Great earthquakes at island arcs and the lithosphere

Abstract

The occurrence of great earthquakes in the northwestern circum-Pacific belt is explained systematically in terms of the interaction between the oceanic and continental lithospheres. The great earthquakes in the Alaska-Aleutian region are considered to be a result of a rebound of the continental lithosphere which is dragged by the underthrusting oceanic lithosphere. The largest earthquakes in the Japan region are about one order of magnitude smaller than those in the Alaska-Aleutian region. This is interpreted as due to the weakening of the continental lithosphere caused by a frictional heating at the interface between the oceanic and the continental lithospheres. When the friction becomes very small because of the subsequent heating, a tensile force begins to prevail in the oceanic lithosphere. This tensile force is caused by a gravitational pull exerted by the sinking lithosphere. When this tensile stress surpasses the strength of the lithosphere, a large-scale normal fault occurs which extends through the entire thickness of the lithosphere. The intermittent slippages of the lithosphere on this fault plane are observed as great normal-fault earthquakes. The Sanriku earthquake of 1933 represents one of these earthquakes. The normal faulting accounts for the sharp bend of the lithosphere at the trench and the rapid increase of the dip angle of the deep seismic zone in going from northern Japan to the Izu-Bonin region. After repeated slippages, the sinking lithosphere becomes detached from the oceanic lithosphere and no further lithospheric interaction can take place. This picture is consistent with the complete lack of great shallow earthquakes in the Izu-Bonin region.