Double seismic zones: Kinematics

Abstract

The kinematics of a double seismic zone of subducting lithosphere at an intermediate depth is studied. A simple geometrical argument shows that the strain rate due to unbending of subducting lithosphere can be as large as 10−15 s−1 and large enough to account for the seismic strain rate in this depth range. The geometrical unbending of subducting lithosphere in the mantle and the presence of a double seismic zone there can be, therefore, considered essentially equivalent phenomenon. We model a double seismic zone required from this unbending in order to investigate what constraint the presence of a double seismic zone can place on the properties of the descending lithosphere. The thickness of a double seismic zone is controlled by the thermal structure and the rheological properties of the subducting slab. A preliminary result for the double seismic zone beneath Tohoku, Japan, indicates a possibility that the currently available flow law of “dry” olivine does not properly describe the double seismic zone or at least presents some inconsistency in our knowledge of the mechanical properties (e.g., thermal structure, rheology) of oceanic lithospheres.