October 13, 2004
Headquarters for Earthquake Research Promotion
Earthquake Research Committee


Seismic Activity Off-shore Southeast of the Kii Peninsula


On September 5, at 23:57 (JST) there was a M7.4 earthquake off-shore southeast of the Kii Peninsula (off-shore of Tokaido). About five hours before that event, there was a M6.9 earthquake. (see the 130th and the 131st Evaluations of the Earthquake Research Committee gSeismic Activity Off-shore Southeast of the Kii Peninsulah) Judging from the sequence of occurrence, it is thought that the seismic activity was a foreshock - main shock - aftershock sequence, with the M7.4 earthquake as the main shock. The aftershocks are declining normally. The largest aftershock so far was a M6.5 event at 23:58 on September 8 that occurred to the east of the main shock. Almost all of these events were distributed near the Nankai Trough, off-shore southeast of the Kii Peninsula in an area that is approximately a 80 km square in size. In a broad view, trends in the aftershocks are seen in the direction along the Trough and in a NW-SE direction perpendicular to the Trough near the location of the main shock. The foreshock, main shock and the largest aftershock are located along the trough in the aftershock area. Focal mechanisms for all three of these events, showed reverse faults with compression axes in a N-S direction. On the other hand, there have been no earthquakes equal to or larger than M6.0 in the aftershocks trending in the NW-SE direction, and the focal mechanisms showed strike-slip faults.

Following the main shock, since September 8, data from the rapid deployment observations of (auto-surfacing) ocean bottom seismometers, show that the aftershock distribution near the epicenter of the main shock is clearly divided into two parts. Within the Philippine Sea plate, one group is centered at a depth of approximately 10km, and the other is centered at a depth of approximately 20km.

According to the GPS data, crustal movements associated with this event were observed widely in an area from the southern Tohoku region to the Shikoku region. Relatively large crustal movements toward the south were observed over an area from Mie prefecture to central Shizuoka prefecture, with displacements of approximately 6cm near the Shima Peninsula. This is consistent with the focal mechanisms of the current seismic activity. No significant postseismic movement has been seen since the main shock.

There is a NW-SE trending liner feature on the ocean floor, near the aftershocks that are also distributed in a NW-SE direction from the vicinity of the main shock. From the analysis of exploration surveys in this area, it is estimated that there is a NW-SE trending structural discontinuity in the crust in the vicinity of the mainshock.