Seismic and aseismic slip along subduction zones and their tectonic implications

Abstract

Results of detailed mechanism studies of great earthquakes are used together with their repeat times to determine the amount of seismic slip along various subduction zones. Comparison of the seismic slip with the rate of plate motion suggests that, in Chile, and possibly Alaska, the seismic slip rate is comparable to the rate of plate motion while, in the Kuriles and Northern Japan, the seismic slip constitutes only a very small portion, approximately 1/4, of the total slip. In the Sanriku region, and to the south of it, the relative amount of seismic slip is even smaller. These results suggest that in Chile and Alaska the coupling and interaction between the oceanic and continental lithosphere are very strong, resulting in great earthquakes with a very large rupture zone, and in break-off of the undergoing lithosphere at shallow depths. In the Kuriles and Northern Japan, the oceanic and continental lithosphere are largely decoupled, so that the slip becomes largely aseismic, and the rupture length of earthquakes reduced. The reduced interaction at the inter-plate boundary may allow the oceanic lithosphere to subduct more easily and to form a continuous Benioff zone extending to depths. It may also facilitate ridge subduction beneath island arcs, which may play an important role in the formation of marginal seas such as the Japan Sea. The decoupling is also evidenced by silent or tsunami earthquakes [e.g., the 1896 Sanriku earthquake], great intra-plate normal-fault earthquakes [e.g., the 1933 Sanriku earthquake], and crustal deformation. A natural extension of this concept of inter-plate decoupling is the spontaneous sinking of the oceanic lithosphere with a consequent retreating subduction. Retreating subduction may be an important mechanism in the format ion of marginal seas such as the Philippine Sea, and explains the complete lack of major shallow earthquake activity along some subduction zones such as the Izu-Bonin-Mariana arc.