Subsurface Structures Around the Subducting Seamount Illuminated by
Local Earthquakes at the Off-Ibaraki Region, Southern Japan Trench
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 event occur along or below the plate interface as the
top of the oceanic crust.