Complex microseismic activity and depth-dependent stress field changes in Wakayama, southwestern Japan

Abstract

We examined the spatial relationship between seismicity and upper crustal structure in the Wakayama region, northwestern Kii Peninsula, Japan, by investigating microearthquake focal mechanisms and the local stress field. The focal mechanisms of most events studied fall into three categories: (1) normal faulting with N-S-oriented T-axes mainly occurring at shallow depths, (2) reverse faulting with E-W-oriented P-axes dominating at intermediate depths, and (3) strike-slip faulting with N-S-oriented T-axes and E-W-oriented P-axes mainly seen at greater depths. The stress field varies with depth: the shallow part is characterized by a strike-slip-type stress regime with N-S tension and E-W compression, while the deep part is characterized by an E-W compressional stress regime consistent with reverse faulting. The depth-dependent stress regime can be explained by thermal stress caused by a heat source, as expected from geophysical observations. Geologic faults, acting as weak planes, might contribute to generate shallow normal fault-type and deeper strike-slip fault-type microearthquakes.

Crustal dynamics: unified understanding of geodynamic processes at different time and length scales

“…The observed complexity is explained as the effect of non-planar fault geometry with a number of branch faults and bends. Maeda et al (2018) explored the spatial relationship between upper crustal structure and seismicity in the Kii Peninsula of southwest Japan, where the stress field and the predominant focal mechanism change with depth. They attribute this stress heterogeneity to localized thermal stress from a buried heat source in the lower crust.…”

The Wakayama earthquake swarm in Japan

“…1). The spatial correlation between this swarm location and the major geological units has previously been investigated, and geological faults have been interpreted as a major cause of this earthquake swarm (Maeda et al 2018(Maeda et al , 2021. However, the MTL and other major geological units extend farther to the west and east of the Wakayama region, and the reason for the con ned location of this earthquake swarm remains elusive.…”

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

“…Focal mechanism solutions are classified as normal, strike-slip, or reverse types using the same scheme as Maeda et al (2018), who introduced two grades (A and B) for each fault type: grade A means that a focal mechanism has an axis with a plunge angle θ ≥ 60°, and grade B indicates a plunge angle 45° ≤ θ < 60°. We classified focal mechanisms satisfying neither criterion (i.e., θ < 45° for all axes) into a group labeled "other".…”

“…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).…”

“…These phenomena also indicate that the maximum shear stress is not much larger than the stress drops of earthquakes there. Maeda et al (2018) showed that microearthquake faulting styles in northwestern Kii Peninsula, southwestern Japan, can vary across a small region, indicating the crustal strength is very low in the region. These results indicate that the earthquakes do not necessarily occur in large differential stress regimes but rather in weak regions.…”