Earth Planets Space, Vol. 57 (No. 4), pp. 315-320, 2005LETTER
Sun-Cheon Park and Jim Mori
Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, 611-0011, Japan
(Received December 4, 2004; Revised March 18, 2005; Accepted April 5, 2005)
We examine the spatial and temporal relationships of the sequence of strong earthquakes that occurred off the Kii Peninsula, Japan, on 5 September 2004. The first event (Mj 7.1) occurred at 10:07:08 (UTC) on a northward dipping plane within the subducting Philippine Sea plate. From 10:16 to 14:47 the seismicity shows a group of earthquakes (Mj 3.2 to 4.8) 35 km to the east which are regarded as foreshocks to the second large earthquake. At 14:57:17, a Mw 6.1 strike-slip event occurred on a northwest trending plane. Some 14 seconds later, a large (Mj 7.4) thrust earthquake started 4.2 km southeast of the initial epicenter of the second earthquake. This largest earthquake is thought to have occurred on a southward dipping plane with the strike in an east-southeastly direction. Using the geometry of faults determined in this study, calculations of the Coulomb failure function show that simple static stress changes do not provide a good explanation for the triggering of the subsequent earthquakes.
Key words: Kii Peninsula, Nankai Trough, slip distribution, static stress, triggering, foreshocks.