M 6.9 - 90 km E of Yujing, Taiwan

Tectonic Summary

The September 18, 2022, M 6.9 earthquake south-east of Lugu, Taiwan, occurred as the result of strike-slip faulting at shallow depth, near the plate boundary between the Philippine Sea and Eurasia plates at the southeast coast of Taiwan. Preliminary focal mechanism solutions for the earthquake indicate rupture occurred on a steep fault striking either east-southeast (right-lateral), or south-southwest (left-lateral). At the location of this earthquake, the Philippine Sea plate converges with the Eurasia plate at a velocity of approximately 73 mm/yr towards the northwest. The location, depth and mechanism of the September 18, 2022 earthquake are consistent with its occurrence along a fault in close proximity and relation to, the complex plate boundary in this region. Earthquakes do not occur at points but rupture an area along a fault. An earthquake of similar size and mechanism to the September 18, 2022 earthquake generally rupture a fault about 50 km long and 15 km wide.

Taiwan lies in a region of complex tectonics near the intersection of three major tectonic plates – the Philippine Sea plate to the east and southeast, the Eurasia plate to the north and west, and the Sunda plate to the southwest. The location of the September 18, 2022 earthquake lies south of the Ryukyu subduction zone, near the plate boundary between the Philippine Sea and Eurasia plates in this region. The plate boundary in Taiwan itself is characterized by a zone of arc-continent collision; whereby the northern end of the Luzon (Philippines) island arc is colliding with the buoyant crust of the Eurasia continental margin offshore China. Along Taiwan’s west coast, and continuing south, this collision zone transitions into the eastward-oriented Manila subduction zone.

The September 18, 2022 earthquake was preceded by an M6.5 earthquake in the same area 17 hours earlier. There have been 7 earthquakes of M5.0 and larger in the sequence (as of September 18, 2022, 20:00 UTC).

Because of its plate boundary location, Taiwan commonly experiences moderate-to-large earthquakes. The region within 250 km of this September 18, 2022 earthquake has hosted 239 other M 6+ earthquakes over the preceding century; 24 of these were M 7+. In 1951 the east coast of Taiwan was rocked by a series of about 20 earthquakes M 6 and larger the largest being a M7.8 that was about 10 km to the southwest of the recent September 18, 2022 earthquake. In September 1999, the M 7.7 Chi Chi earthquake occurred in central Taiwan and resulted in at least 2,297 fatalities, and caused damage estimated at $14 billion.