Weiren Lin (Group Leader) from Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology (JAMSTEC: Asahiko Taira, President) and other researchers have analyzed the geophysical logging data collected by the Integrated Ocean Drilling Program (IODP) Expedition 343 (JFAST: Japan Trench Fast Drilling Project) conducted by Deep Sea Drilling Vessel Chikyu and have determined the in-situ stress state in the frontal prism of the Japan trench subduction zone.

On the basis of the horizontal stress orientations and magnitudes estimated from borehole breakouts, and the increase in coseismic displacement during propagation of the rupture to the trench axis, in-situ horizontal stress decreased during the earthquake. The stress change suggests an active slip of the frontal plate-interface consistent with coseismic fault weakening and a nearly total stress drop.

They found that such coseismic stress release is able to occur in the shallow portion of the subduction zone, where previously it had been thought that the stress and energy would not accumulate; consequently seismic slip would not occur. These results are the first to substantiate that energy does accumulate and to explain that a large slip could occur even for a shallow portion of the plate-boundary fault.

Press Release, February 14, 2013