Biggest earthquake of year rocks east coast city

An earthquake with a magnitude of 4.5 struck off the east coast city of Donghae, Gangwon, on Monday morning, the strongest quake to hit the country this year. The earthquake was detected 52 kilometers (32 miles) northeast of Donghae at a depth ...

Officials at the Korea Meteorological Administration observe data about a 4.5 magnitude earthquake that struck the waters off Donghae in Gangwon on Monday morning. [YONHAP]

An earthquake with a magnitude of 4.5 struck off the east coast city of Donghae, Gangwon, on Monday morning, the strongest quake to hit the country this year.

The earthquake was detected 52 kilometers (32 miles) northeast of Donghae at a depth of 32 kilometers, according to the Korea Meteorological Administration (KMA).

The earthquake reached Level 3 on the Korean seismic intensity scale of 12 in Gangwon and 2 in North Chungcheong.

During an earthquake with an intensity scale of 3, people indoors — particularly those on upper floors — can feel the tremor, and parked cars shake. Level 2 is when just a few people in quiet places and on upper floors can feel the shaking.

No casualties or damages have been reported, but residents said they woke up in bed with their homes shaking.

Monday's earthquake worried many residents living along the east coast, as over 50 quakes have struck the area in the past month.

On Sunday, a 3.1 magnitude earthquake occurred in waters 51 kilometers northeast of Donghae, and two earthquakes with a magnitude of 2.5 struck in waters 4 kilometers south and southeast of Donghae Wednesday.

"I was in my yard in the morning and was surprised to see the entire yard tremble," a 69-year-old living in Mukho-dong in the northern area of Donghae told the JoongAng Ilbo.

"My family who was at home could feel the earthquake, too."

The Interior and Safety Ministry raised its earthquake alert level by one notch from "interest" to "alert" at 7:30 a.m. Monday.

The ministry had issued the alert level of its four-level warning system on April 25 as multiple earthquakes struck Gangwon.

It also asked the public "to be aware of earthquake safety measures in case of emergency."

Students at Chilbo Middle School in Suwon, Gyeonggi, participate in an earthquake evacuation drill on Monday. [YONHAP]

"I am worried because earthquakes' magnitude is rising," a 50-year-old resident of Mukho-dong said.

"It is more concerning because there are power plants and gas facilities in Uljin, North Gyeongsang, which is close."

Gangwon Gov. Kim Jin-tae on Monday also advised related authorities to closely monitor the situation and swiftly respond to emergencies as earthquakes continued to occur in the area.

Monday's earthquake is believed to have resulted from a reverse fault which occurs when one plate moves up relative to another plate due to compression, according to the KMA.

Experts estimate that the earthquake started from an unknown fault located north of Donghae as it was detected emanating from an area relatively distant from the Hupo Fault or the Ulleung Fault, two large underwater faults of the East Sea.

"There is a fault that extends to the northeast direction from the northern end of the Ulleung Fault," Kim Young-seog, a geoscience professor at Pukyong National University, said.

"The fault has never been reported in Korea, but I found the information from a case compiled by a Japanese scholar."

Kim added that if the latest earthquake was a foreshock, a 6.0 magnitude earthquake may be coming as the unknown fault is connected to two big faults in the area.

"But it's also possible that no additional earthquakes will occur because the focal depth is too deep."

The focal depth of earthquakes that normally occur on the Korean Peninsula is usually less than 10 kilometers, but Monday's earthquake was detected at a depth of 32 kilometers.

BY PARK JIN-HO,CHO JUNG-WOO [cho.jungwoo1@joongang.co.kr]