Causes of the N–S compressional aftershocks of the E–W compressional 2008 Iwate–Miyagi Nairiku earthquake (M7.2) in the northeastern Japan arc

In this study, we investigated the influence of local structural heterogeneities on aftershocks of the 2008 Iwate–Miyagi Nairiku earthquake (M 7.2) that occurred in the northeastern Japan arc. Although this area is characterized by an E–W compressional thrust faulting stress regime, many N–S compressional thrust-type and strike-slip-type earthquakes were also observed in the aftershock activity of the 2008 event. The occurrence of such N–S compressional aftershocks indicates that the differential stress was very low before the main shock. We investigated V_p/V_s structure in the aftershock area in detail to find most of the area showed V_p/V_s as low as less than 1.70, which is consistent with the previous studies. Our result shows, however, the aftershock area was dotted with high V_p/V_s small areas, which suggests the locations of abundant fluid-filled cracks. The comparison of the distributions of V_p/V_s, the stress changes due to the mainshock, and focal mechanisms of the aftershocks indicate that the N–S compressional aftershocks are concentrated in regions with high V_p/V_s and large N–S compressional stress change. From these results, we concluded that the N–S compressional aftershocks occurred in regions with low strength due to the high pore pressure and with N–S compressional stress caused by the stress change due to the mainshock in a low differential stress regime.[Figure not available: see fulltext.]