A 3.7 magnitude earthquake struck off the western county of Ganghwa early Monday, the first quake stronger than 3.0 magnitude to hit South Korea this year, but no damage was reported, the state weather agency said.

The quake occurred at 1:28 a.m. and was centered in waters 25 kilometers west of Ganghwa with a depth of 19 km, the Korea Meteorological Administration said.

The earthquake was earlier estimated to be magnitude 4.0 but was later revised down.

The seismic intensity of Monday's quake, measuring the severity of ground shaking at a given location, reached Level 4 in Incheon, and Level 3 in Seoul and the surrounding Gyeonggi Province.

In the 12-level intensity scale used by South Korea, Level 4 is a level when many people indoors sense shaking, people at night are awakened, or windows or dishes become wobbly.

By 2 a.m., a total of 30 earthquake-related reports had been filed with fire authorities in Incheon, but no damage had been reported.

Monday's quake marks the first one over magnitude 3.0 that has struck South Korea this year. The last quake of a similar size was reported in the county of Goesan, 110 km southeast of Seoul, on Oct. 29 at 4.1 magnitude. (Yonhap)