Double seismic zones: kinematics,

Kawakatsu, H.
J. Geophys. Res., 91, 4811-4825, 1986.

Abstract

Oceanic lithosphere bends above a depth of \*~50 km, when it subducts into the mantle. The fact that the subducting lithosphere is almost straight in the mantle below a depth of \*~200 km indicates that the lithosphere must unbend in a depth range from \*~50 km to \*~200 km to become straight. Therefore unbending of the subducting oceanic lithosphere is taking place at intermediate depth range of the Wadati-Benioff zones. A simple geometrical argument shows that the strain rate due to this unbending can be as large as $10 sup -15 s sup -1$ and large enough to account for the earthquake energy release in this depth range. This paper suggests that the so-called double seismic zone of intermediate earthquakes is a result of this unbending of the subducting lithosphere and therefore is a natural consequence of the theory of plate tectonics. Following Tsukahara [1980], a double seismic zone caused by this unbending is modeled in order to investigate what constraint the presence of a double seismic zone can place on the properties of the descending lithosphere. It will be shown that the thickness of a double seismic zone is strongly influenced 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 of oceanic lithospheres.