Very-low-frequency earthquakes indicate a transpressional stress regime in the Nankai accretionary prism

Abstract

We investigated the stress field within the Nankai accretionary prism of southwestern Japan, where very-low-frequency (VLF) earthquakes occur associated with thrust faulting. A northwest-southeast azimuth of the maximum horizontal principal stress previously estimated from borehole breakouts in wells drilled in the region by deep-sea drilling vessel Chikyu suggests trench-normal shortening, although strike-slip and normal faulting are also possible within a thrust-dominated tectonic environment. We estimated stress orientations and stress ratios by using stress tensor inversion to derive moment tensor solutions for VLF earthquakes in three regions along the Nankai Trough: off Kumano, off Muroto, and Hyuga-nada. The stress orientations we obtained indicate that the regions off Kumano and off Muroto are within a transpressional stress regime with trench-normal shortening, whereas the Hyuga-nada region on the westernmost edge of the Nankai accretionary prism is in a reverse-faulting regime. Copyright 2009 by the American Geophysical Union.