It occurred about 50 minutes after a 5.1-magnitude quake hit the city at 7:44 p.m.
The Ministry of Public Safety and Security organized a Disaster Management Office, an emergency team that monitors damage and takes necessary emergency measures.
At least 22 aftershocks with magnitudes of 2 to 3 were detected by 10 p.m. with more expected throughout the night.
The epicenter for the first quake was the Naenam Elementary School in Gyeongju. The second quake's epicenter was 1.4 kilometers away, according to the Daegu Regional Meteorological Administration.
Tremors spread throughout the country, including Seoul, terrifying people who felt them. They were even detected in Kyushu, Japan.
The government has ordered people in the quake-hit areas to evacuate to safe areas.
As of 1 a.m., six people had been injured, including an elderly woman injured by her television that fell from its mounting. The government said more than 30 minor cases property damage, such as cracks in walls, were also reported.
Nuclear power plants near Gyeongju are operating normally, according to Korea Hydro and Nuclear Power. Major manufacturing facilities in Pohang and Ulsan, many of which are owned by Hyundai Motor, SK, Hyundai Heavy Industries, were not affected by the quakes.
A liquefied natural gas-fueled power station in Ulsan, just kilometers away from Gyeongju, was shut down in the wake of the quakes.
Subway trains in Busan temporary stopped operation, and KTX bullet trains travelling through the quake-hit area reduced speed in case additional aftershocks damaged rail lines. Busan International Finance Center said it evacuated most people from its 63-story building as a precautionary measure.
As people in the affected areas called their family and friends, some regions had communication disruptions, including on KakaoTalk.
"I ran out of my office after feeling violent shaking," a worker living in Pohang, several kilometers north of Ulsan, told The Korea Times over the phone. He said the tremors were "more powerful than the previous one," referring to a 5-magnitude quake detected in waters 52 kilometers off the southeastern port city of Ulsan on July 5.
A person living near the epicenter told local broadcaster KBS that all people living in his village had evacuated to safe areas.
"The ground started shaking strongly and a loud noise was heard," he said, recollecting the first quake. "There were no signs before the quake. The ground started shaking all of a sudden."
Experts say there also have been more than 20 tremors of magnitude 2 or higher on the Korean Peninsula this year. Statistically, however, it has been extremely rare for the country to experience quakes with magnitudes of 5 or stronger.
They speculate that the latest quakes indicates the Korean Peninsula is no longer in the "safe zone."
Experts say that if an earthquake with magnitude 6 or above hits Korea, mass casualties are possible because many buildings are not built to withstand temblors.
A 5.8-magnitutde earthquake, the most powerful seismic activity here since 1978 when the nation started measuring tremors, struck the historic city of Gyeongju, North Gyeongsang Province, at 8:32 p.m., the Korea Meteorological Administration (KMA) said Monday.
El epicentro tuvo lugar en Gyeongju.
El 12 de septiembre a las 8:32 de la noche hora coreana, un terremoto de 5.8 grados de magnitud sacudió el sudeste de , con epicentro en una región 8 kilómetros al sur de la ciudad de . Este terremoto —el más fuerte registrado desde 1978— fue precedido por un terremoto de 5.1 de magnitud 50 minutos antes.

Los temblores se sintieron en  e incluso en , . Luego del sismo, se sucedieron réplicas de magnitud 2 a 3.

El Ministerio de Administraciones Públicas y Seguridad informó de seis heridos y daños menores a moderados a la propiedad, y organizó un equipo de emergencia para evaluar los daños y tomar las medidas de emergencia necesarias. Los habitantes cerca del epicentro fueron evacuados a áreas seguras. El sistema de metro fue suspendido temporalmente cerca de las áreas de epicentro de los dos sismos. La presidenta de Corea del Sur, Park Geun hye, prometió ayer tomar medidas para proteger los edificios más importantes —entre ellos las plantas nucleares— de los terremotos.

Ha crecido entre los expertos el temor de que Corea del Sur se convierta en una zona con fuerte actividad sísmica, considerada antes un refugio del Anillo de Fuego sobre el que se encuentra Japón, país frecuentemente azotado por la actividad sísmica. Los expertos ahora advierten que los recientes terremotos en el vecino Japón hayan podido desestabilizar las líneas de falla en Corea, lo que haría que la región fuera más inestable y los temblores más frecuentes.