The International Information Center for Geotechnical Engineers

Friday, 24 November 2017 01:00

Aftershocks continue after the rare M5.4 quakes in South Korea

At least 58 aftershocks have been recorded after the two destructive M5.4 seismic events that struck the southeastern part of the Republic of Korea on September 12 and November 15, 2017, while concerns have been raised about the three nuclear reactors that are located in the vicinity of the Yangsan Fault, responsible for the quakes.

The M5.4 earthquakes on September 12 and November 15, 2017 were the two largest quakes to hit the country since Global CMT and USGS ANSS catalogs began recording in 1977. According to the USGS, the November 15th M5.4 quake struck 9.3 km (5.8 miles) northwest of the industrial city of Pohang (population of 500,000) at 05:29 UTC, at a depth of 10 km. The M5.4 shock on 12 September 2016 was recorded only 25 km away, to the south.

In total, 89 people were injured from the events, including one in critical condition due to an injury from a rockfall. State officials reported 5,569 cases of facility damage, with most of the damaged buildings being houses that sustained collapsed roofs or partial or full destruction. Moreover, 300 business buildings were damaged, including about 230 schools.

Of the 58 aftershocks registered by late November 20, two were in the mid-3 magnitude range, including the second strongest event of magnitude M3.6.

In an article posted by Temblor, Inc., Dr. Ross Stein (Temblor, Inc.) and Dr. Jaesung Park (Nephila Advisors, LLC.) state that both M5.4 quakes appear to lie on or close to the 120 km long active Yangsan Fault, with their focal mechanisms being consistent with a fault that dips (or is inclined to) the southeast by 50°-70°. The length of Yangsan Fault makes it capable of producing a future M7.3 earthquake.

Stein and Park examined the seismic catalogs, Google Earth imagery of the faulted landforms, the Global Earthquake Activity Rate (GEAR) model, and the GPS-derived crustal strain rates (GSRM2.0), to assess the significance of the seismic threat. And although the fault's slip rate is probably low (<1 mm/yr), they suggest that further investigation by the national authorities should now take place, as three nuclear reactors are located within 12 km from the fault. 

In their article, they state: "We recommend that the seismic hazard be reassessed in light of the two recent M5.4 shocks, because their locations and focal mechanisms are consistent with the existence of an active fault capable of much larger earthquakes, whose shaking could be larger than was anticipated in the design of the nuclear power plants."

Sources: The Watchers, USGS,

Read 389 times Last modified on Friday, 24 November 2017 16:04

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