A 4.9-magnitude quake hit an area near Baeknyeong Island off Incheon, west of Seoul on Saturday.
It follows a series of temblors in Northeast Asia, including a 7.0-magnitude earthquake in the Chinese province of Sichuan on April 20 and a 5.9-magnitude quake in waters off Sendai, Japan last Saturday.
The Korean quake took place in waters 31 km south of Baeknyeong Island at 7:02 a.m. Saturday. A total of nine aftershocks with a magnitude of 2.2-3.3 occurred over the next two days.
That was the largest number of aftershocks since the Korea Meteorological Administration began earthquake observation in 1978.
"We received reports from residents in Hongseong and Seosan in South Chungcheong Province as well as Seoul about 200 km away saying that they felt the ground tremble," a KMA official said.
Some residents on Baeknyeong and Socheong Islands evacuated to shelters, fearing that war had broken out.
Earthquakes are becoming more frequent. In the 1980s, an average of 15.7 quakes occurred here, in the 1990s 25.5 and in the 2000s 43.6. The first two years of the new decade already saw about 50, according to the KMA.
Some pundits worry that the latest quake could be a prelude to a massive quake with a magnitude of more than 6.