Stress Map of Japan: Detailed Nationwide Crustal Stress Field Inferred From Focal Mechanism Solutions of Numerous Microearthquakes

Crustal stress is a crucial parameter for understanding tectonics and assessing the occurrence of large earthquakes. Stress at seismogenic depths over a wide area has been inferred from focal mechanism solutions. The existence of focal mechanism solutions determines the area that can be potentially studied. Therefore, we studied the crustal stress orientations in Japan from focal mechanism solutions of small earthquakes. First, we obtained about 220,000 focal mechanisms using the P-wave first-motion polarities picked by a deep learning model. Next, we performed a linearized inversion on the nationwide stress orientation model at a grid of 0.2-degree intervals. The estimation errors reflect the quality and quantity of focal mechanism solutions. Overall, the results indicate that Japanese islands are under an east-west compressional stress, whereas some areas along the Pacific coast of northeast Japan are under east-west extensional stress. A closer look shows the spatial trends of stress orientation may vary near the geological borders, including the Hinagu and Futagawa faults in Kumamoto Prefecture (Kyushu Island) and the Median Tectonic Line in Shikoku Island. Our stress model also revealed the existence of stress anomalies around Cape Shionomisaki in the Kii peninsula. Most of the active faults are at approximately optimal orientation for the derived stress orientation and stress ratio, although uncertainties in the fault geometry, particularly the dip angle, substantially affect this evaluation. The stress map helps understand the overall vision of the crustal stress in Japan and further study the local stress distribution.