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Crustal Stress Orientations in Japan Islands Inferred From Focal Mechanism Solutions of Small and Microearthquakes

Session: Crustal Stress and Strain and Implications for Fault Interaction and Slip [Poster]

Type: Poster

Date: 4/22/2021

Presentation Time: 11:30 AM Pacific

Description: 

Understanding of crustal deformation and forecast of inland earthquakes need knowledge on the crustal stress orientation. Estimated stress orientations have been compiled as the World Stress Map (Heidbach et al., 2018). In the case of Japan Islands, Terakawa et al. (2010) estimated the stress orientations using moment tensor solutions from the routine catalog of NIED F-net for typically M ≥ 3.5. The resolution of the stress field estimation is limited by data. Moreover, there are many areas with no focal mechanism solutions. The stress orientations in no data areas may be different from their surroundings (e.g., Imanishi et al. (2012)). Therefore, we desire the focal mechanisms of even smaller earthquakes. Thanks to the recent development of deep learning, Uchide (2020) studies focal mechanisms of ~ 110 thousand events with M 1.5 or larger beneath Japan Islands. Using this technique, we estimate the crustal stress orientation of the Japan Islands.

First, we determined focal mechanisms of small to microearthquakes around Japan Islands by HASH method (Hardebeck and Shearer, 2002) using P-wave first-motion polarities. Seismic data were from NIED Hi-net, JMA, and GSJ/AIST. P-wave arrival times were from the JMA catalog and first-motion polarities were picked by the neural network model of Uchide (2020). We selected earthquakes with JMA magnitudes greater than 0.5, inside of or within 50 km from coastlines of Japan Islands, and at depths less than 20 km. As a result, we analyzed ~ 573 thousand events.

The stress orientation was estimated using the SATSI code (Hardebeck and Michael, 2006). The grid interval was 0.2° for both latitude and longitude. Several small-scale stress anomalies were seen: normal faulting areas in northeast Japan; stress orientation changed at the southern end of Itoigawa-Shizuoka Tectonic Line; and stress anomaly around Cape Shionomisaki of Kii peninsula, beneath which a high-density block was detected (Honda and Kono, 2005). This study will help discover and understand seismotectonics.

Presenting Author: Takahiko Uchide

Student Presenter: No

Authors