Triggering sequence of large aftershocks of the Mid Niigata prefecture, Japan Earthquake in 2004 by static stress changes

Following the Mid Niigata prefecture Earthquake (M_JMA 6.8) in 2004, 4 large aftershocks (M_JMA 6.3, 6.0, 6.5, 6.1) occurred: three within 40 minutes and one after 4 days. We examine the possibility for the triggering of this sequence of large aftershocks by static stress changes. For the close spatial triggering, it is important to have information about the fault geometries, slip distribution, and focal mechanisms. We determine the fault plane orientations from the aftershock distributions. Slip distributions of the mainshock and the largest aftershock are obtained by seismic waveform inversions of local strong-motion records. Mechanisms for the events are taken from MT solutions. The temporal variations of Coulomb failure function changes (ΔCFF) are calculated on the fault planes of the aftershocks before their rupture. Positive ΔCFF values (0.06-0.3 MPa) are obtained around the hypocenters on the fault planes, indicating the possibility that static triggering from the main event and following aftershocks can explain the occurrence of subsequent aftershocks.