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 Vp/Vs structure in the aftershock area in detail to find most of the area showed Vp/Vs 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 Vp/Vs small areas, which suggests the locations of abundant fluid-filled cracks. The comparison of the distributions of Vp/Vs, 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 Vp/Vs 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.
Highlights
The spatial distribution and focal mechanisms of inland earthquakes provide important insight into local structural heterogeneities in the crust (e.g., Magistrale and Zhou 1996; Umino et al 1998; Hardebeck and Hauksson 1999; Maeda et al 2018; Hauksson and Meier 2019)
Their quartz-rich samples are taken from regions that correspond to the low-Vp/Vs regions estimated in this study, while samples with quartz contents of < 25% seems to be distributed in the region of high Vp/Vs ratios revealed by Okada et al (2012) (Additional file 1: Figure S2)
The present analysis has revealed that the area of the Iwate–Miyagi Nairiku earthquake have low Vp/Vs ratios, but Vp/Vs > 1.90 in some regions, which is consistent with the results of Okada et al (2012)
Summary
The spatial distribution and focal mechanisms of inland earthquakes provide important insight into local structural heterogeneities (e.g., fluid, lithology, stress field, temperature, weak planes) in the crust (e.g., Magistrale and Zhou 1996; Umino et al 1998; Hardebeck and Hauksson 1999; Maeda et al 2018; Hauksson and Meier 2019). Earthquakes with fault types very different from those expected from the regional stress field have been reported by many researchers (e.g., Terakawa et al 2013; Yoshida et al 2014b; Vavryčuk and Adamová 2018). Causes of such strange earthquakes are usually attributed. Our study is composed of the following three steps: First, we examine the distribution of aftershocks of the 2008 Iwate–Miyagi Nairiku earthquake and their faulting styles in detail. We discuss the relationship between the N–S compressional aftershocks of the 2008 earthquake and the inhomogeneous Vp/Vs structure in the last step
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