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

Abstract

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. which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

Special issue "Crustal dynamics: toward integrated view of island arc seismogenesis"

“…In and around many fault zones, Sibson (2020) found many geological and geophysical characteristics that support his "fault-valve" model, especially in compressional-transpressional tectonic regimes, which are better at containing overpressure and are 'load-strengthening' (mean stress rising with increasing shear stress). Maeda et al (2019) investigated the focal mechanisms of aftershocks of the 2008 Iwate-Miyagi Nairiku earthquake (M 7.2) in the northeastern Japan arc and detailed Vp/Vs structure there. They concluded that exceptional 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.…”