September 8, 2004 |
Seismic Activity Off-shore Southeast of the Kii Peninsula
On September 5, at 19:07 (JST) there was a M6.9 (maximum seismic intensity 5 Lower)@earthquake off-shore southeast of the Kii Peninsula (off-shore of the Kii Peninsula). This earthquake caused tsunamis on the Pacific coast from the Izu islands to the Shikoku region, with tsunami heights of 0.5m at Kozushima. Also, at 23:57 (JST) on the same day, there was a M7.4 (maximum seismic intensity 5 Lower) earthquake off-shore southeast of the Kii Peninsula (off-shore of Tokaido), to the east of the previous event. The second earthquake caused tsunamis on the Pacific coast from the Izu islands to the Shikoku region, with tsunami heights of 0.9m at Kushimoto. (see the 130th Evaluation of the Earthquake Research Committee gSeismic Activity Off-shore Southeast of the Kii Peninsulah) 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. As of 16:00 on September 8, the largest aftershock was a M6.4 (preliminary) event that occurred to the northeast of the main shock at 08:29 on September 7. All of these events were distributed about 100km southeast of the Kii Peninsula, near the Nankai Trough in an area that is approximately a 50 km square in size. The foreshock, main shock and the largest aftershock are located along the trough. All focal mechanism showed reverse faults with compression axes in a N-S direction. It is thought that these events occurred within the Philippine Sea plate, because the dip angle of the estimated fault plane is steeper 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, with displacements of approximately 5cm near the Shima Peninsula, which is roughly north of the main shock. This is consistent with the focal mechanisms of the current seismic activity. No significant postseismic movement has been seen since the main shock.
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 from that of the expected Tonankai earthquake. So, it is inferred 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.
(reference)
The values for the probability of earthquake occurrence have become increasingly higher over time in the long-term evaluation for the Nankai Trough earthquakes, announced by the Earthquake Research Committee on September 27, 2001. For the expected Tonankai earthquake (approximately M8.1) and Nankai Earthquake (approximately M8.4), the following table shows a comparison of the earthquake occurrence probabilities calculated beginning on January 1, 2001 and beginning on September 1, 2004.
| @ | Evaluation time | Within 10 years |
Within 30 years |
Within 50 years |
Elapsed time since last earthquake |
| Tonankai (approximately M8.1) |
Jan. 1, 2001 | About 10 % | About 50 % | 80 - 90 % | 0.65 |
| Sept. 1, 2004 | 10 - 20 % | About 60 % | About 90 % | 0.69 | |
| Nankai (approximately M8.4) |
Jan. 1, 2001 | Less than 10 % | About 40 % | About 80 % | 0.60 |
| Sept. 1, 2004 | About 10 % | About 50 % | About 80 % | 0.64 |
(Elapsed time since last earthquake: Ratio of elapsed time since the last
earthquake to the average recurrence interval)