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Mw 5.4 Earthquake Strikes the Southeastern Korean Peninsula

13 Sep 2016

In the early evening of 12 September 2016, a moderate earthquake (Mw 5.4) struck the southeastern coast of South Korea, about 50 minutes after a Mw 4.9 foreshock. The tremor was strong enough to be felt across South Korea and the northern coast of Kyushu, Japan. Photographs shared on social media showed minor damage in nearby cities, mostly in the form of fallen plaster and ceilings, and minor ground cracks.

The tremor also resulted in panic in the Gyeongju area, a popular tourist destination in South Korea close to the earthquake’s epicentre. Usually, magnitude-5.4 earthquakes do not cause significant damage. However, in a region where seismic hazards are not properly mitigated, which is not uncommon in regions where earthquakes are rare, a magnitude-5.4 event can produce considerable damage.

According to the United States Geological Survey, the earthquake’s epicentre is 8 kilometres (km) south of Gyeongju, also known as Kyongju (35.769°N, 129.227°E). The magnitude of this earthquake (Mw 5.4) is about 1.4 times greater than the recent man-made explosion in North Korea, which was felt in the northern part of the Korean Peninsula and along the North Korea-China border. Although the timing of both events are close, the distance between them and their magnitudes suggest that they are unrelated.

The Mw 5.4 earthquake’s epicentre is located close to the NNE-SSW fault system in the eastern coast of South Korea. The fault slip rate in this region is generally very low, and difficult to be detected by modern GPS and satellite technology. A seismic analysis from Japan suggests that the earthquake is a strike-slip event, with a source orientation similar to the orientation of the area’s geological faults. (Source: The tectonic map of South Korea is from Choi et al., 2005. The epicentre location was taken from USGS. The focal mechanism is from NIED’s automatic focal solution.)

The seismic analysis of the Mw 5.4 earthquake, conducted by Japan’s National Research Institute for Earth Science and Disaster Resilience (NIED), suggests that this quake was caused by rupture of a strike-slip fault within the shallow layers of the Earth’s crust, where the ground on both sides of the fault move horizontally in opposite directions.  Given the location of the epicentre and its strike-slip focal solution, it is believed that this earthquake is likely to have been generated by one of the NNE-SSW right-lateral active faults in the southeastern coast of South Korea.

Although the slip rates of active faults in South Korea are generally very low (i.e. 

The slow activity of the fault in this area, together with historical records of the region's earthquake occurrence, suggest that the Korean Peninsula is not completely seismically quiet. Even though it is more than 500 km away from major plate boundary faults, the on-going tectonic motions between the Philippine Sea plate and the Eurasian plate, as well as the tectonic deformation within northern China, are producing notable deformations in the peninsula, and reactivating the ancient fault in South Korea’s intraplate environment.