Three-dimensional S-wave velocity structure of the Kinki Region, southwestern Japan with ambient seismic noise tomography using a dense seismic array

Abstract

Research interest in the Kinki region, southwestern Japan, has been aroused by the frequent occurrence of microearthquake activity in the area, particularly on the northern side of the Arima-Takatsuki Tectonic Line. Previous studies targeting this area focused mainly on deep, large-scale structures and could not efficiently resolve fine-scale shallow crustal structures, hence the need for an improved spatial resolution characterization of the upper crustal structure of this region. By cross-correlating the vertical components of the continuous ambient seismic noise data recorded between April 1st and September 30th, 2019, we estimated phase velocities using a frequency domain method to construct the three-dimensional S-wave velocity model of the Kinki region. We used data recorded by the dense Hi-net high-sensitivity seismographs and the Manten seismic network administered by Kyoto University, AIST, NIED, and the JMA. Rayleigh wave dispersion data were determined from zero crossings of the real component of the cross spectra using Aki’s spectral formulation, and a direct surface-wave inversion method was utilized to estimate the 3D S-wave velocity structure. This study revealed a low-velocity tectonic structure coinciding with the location of the reactivated Niigata-Kobe Tectonic Zone coexisting with fine-scale low-velocity tectonic structures on its flanks, which are likely to be indicating active fault locations. Additionally, these results indicate the presence of high-velocity zones on the northwestern and the southeastern portions of the study area attributable to very shallow basement material, mountainous regions, or sedimentary complexes. Sedimentary basins manifest as low-velocity zones extending to depths of about 2.2 km. These results contribute to deepening fundamental discerning of earthquake faulting and will be useful in hazard assessment and disaster mitigation.