Stress state in the upper crust around the source region of the 1891 Nobi earthquake through shear wave polarization anisotropy

Abstract

We investigate shear wave polarization anisotropy in the upper crust around the source region of the 1891 Nobi earthquake, central Japan. At most stations, the orientation of the faster polarized shear wave is parallel to the axes of the maximum horizontal compressional strain rate and stress, indicating that stress-induced anisotropy is dominant in the analyzed region. Furthermore, near the source faults, the orientation of the faster polarized shear wave is oblique to the strike of the source faults. This suggests that microcracks parallel to the strike of the source fault, which would be produced by the fault movement of the Nobi earthquake, have healed with the healing of the faults. For stress-induced anisotropy, time delays normalized by path length in the anisotropic upper crust as a function of the differential strain rate are coincident with those in the inland high strain rate zone, Japan. These data, together with those of a previous study, show that the variation in the stressing rate, estimated from shear wave splitting, is close to that estimated from geodetic observation. This implies that the variation in the stressing rate in the brittle upper crust is linked to that in the strain rate on the ground surface.

Preliminary analysis of crustal anisotropy in the southernmost part of Simeulue Island

“…In a region where the anisotropy source is associated with structural anisotropy, the fast direction of the splitting parameter should be parallel with the strike of the major structural feature, but our measurements show that the fast orientation is perpendicular to the strike of 12th International Physics Seminar 2023 Journal of Physics: Conference Series 2596 (2023) 012035 the features. Second, anisotropy caused by this mechanism usually occurs at shallow depths [4]. Our analysis shows that most of the earthquake data are from the lower crust; thus, it may be possible that the anisotropic source is also originally from this layer.…”

“…Stress-induced anisotropy is often attributed to the maximum horizontal stress allowing microcracks in the surrounding area to open, causing the orientation of the fast wave to be aligned to the maximum stress direction [3]. This anisotropic source is usually present in the upper crust, and the effect tends to lessen with the depth, as the increasing pressure may close the microcracks in all directions [4]. The second anisotropic cause, structuralinduced anisotropy, is attributed to macroscopic geological features such as fault zone fabric [5], bedding sedimentary planes [6], and anisotropic mineral alignment [7].…”

Shear wave splitting of the 2018 Lombok earthquake aftershock area, Indonesia

“…In certain cases, an increase in delay time at large depths can occur due to the presence of fracture filling fluid at that depth. The fluid can increase the pore pressure and prevent the fracture from closing again due to high overburden pressure (e.g., Syuhada et al 2017;Hiramatsu and Iidaka 2015). However, we still need more findings to prove our interpretation.…”

“…5. Through the analysis of the spatial average of ф, we will further analyze the causes of variations in the direction of fast polarization in this study, whether it is controlled by regional stress conditions or local geological structures (e.g., Hiramatsu and Iidaka 2015).…”

“…At deeper crust, the effect of this anisotropy usually decreases with increasing the confining pressure closing the microcrack in all directions. However, the presence of fluid-filled microcracks may prevent the cracks from closing contributing to crustal anisotropy (e.g., Hiramatsu and Iidaka 2015). For the second mechanism, structure-induced anisotropy is seismic anisotropy related to aligned macroscopic fabrics such as fault or strike slip filled with fluid (Zhang et al 2007;Tadokoro and Ando 1999), foliation plane of anisotropic rocks, and sedimentary bedding system (Valcke et al 2006;Kern and Wenk 1990).…”