A magnitude-5.1 earthquake struck the west coast of South Korea early Tuesday, marking the third-largest quake since the country began keeping records in 1978.

The tremor, a relatively moderate one by seismic standards, struck at 4:48 a.m. about 100 kilometers off West Gyeongnyeolbi Archipelago near Taean, South Chungcheong Province, according to the Korea Meteorological Administration. No tsunami, damage or injuries were reported.

The quake was followed by smaller aftershocks that were felt in adjacent areas including Taean, and Seosan in South Chungcheong Province, Seoul and Incheon.

“I live on the 12th floor of an apartment and I felt a shake while sleeping last night,” a Twitter user in Seoul wrote on his account.

An aftershock measuring at 2.3 magnitude followed at 9:25 a.m. in Seoul, the KMA said.

“A small number of facilities and buildings were briefly affected,” Park Joon-soo, a KMA official said. “There could be more aftershocks but they will not be as strong as the first quake.”

Due to the temblor, several facilities at semiconductor lines at Samsung Electronics plant in Giheung, Gyeonggi Province, suspended operations.

“A minor mechanical error occurred after receiving a shock from the quake,” a Samsung PR person said.

The latest quake came on the heels of a magnitude 4.9 tremor that hit near Baengnyeongdo Island about 10 months ago. Palpable quakes were felt in nearby cities but no damage was reported.

Despite the generally light seismic activity, South Korea has recently seen a rising number of earthquakes.

According to the meteorological agency, the peninsula was struck by 93 quakes with a magnitude of 2.0 or higher last year, almost twice the annual average of 44.5 quakes monitored between 1999 and 2012.

Earthquakes largely happened in and around the West Sea area, with 52 occurrences observed in that period. The East Sea area saw 15 quakes and North Korea seven quakes last year.

The strongest, based on official records, were two magnitude-5.2 earthquakes that hit areas near Mount Songnisan on Sept. 16, 1978, and in waters about 80 kilometers east of Uljin, North Gyeongsang Province, on May 29, 2004.

By Suk Gee-hyun (monicasuk@heraldcorp.com)