Aftershock distribution of the February 27, 2010 Okinawa-honto Kinkai earthquake (Japan) using sP depth phase

Abstract

A major earthquake (moment magnitude; Mw = 7.0) occurred on February 27, 2010 in the central Ryukyu Trench, southwestern Japan. A slab-parallel extension-type mainshock and its aftershocks occurred on a strike-slip fault, whose extension axis was parallel to the dip of the Philippine Sea plate. The epicenters of the mainshock and the aftershocks were located in the cluster of slab-parallel compression-type earthquakes, whose compression axes were parallel to the dip of the Philippine Sea plate. To clarify the relationship between the slab-parallel extension-type mainshock and aftershocks and the slab-parallel compression-type earthquakes, I determined the depths of the hypocenters of 114 aftershocks (M ≥ 3.0) of the mainshock and 29 background earthquakes (M ≥ 3.5) from 2006 to 2009 near the central Ryukyu Trench using the arrival times of the depth phase. Then, I determined the distribution of the aftershocks using the double-difference method. The relocated aftershock distribution extends along the ENE–WSW direction. This suggests that the mainshock fault lies on a left-lateral strike-slip fault extending along ENE–WSW direction. On the basis of the relocated hypocenters, it is found that reverse-fault-type (slab-parallel compression-type) earthquakes occur in the oceanic crust of the subducted Philippine Sea plate, and strike-slip fault-type (slab-parallel extension-type) earthquakes occur in the mantle of the subducted Philippine Sea plate. This suggests that a double seismic structure exists locally in the shallow part of the subducted Philippine Sea plate of the central Ryukyu Trench.