The 2004 sequence of triggered earthquakes off the Kii peninsula, Japan

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.