Downdip Variations in Megathrust Seismogenic Behavior in the Japan Trench Subduction Zone Affected by Mantle Wedge Serpentinite

It is important to seek geological context for the geophysically observed downdip segmentation of seismogenic behavior of subduction megathrusts. Here we focus on how mantle wedge serpentinite may affect megathrust seismogenesis at Japan Trench—an end-member cold subduction zone. Based on thermal modeling constrained by heat flow observations and serpentine frictional behavior reported in the literature, we hypothesize the presence of a lizardite-rich soft barrier starting at the mantle wedge corner (MWC) and an antigorite-rich seismic zone farther downdip. This soft-barrier hypothesis can explain why two groups of megathrust events are roughly divided by the MWC yet the giant 2011 Tohoku-oki earthquake ruptured through the MWC. Seismogenic antigorite patches embedded in semi-viscous to viscous fault zone materials can explain other observed seismic phenomena of the deeper part of the megathrust such as aftershocks, persistent repeating earthquakes, and strong radiation of short-period energy in great earthquakes.