Rare 4.8 quake in Korea’s southwest rattles more than the ground

Posted on : 2024-06-13 17:30 KST Modified on : 2024-06-13 17:30 KST
With no known faults in the area, why the quake hit where it did will require further examination
A crack can be seen in a concrete wall that stands around a middle school in Buan County, North Jeolla Province, after a 4.8 magnitude earthquake struck there on the morning of June 12, 2024. (courtesy of the Jeonbuk State Office of Education)
A crack can be seen in a concrete wall that stands around a middle school in Buan County, North Jeolla Province, after a 4.8 magnitude earthquake struck there on the morning of June 12, 2024. (courtesy of the Jeonbuk State Office of Education)

The 4.8 magnitude earthquake that struck Buan County, North Jeolla Province, on Wednesday morning was the most powerful earthquake on the Korean Peninsula and its neighboring waters so far this year. Notably, this was also the only one of nine earthquakes of 4.5 magnitude and above over the past 10 years to occur in the country’s western region (excluding Jeju Island).

Earthquakes are rare in Korea’s southwest Honam region. The Korea Meteorological Administration (KMA) said this was the strongest earthquake to occur within a 50 km radius of the epicenter since 1978, when the Korean government began to systematically monitor earthquakes. Since the earthquake hit an area that has typically been regarded as safe from earthquakes, the relevant authorities are closely monitoring developments there.

In regard to the immediate cause of this earthquake, the KMA said, “For the moment, there are no faults about which we have accurate information in the area in question.” That means there are no specifically named faults in the area because of insufficient stratigraphic research in the area.

A person points where the 4.8 quake hit Korea on a screen at the Korean Meteorological Agency’s Seoul metro area branch in Suwon on June 12, 2024. (Yonhap)
A person points where the 4.8 quake hit Korea on a screen at the Korean Meteorological Agency’s Seoul metro area branch in Suwon on June 12, 2024. (Yonhap)

After analyzing the fault movement around the earthquake, the KMA explained that the earthquake was apparently the result of strike-slip fault movement, in which plates moved horizontally from northeast to southwest or from southeast to northwest along the fault line.

Some experts suspect the influence of the Hamyeol fault line, which stretches from Buyeo County, South Chungcheong Province, to Buan County, North Jeolla Province, and the Sipjaga (Cross) fault line, which runs from Buyeo County to Tancheon Township, Gongju.

“The Hamyeol and Sipjaga fault lines run north-northeast and south-southwest, and if you follow them, they are linked to the area where the Buan earthquake occurred,” explained Son Moon, a professor in the Department of Geological and Environmental Sciences at Pusan National University.

But an official from the KMA’s bureau of seismology and volcanology noted that the two faults are more than 50 km away from the quake’s epicenter. “We will need to confirm whether [the earthquake] occurred in line with those faults, or whether it occurred at a completely new fault line,” the official commented.

Rock in the Honam area is covered by sweeping plains that support one of Korea’s major breadbaskets, which makes it very hard to investigate the faults there. And given the weakness of the ground in that area, there are concerns an earthquake there could cause even more damage than is typical for its magnitude.

“The reason the 2017 Pohang earthquake (5.4 magnitude) did more damage despite being weaker than the 2016 Gyeongju earthquake (5.8 magnitude) is because the ground is weaker in Pohang. There are many weak parts in Honam, a grain-producing area where the soil goes deep. Stratigraphic research is also needed in the reclaimed land behind the Saemangeum Seawall, since the ground there is weak as well,” said Oh Chang-whan, professor emeritus at the Department of Earth and Environmental Sciences at Chonbuk National University.

The quake wrought some damage, including this shelf that had been holding beverages at a convenience store in Buan’s Haengan Township. (courtesy of the Jeonbuk Fire Service)
The quake wrought some damage, including this shelf that had been holding beverages at a convenience store in Buan’s Haengan Township. (courtesy of the Jeonbuk Fire Service)

The initial quake on Wednesday was followed by 17 aftershocks, including one with a magnitude of 3.1, as of 6 pm.

“There’s still stress in the surrounding area even after the earthquake, and that energy will have to be released one way or another,” said Lee Yun-su, an adjunct professor in the Division of Environmental Science and Engineering at POSTECH.

Given how unusual it is for a substantial earthquake to hit Honam, experts say the situation needs to be watched closely going forward.

“Historical records from the Joseon dynasty and elsewhere indicate that earthquakes with a magnitude of 5.0–6.0 do occur on Korea’s western coast, though on a very long cycle measured in centuries,” said Oh Chang-hwan.

By Shin So-yoon, staff reporter

Please direct questions or comments to [english@hani.co.kr]

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