench - Hokkaido Rise System: Large Shallow Earthquakes and Simple Models of Deformation

Large shallow earthquakes of the Kuril Trench are distinct from those of the adjacent Aleutian Trench in two important ways: in the case of the Kuril events, (1) the rate of occurrence of events near or oceanward of the trench axis with tensional focal mechanisms is markedly lower and (2) the average apparent stress of 14 shallow events is almost an order of magnitude higher. An explanation of these observations is that the oceanic lithosphere forming the Kuril Trench-Hokkaido Rise system is acted upon by a horizontal compressive stress of several kilobars in a direction normal to the Kuril Trench axis. Calculations based on simple models of elastic deformation support this explanation. Unless the Kuril oceanic lithosphere has a strong nonelastic response on a time scale of a few million years, a horizontal compressive stress of approximately several kilobars is necessary to maintain the Kuril Trench-Hokkaido Rise bathymetry. The oceanic lithosphere in this region has an effective (elastic) thickness of 30–40 km and is capable of supporting deviatoric compressive stresses of several kilobars. The elastic thickness of the Kuril oceanic lithosphere agrees well with the seismic thickness of the lithosphere descending beneath the Tonga arc.