Abstract
Abstract The 2004 off the Kii peninsula earthquakes (M w 7.5 for the main shock) occurred within the subducting Philippine Sea (PHS) plate near its boundary, the Nankai trough, southwest Japan. The rupture mode of the foreshock-main shock-aftershock sequence was complicated, a combination of ENE-WSW striking (almost trough parallel) reverse faulting beneath the trough and NW-SE trending (almost trough normal) strike-slip faulting mostly on the landward side of the former. In this paper, we discuss the tectonic meaning of this NW-SE running strike-slip fault. We examined hypocenter distribution and focal mechanisms of slab earthquakes from October 1997 through September 2004 and confirmed a NW-SE striking tear of the PHS slab beneath the middle part of the Kii Peninsula pointed out by Miyoshi and Ishibashi (2004). According to the Earthquake Research Committee (2004) there is a NW-SE trending structural discontinuity in the PHS crust to the southeast of the main shock epicenter. Putting all features together, we interpret that there is a NW-SE striking fracture within the PHS plate continuously from the Nankai trough region to the slab beneath the Kii Peninsula, and that a partial rupture of this fracture occurred during the off the Kii peninsula earthquakes together with trough-parallel reverse faulting. It should be noted that two disastrous M 7-class slab earthquakes seem to have occurred along this tear beneath the peninsula in 1899 and 1952.
Highlights
(magnitude assigned by the Japan Meteorological Agency (JMA); e.g., Utsu, 2002) 7.4 took place just beneath the Nankai trough off the southeastern coast of the Kii Peninsula, southwest Japan (Fig. 1)
The Nankai trough is the boundary between the Philippine Sea (PHS) and the Eurasian (EUR) plates, where the former is being subducted beneath the latter northwestward and along which great interplate earthquakes have recurred with spatio-temporal regularity (e.g., Ando, 1975)
Miyoshi and Ishibashi (2004) delineated the geometry of the upper surface of the seismic PHS slab beneath the middle part of southwest Japan including the Kii Peninsula based on hypocentral distribution of slab earthquakes, assuming that the upper boundary of slab earthquake distribution roughly coincides with the slab upper surface
Summary
On September 5, 2004, a large earthquake of MJ (magnitude assigned by the Japan Meteorological Agency (JMA); e.g., Utsu, 2002) 7.4 took place just beneath the Nankai trough off the southeastern coast of the Kii Peninsula, southwest Japan (Fig. 1). According to Sakai et al (2005), who carried out urgent aftershock observation using ocean bottom seismometers, the depth range of aftershocks is about 5–25 km Focal mechanisms of these four large events (Fig. 2) show E-W striking reverse faulting, not low-angle, due to P-axes in a N-S direction. Judging from these mechanisms, epicentral locations, focal depths, and the hypocentral distribution of the foreshock–main shock–aftershock sequence, this seismic activity is considered basically troughparallel reverse faulting within the PHS plate that is just starting subduction at the Nankai trough. Baba et al (2005) and Satake et al (2005), who analyzed tsunami waveform data, and Hashimoto et al (2005), who analyzed GPS data, concluded the necessity of the strike-slip fault for their source modeling
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