September 6, 2004 |
Seismic Activity Off-shore Southeast of the Kii Peninsula
On September 5, at 19:07 (JST) there was a M6.9 (preliminary) earthquake off-shore southeast of the Kii Peninsula (off-shore of the Kii Peninsula). This event had a maximum seismic intensity 5 Lower in Nara and Wakayama prefectures. This earthquake caused a tsunami on the Pacific coast from the Izu islands to the Shikoku region, with tsunami heights of 0.5m in Kozushima and 0.3m in Irozaki, Owase, Kushimoto and Murotomisaki. Also, at 23:57 (JST) on the same day, there was a M7.4 (preliminary) earthquake off-shore southeast of the Kii Peninsula, to the east of the previous event (off-shore of Tokaido). This event had a maximum seismic intensity 5 Lower in Mie, Nara and Wakayama prefectures. This earthquake caused a tsunami on the Pacific coast from the Izu islands to the Shikoku region, with tsunami heights of 0.9m in Kushimoto, 0.8m in Kozushima, 0.7m in Irozaki, 0.6m in Owase, and 0.5m in Murotomisaki. Including these events, there were 18 events with intensity equal to or greater than 1 by 14:00 (JST) on September 6. Those events were distributed about 100km southeast of the Kii Pennisula, near the Nankai Trough in an area that is approximately a 50 km square in size. Judging from the sequence of occurrence, it is thought that the seismic activity was a foreshock - main shock - aftershock sequence, with the event at 23:57 as the main shock. The focal mechanisms of the foreshock and the main shock showed reverse faults with compression axes in a N-S direction. It is thought that this event occurred within the Philippine Sea plate because the dip angle of the estimated fault plane is larger than that of the plate boundary between the continental and the Philippine Sea plates.
According to the GPS data, crustal movements toward the south associated with this event, were observed over a large area from Mie prefecture to Aichi prefecture (maximum displacements of approximately 4cm near the Shima Peninsula). This is consistent with the focal mechanisms of the current seismic activity.
These earthquakes occurred outside of the expected focal region of the so-called Tonankai earthquake, as evaluated by the Earthquake Research Committee, and the focal mechanisms are different than that of the expected Tonankai earthquake. So, it is thought that the current activity did not rupture the focal region of the expected Tonankai earthquake. Therefore, it is thought that this seismic activity does not have a direct affect on the expected Tonankai earthquake.
According to the long-term evaluation announced by the Earthquake Research Committee on September 27, 2001, there is an approximately 50 percent chance of occurrence of the expected Tonankai earthquake (approximately M8.1) within 30 years from January 1, 2001. In addition, there is an approximately 60 percent chance of occurrence within 30 years from September 1, 2004.