Aftershock observation of the 2004 off the Kii Peninsula earthquake using ocean bottom seismometers

The 2004 off the Kii Peninsula earthquake (Mj7.4) occurred near the Nankai trough axis, southeast off the Kii Peninsula, Japan, on September 5, 2004. The earthquake was estimated to have a thrust-type focal mechanism with N-S striking P-axis and was regarded as an intra-plate earthquake in the uppermost mantle of the Philippine Sea plate (PHS). In order to investigate the precise distribution and time change of the aftershock, we conducted pop-up ocean bottom seismometer (OBS) observations around the aftershock region. The observations were conducted three times intermittently for almost one year after the earthquake. A one-dimensional seismic velocity structure for hypocenter determination was derived from the survey result of the previous seismic refraction study. A station correction method using a PS conversion wave was applied to improve the hypocenter determination. Consequently, we could determine a more detailed and precise aftershock distribution compared with that of the Japan Meteorological Agency (JMA). The depth range of the OBS-located hypocenters was 5 to 30 km, about 20 km shallower than that of the JMA, and the OBS-derived epicenters were shifted about 10 km to the southeast compared to those of the JMA. The aftershocks, according to the JMA hypocenters, seem to be distributed mainly landward of the trough axis, whereas OBS-located hypocenters were distributed mainly along the trough axis. It was also found that the aftershock distribution can be divided roughly into two groups, a relatively shallower group with a depth range of 5 to10 km, and a deeper group with a depth range of 15 to 30 km. The shallower group, which is located inside the PHS or the accretionary prism just over the PHS, was distributed from the center to the north of the aftershock region. The deeper group is located in the uppermost mantle of the PHS near the Nankai trough axis, which is inferred to be the main ruptured zone of the main shock. We also detected several seismic clusters in the shallower earthquake group. They form vertical planes going down from the accretionary prism to the PHS. We are interested in the relation between the detected seismic clusters and the splay fault system in the accretionary prism.