Stress Field in the 2016 Kumamoto Earthquake (M 7.3) Area

Abstract

We used 1‐D and 3‐D velocity models to determine focal mechanism solutions (FMSs) of 349 crustal earthquakes (M 2.7–7.3) and stress tensors in the source area of the 2016 Kumamoto earthquake (M 7.3) that occurred on the Futagawa‐Hinagu fault zone in Kyushu, Southwest Japan. There are some differences in the FMSs determined with the 1‐D and 3‐D velocity models. The use of the 3‐D velocity model leads to better results of stress tensors, which are determined by inverting the FMSs. The orientation of the minimum stress (σ3) axis is more accurately determined, which trends NNW‐SSE to N‐S nearly horizontally. In contrast, the axes of the maximum and intermediate stresses (σ1 and σ2) trend WSW‐ENE to E‐W with wide ranges. Significant spatiotemporal variations of the stress field are revealed in the Kumamoto source zone, indicating a small magnitude of deviatoric stress. The friction coefficient of the faults is estimated to be relatively small (~0.4), indicating that the seismogenic faults in central Kyushu are weak. The fault weakening may be caused by fluids beneath the source area and arc magma under the nearby Aso active volcano.

Absolute Tectonic Stress Inversion in the 2016 Mw 7.0 Kumamoto Earthquake Source Region Using Topography and Rupture Models

“…Finally, we apply the inversion scheme to estimate the tectonic stress field using the coarse model. The tectonic stress is assumed to be uniaxial tensional because numerous researches have demonstrated that tensional stress with a nearly north‐south orientation is dominant throughout the whole Kyushu island (Matsumoto et al., 2015; Mitsuoka et al., 2020; Savage et al., 2016; Yoshida et al., 2016; Yu et al., 2019). Thus, the parameter $$\boldsymbol{m}$$ is composed of the amount and direction of the horizontal minimum compressional stress ($${S}_{h\text{min}}$$).…”

“…Therefore, based on the inversion result of the uniaxial tectonic stress, we can conclude that 𝐴𝐴 𝐴𝐴3 is horizontal and nearly in the direction of N-S (N6.5°W), and the magnitude of 𝐴𝐴 𝐴𝐴2 is almost equal to that of 𝐴𝐴 𝐴𝐴1 . Yoshida et al (2016) and Yu et al (2019) have investigated the normalized deviatoric stress (orientations and relative magnitudes of the principal stresses) near the source region of the Kumamoto earthquake using large amounts of focal mechanisms based on the Wallace-Bott assumption. They suggested that the 𝐴𝐴 𝐴𝐴3 axis trends NNW-SSE or N-S horizontally, which is the same as our results.…”

“…(2016) and Yu et al. (2019) have investigated the normalized deviatoric stress (orientations and relative magnitudes of the principal stresses) near the source region of the Kumamoto earthquake using large amounts of focal mechanisms based on the Wallace‐Bott assumption. They suggested that the $${\sigma }_{3}$$ axis trends NNW‐SSE or N‐S horizontally, which is the same as our results.…”

“…Previous studies about temperature, velocity tomography, viscosity, and so on have demonstrated that rich fluids and arc magma exist under the Aso volcano (Moore et al., 2017; Okubo & Shibuya, 1993; Zhao et al., 2018). The overpressured fluids probably lead to the low‐magnitude shear stress and likely promoted the generation of the Kumamoto earthquake (Mitsuoka et al., 2020; Yoshida et al., 2016; Yu et al., 2019).…”

Structural Heterogeneity of the Alaska‐Aleutian Forearc: Implications for Interplate Coupling and Seismogenic Behaviors

“…Enormous strain energy can be accumulated in the megathrust zone due to the plate convergence along the frictional plate interface, which can intermittently host huge earthquakes with magnitudes up to Mw 9.0 or greater. Due to the existence of strong structural heterogeneities and complex stress field in subduction zones (e.g., Hua et al., 2020; Huang & Zhao, 2013; Yu et al., 2019; Zhao et al., 2011), uneven interplate seismic coupling, segmentation of great megathrust ruptures and heterogeneous distribution of various type earthquakes (e.g., interplate, intraslab, and slow earthquakes, etc.) have been observed in many subduction regions (Chlieh et al., 2008; Hasegawa et al., 2009; Sato et al., 2013; Schmalzle et al., 2014; Sippl et al., 2021; Yokota et al., 2016).…”