Shear wave splitting and seismic velocity structure in the onshore focal area of the 2024 Noto Peninsula earthquake, central Japan, as a fluid-promoted multifault rupture in a compressional inversion area

The M7.6 Noto Peninsula earthquake occurred on 1 January 2024 in the Noto region of Ishikawa Prefecture. Several faults successively ruptured during the 2024 Noto Peninsula earthquake. Seismic activity in the area has increased since approximately August 2020. Possible causes of this increased seismic activity include stress changes due to crustal deformation and increased fluid pressure. In this study, following an analysis of the seismic swarm area northeast of the Noto Peninsula, S-wave splitting in the epicentral area was investigated using a dense seismic network of temporary and permanent stations. Seismic wave velocity tomography analysis was also conducted to investigate the isotropic seismic wave velocity structure in the onshore source region. The predominant fast direction of anisotropy throughout the peninsula was NE‒SW. This direction generally coincided with the strike direction of faults and folds and may be due to structural anisotropy.. In addition, NW‒SE anisotropy, whose direction generally coincided with the axis of maximum horizontal compression, and north‒southanisotropy were also observed in some areas. In the shallow crust, the hanging wall of the fault has low velocities. This shallow low-velocity hanging wall may be associated with structures that developed during past normal faulting that formed in this region. The low velocity in the shallow part of the hanging wall side of the fault and the anisotropy due to structural properties indicate a complex structure in which the structure of normal faults that developed in this region coexists with a compressional structure due to the present stress field. Low-Vs and high-Vp/Vs regions were identified beneath or at deeper extensions of hypocenters and faults at a depth of 18 km. Slightly larger P-wave velocities were obtained in this region than in the surrounding area. Tertiary igneous rocks are distributed in the target area, suggesting that the low-Vs and high-Vp/Vs regions may represent an old magma reservoir and that fluids released from/through this reservoir were involved in the 2024 Noto Peninsula earthquake and preceded seismic swarm activity that occurredbefore the 2024 earthquake.