Inelastic strain rate in the seismogenic layer of Kyushu Island, Japan

Abstract

Seismic activity is associated with crustal stress relaxation, creating inelastic strain in a medium due to faulting. Inelastic strain affects the stress field around a weak body and causes stress concentration around the body, because the body itself has already released stress. Therefore, the understanding of inelastic deformation is important as it generates earthquakes. We investigated average inelastic strain in a spatial bin of Kyushu Island, Japan, and obtained the inelastic strain rate distribution associated with crustal earthquakes, based on the analysis of fault plane solutions and seismic moments. Large inelastic strains (>10) were found in the Beppu-Shimabara area, located in the center of Kyushu Island. The strain rate tensor was similar to that of the stress tensor except the absolute value in the area, implying that the inelastic strain was controlled by the stress field. The 2016 Kumamoto earthquake sequence (maximum magnitude 7.3) occurred in the Beppu-Shimabara area, with the major earthquakes located around the high inelastic strain rate area. Inelastic strain in the volume released the stress. In addition, the inelastic strain created an increment of stress around the volume. This indicates that the spatial heterogeneity of inelastic strain might concentrate stress.

Seismic velocity structure in the source region of the 2016 Kumamoto earthquake sequence, Japan

“…The hypocenters of the two main earthquakes are located near the bottom of the seismogenic zone. This relatively thick seismogenic zone was also reported by Matsumoto et al [2016]. Ito [1990Ito [ , 1993Ito [ , 1999 reported that the thickness of a seismogenic zone is related to the thermal structure and strength of the crust, since the seismic-aseismic boundary is related to the brittle-ductile transition.…”

Crustal Extension and Graben Formation by Fault Slip‐Associated Pore Opening, Kyushu, Japan

“…These different mechanisms share the common minimum compressional axes in the north‐south direction (Yoshida et al, ). The resultant north‐south crustal extension rate in BSG is seismologically estimated (Matsumoto et al, ) to be on the same order of magnitude as the rate geodetically measured (El‐Fiky et al, ).…”

“…The mechanisms so classified are illustrated by colored dots at the epicenters in the tomographic maps (Figure ) and cross sections (Figures and b). Normal fault (green) is the dominant mechanism for earthquakes in BSG but strike‐slip mechanism (blue) is significant as well (Matsumoto et al, ; Yoshida et al, ). For both types of mechanisms the minimum principal compression axis is in the north‐south direction.…”

“…The BSG is now opening in the north‐south direction, roughly perpendicularly to the Nankai Trough, as revealed by repeated geodetic surveys (e.g., El‐Fiky et al, ), possibly in relation with the back‐arc spreading of the Okinawa Trough (Ikeda et al, ; Okino et al, ; Tada, ). This crustal extension has been suggested to take place largely by earthquakes with normal fault and strike‐slip mechanisms with the minimum compression axes approximately in the north‐south direction, perpendicular to the Trench (Matsumoto et al, ).…”

Special issue “2016 Kumamoto earthquake sequence and its impact on earthquake science and hazard assessment”

“…Miyakawa et al (2016) invert gravity data to reveal a low-density body beneath the Aso caldera and discuss its possible effect on the termination of rupture. Matsumoto S et al (2016) perform a stress tensor inversion of fault plane solutions and seismic moment tensors in central Kyushu and reveal a zone of large inelastic strain.…”