Extreme plate boundary localization promotes shallow earthquake slip at the Japan Trench.

The 2011 moment magnitude 9.1 Tohoku-oki earthquake is exceptional among great earthquakes for having peak slip of ~50 to 70 meters on the shallowest portion of the plate boundary megathrust. Drill cores and geophysical logs from International Ocean Discovery Program Expedition 405 demonstrate that the megathrust preferentially develops at the top or base of the pelagic clay in the sedimentary layers present on the incoming Pacific Plate, where pronounced contrasts in physical properties occur. This preference results in a narrow, weak fault located at a major mechanical contact between frontal prism mud and subducted sediments, which enhances the tendency for shallow seismic slip, suggesting that the Japan Trench may be more susceptible to ruptures with large shallow slip than are margins without weak clays. Editor's summary: The 2011 moment magnitude 9.1 Tohoku-oki earthquake had the perfect storm of energetic, long slip on a shallow, subsea fault. The devastating tsunami that followed motivated efforts to understand the risk for shallow earthquakes in subduction zones. Kirkpatrick et al. report findings from the final cruise of the International Ocean Discovery Program, which drilled into the fault and undeformed seafloor. Sediment cores from both sites showed that the fault developed where a marine clay layer is juxtaposed against silica-rich mud or chert. The contrasting physical properties of sediment entering the subduction zone may facilitate shallow slip along the Japan Trench. —Angela Hessler [ABSTRACT FROM AUTHOR]