3/11 seabed ruptures rattle quake-fault theory

Kyodo

Fault ruptures in the seabed caused by the Great East Japan Earthquake in March 2011 occurred in shallows where two plates meet, calling into question the common notion that fault slips are unlikely, a study says.

The study by the Japan Agency for Marine-Earth Science and Technology, published online Sunday by Nature Geoscience, also may reveal another factor behind the massive tsunami generated by the magnitude 9.0 quake, the agency said.

A research team from the agency conducted an acoustic wave survey last year of the seabed near the Japan Trench, some 250 km off the coast of Miyagi Prefecture, and compared its data to the data from 1999.

The agency found upheavals on the seabed, which was nearly flat in 1999, and signs that fault ruptures reached the seabed’s surface from the quake’s epicenter, which was in a deep part of the boundary where the plates met.

The prevailing belief is adjacent plates are not strongly bonded in the shallow parts of their boundaries and major fault ruptures do not occur there in quakes.