South Korea

South Korea sits on the eastern margin of the Eurasian Plate, underlain by the stable Sino-Korean craton. The primary seismic stresses acting on the Korean Peninsula are transmitted from the Pacific Plate subduction occurring along the Japan Trench to the east, creating a northwest-directed compressional regime within the Korean crust. This manifests as reactivation of ancient northeast-southwest trending faults in the Precambrian and Palaeozoic basement, producing intraplate earthquakes of generally moderate magnitude. The Yangsan Fault and Ulsan Fault systems in southeastern Korea, near the city of Gyeongju, are the most clearly active fault zones identified in South Korea, with evidence of Quaternary offset and historical seismicity. The East Sea (Sea of Japan) to the east provides some seismicity from back-arc extension and offshore fault systems.

South Korea's modern seismic history has been marked by a general expectation of low hazard, disrupted by several recent events. The 2016 Gyeongju earthquake (magnitude 5.8) was the strongest recorded in South Korea since modern seismology began, causing damage to traditional structures and generating public debate about seismic design standards for nuclear power plants in the region. The 2017 Pohang earthquake (magnitude 5.5) killed one person and injured hundreds, and was subsequently linked to water injection at an enhanced geothermal power plant — making it one of the most significant cases of confirmed induced seismicity outside the oil and gas industry. Earlier, the 1978 Hongseong earthquake (magnitude 5.0) and the 1980 Uljin earthquake (magnitude 5.3) were the notable pre-digital era events.

The tectonic context of Korean seismicity reflects the position of the peninsula within the broad zone of intraplate deformation in eastern Asia. The Pacific slab descending beneath Japan generates compressional stresses that propagate westward into the Korean Peninsula, reactivating northeast-trending Cretaceous and Jurassic faults in the basement. The Yangsan Fault, running for 170 kilometres through southeastern Korea, has a complex history of both right-lateral and reverse motion and is considered the highest-priority seismic risk in South Korea given its proximity to the nuclear power clusters at Gyeongju and Ulsan. The [[seismic-wave]] propagation efficiency across the old, cold Precambrian crust is high, meaning that even moderate earthquakes can be felt across a large area of the peninsula.