Subsurface Structures Around the Subducting Seamount Illuminated by Local Earthquakes at the Off-Ibaraki Region, Southern Japan Trench

Abstract

Abstract The off-Ibaraki region is a convergent margin at which a seamount subducts. An intensive event location was performed around the subducting seamount to reveal the regional seismotectonics of this region. By applying a migration-based event location to an Ocean Bottom Seismic network record of both P- and S-waves, over 20,000 events were determined in the off-Ibaraki region below ~ M4. The seismicity showed clear spatiotemporal patterns enough to identify the seismicity changes and geometry of the interface. At the updip side, the shallow tectonic tremors and earthquakes are shown to be spatially complementary bounded by an updip limit of the seismogenic zone. At the downdip side, a semicircular low-seismicity zone was identified, which is possibly a rupture area of the Mw7.9 event. The event depth profile exhibited a gently sloped planar downdip interface subparallel to the subducting slab. This plane appears to be stably active from 2008 to 2011. Comparison with the active source seismic survey profiles exhibits that this planar downdip interface is several kilometers deeper than the top of the oceanic crust. After the Mw7.9 event, a high-angle downdip seismic interface was activated above the planar interface. Further, below the planar downdip interface, broadly scattered events occurred with a swarm manner. We successfully illuminated the complicated subsurface structures around the subducting seamount. It is suggested that most of the events occur along or below the plate interface as the top of the oceanic crust.