Stress Field Estimation From S‐Wave Anisotropy Observed in Multi‐Azimuth Seismic Survey With Cabled Seafloor Seismometers Above the Nankai Trough Megathrust Zone, Japan

Abstract

The spatial variation of azimuthal S‐wave phase velocity anisotropies caused by differential horizontal stress along the Nankai Trough was analyzed to understand the stress state of the overhung block of the forearc region, off Kii Peninsula, Japan. We conducted controlled‐source seismic surveys along the circumference of a 3 km radius circle centered at each seismometer of a cabled seafloor observatory installed in the Nankai subduction zone. We applied an anisotropy semblance method to estimate the orientation of fast and slow S‐wave velocities of shallow sediments and deep accretionary prism using the multi‐azimuth seismic dataset acquired at each seismometer location. The estimated orientations of fast S‐wave velocity are parallel to the convergent direction of the subducting place beneath the Kumano basin in the deeper accretionary prism while perpendicular to the convergent direction in the shallow sediments inside the Kumano basin. These fast S‐wave polarization orientations in shallow sediments show good agreement with horizontal maximum stress orientations estimated in situ borehole measurements in the observation area. We estimated the differential horizontal stress field in the Nankai Trough region from obtained S‐wave anisotropy using a simple crack model. Using this method, we first suggest the convergent‐parallel horizontal maximum compression in the deeper part of the accretionary prism above the locked portion. Assuming obtained S‐wave anisotropy is attributed to the tectonic loading by the subducting plate, these results suggest that the S‐wave azimuthal anisotropy measurements can be used to monitor the subsurface stress field as a function of time.

Monitoring the 2021 Mw 8.2 Alaska Earthquake by an Offshore Seismic and Fluid Pressure Observation Network and Implications for Ocean‐Crust Dynamic Coupling

“…Over the past few decades, rapid advances of offshore geophysical observations have revolutionized studies of submarine earthquakes and the associated geohazards such as tsunamis. Among the more recently developed techniques, continuous real‐time monitoring via cable‐connected networks at multiple subduction plate boundaries (Aoi et al., 2020; Barnes et al., 2012; Kaneda, 2014; Smith et al., 2018) has shown great advantages in various aspects of seismotectonic studies, including studying source characteristics of regular and slow earthquakes (Araki et al., 2017; Ariyoshi et al., 2021; Kubota, Kubo, et al., 2021; Nakano et al., 2018; Nishikawa et al., 2019; Takemura et al., 2022; Tanaka et al., 2019; Tréhu et al., 2018; Wallace et al., 2016), characterizing subduction upper‐plate structure and stress states (Akuhara et al., 2020; Kimura et al., 2021; Tonegawa et al., 2017) or larger‐scale forearc structure and geodynamic processes (Hua et al., 2020; Uchida et al., 2020; Yu & Zhao, 2020), and enabling real‐time earthquake monitoring and tsunami early warning (Aoi et al., 2019; M. Inoue et al., 2019; Kubota, Saito, & Suzuki, 2020; Mulia & Satake, 2021; Tanioka, 2020; Thomson et al., 2011). To date, studies of offshore earthquake monitoring in real time via cable‐connected networks were mostly focused on small earthquakes (up to M w 7.1) (e.g., Kubota, Kubo, et al., 2021; Tréhu et al., 2018; Wallace et al., 2016).…”