5:15 PM - 7:15 PM
[SCG61-P06] Stress map of the inland Japan inferred from focal mechanism solutions of small earthquakes
★Invited Papers
Keywords:Earthquakes, Crustal Stress Field, Stress Inversion Analysis, Deep Learning, Japan Islands
Stress is essential for understanding the tectonics related to landform creation and earthquake generation and so on. Conversely, the topography and seismicity help us investigate the stress field. In particular, focal mechanisms are useful for estimating the stress orientation at depth. Michael (1987) proposed a method to determine the stress orientation from slip vectors indicated by focal mechanisms, based on the Wallace Bott hypothesis (Wallace, 1951; Bott, 1959) in which faults slip in parallel to in the direction of the maximum shear stress on the fault. The stress inversion method enabled us to produce stress maps (e.g., Heidbach et al., 2008, 2016, 2018; Hardebeck and Hauksson, 2001; Terakawa and Matsu’ura, 2010, 2023; Yukutake et al., 2015).
One of the ways to improve the stress estimation is the increase of focal mechanism solutions. This requires focal mechanism estimations for numerous numbers of small earthquakes. The bottleneck is the pick of P-wave first-motion polarities which are usually done manually. This problem was solved by recently developing AI technologies (e.g., Ross et al., 2018; Hara et al., 2019, 2020; Uchide, 2020).
We studied the stress field of the shallow inland around the main islands of Japan. First, we estimated the focal mechanisms of 660,332 earthquakes that occurred from 2003 to 2020, shallower than 20 km, inside or within 50 km of the coastline of Japanese Islands, and with JMA magnitudes of 0.5 or greater, by analyzing more than 9 million seismic waveform data using the deep learning model for picking P-wave first-motion polarities (Uchide, 2020). Next, the HASH program (Hardebeck and Shearer, 2002, 2008) succeeded in well constraining 216,528 focal mechanism solutions (rank A to C).
The stress inversion using these focal mechanism solutions illustrates the nationwide distribution of stress orientations and stress ratios (Uchide et al., 2022). The overall east-west compressional stress and local heterogeneities are observed. The spatial trend in stress orientation sometimes changes around the geological borders, such as the Hinagu and Futagawa faults in Kumamoto, the Median Tectonic Line in Shikoku island, and Itoigawa-Shizuoka Tectonic Line around Shizuoka city.
This stress map is useful for understanding the tectonics and assessing the seismic potential of faults by the slip tendency (Morris et al., 1996) and other measures. In the future, we will expand the study area using the same method.
Acknowledgements: We used seismic data from NIED Hi-net, JMA, and GSJ at AIST, and the JMA Unified Earthquake Catalog.
One of the ways to improve the stress estimation is the increase of focal mechanism solutions. This requires focal mechanism estimations for numerous numbers of small earthquakes. The bottleneck is the pick of P-wave first-motion polarities which are usually done manually. This problem was solved by recently developing AI technologies (e.g., Ross et al., 2018; Hara et al., 2019, 2020; Uchide, 2020).
We studied the stress field of the shallow inland around the main islands of Japan. First, we estimated the focal mechanisms of 660,332 earthquakes that occurred from 2003 to 2020, shallower than 20 km, inside or within 50 km of the coastline of Japanese Islands, and with JMA magnitudes of 0.5 or greater, by analyzing more than 9 million seismic waveform data using the deep learning model for picking P-wave first-motion polarities (Uchide, 2020). Next, the HASH program (Hardebeck and Shearer, 2002, 2008) succeeded in well constraining 216,528 focal mechanism solutions (rank A to C).
The stress inversion using these focal mechanism solutions illustrates the nationwide distribution of stress orientations and stress ratios (Uchide et al., 2022). The overall east-west compressional stress and local heterogeneities are observed. The spatial trend in stress orientation sometimes changes around the geological borders, such as the Hinagu and Futagawa faults in Kumamoto, the Median Tectonic Line in Shikoku island, and Itoigawa-Shizuoka Tectonic Line around Shizuoka city.
This stress map is useful for understanding the tectonics and assessing the seismic potential of faults by the slip tendency (Morris et al., 1996) and other measures. In the future, we will expand the study area using the same method.
Acknowledgements: We used seismic data from NIED Hi-net, JMA, and GSJ at AIST, and the JMA Unified Earthquake Catalog.