[S-CG49] Structure, evolution and deformation of island arcs associated with subduction processes

[SCG49-P07] Interpretations to heterogeneous stress field in Chugoku region based on an estimation of seismic velocity structure

*Takahiro Shiina¹, Takahiko Uchide¹, Kazutoshi Imanishi¹ (1.National Institute of Advanced Industrial Science and Technology (AIST))

Keywords:Seismic velocity structure, Tomography, Crustal stress fields

The crustal stress field is important for understanding the generation mechanism of earthquakes and the characteristics of background seismotectonic. Recently, the crustal stress fields in the Japanese island arc are well investigated using focal mechanisms of microearthquakes [e.g., Imanishi et al., 2019, 2021; Uchide, 2020]. Imanishi et al. [2021] revealed detailed spatial variations of the crustal stress fields in the Chugoku region, Japan, by determining the focal mechanism of microearthquakes with magnitudes of 1.5 or larger, and clarified that the azimuth of maximum horizontal compressive stress is estimated along the coastal area of Japan sea in Sanin district (southern part of Chugoku region) rotates clockwise about 20 degrees from that of Sanyo district (north). They discussed that the rotation of the maximum horizontal compressive stress axis could be related to the concentration of strain [Nishimura and Takada, 2017] and the density distribution [Shioda et al., 2002]. The identification of the origin of the observed stress heterogeneity needs further studies on underground structure. In this study, we try to discuss the relations between the crustal stress fields and heterogeneous structure in the Chugoku region by estimating three-dimensional velocity structures.
The P- and S-wave velocity structures around the Chugoku region are estimated by using the double-difference tomography technique [Zhang and Thurber, 2006]. From the unified earthquake catalog of the Japan Meteorological Agency in the period between 2005 and 2020, we collected earthquakes that occurred by a depth of 50 km and their manually picked arrival times. Magnitude of the earthquakes is larger than 1.5. The number of arrival time data of P and S waves are 427,965 and 330,404, respectively. Intervals of grid nodes representing velocity values are about 20 km in horizontal directions and 10 km in depth direction. After 20 iterations, the travel time residuals reduce to 0.08 s from 0.18 s in P wave and to 0.13 s from 0.26 s in S wave.
At the seismogenic depths, the velocity structure estimated in the Chugoku region is almost homogeneous, and some low-velocity zones are identified just beneath the seismogenic layer. These results mean that there would not be clear regional differences in the crustal structure that correspond to the stress fields discussed in Imanishi et al. [2021]. The seismogenic layer changes its thickness in the Chugoku region, and its lower limits become to be deeper in the Sanyo district than those in the Sanin district. Thereby, the spatial variations in the crustal stress fields in the Chugoku region may be related to the thickness of seismogenic layer and heterogeneity below the layer, in addition to structures at depths where crustal earthquakes mainly occur.