A 4.0 magnitude earthquake hit the southeastern city of Gyeongju at 4:55 a.m. on Thursday, the state weather agency said, prompting mobile phones across Korea to blare en masse from the emergency text alert.

No damage has been reported as of Thursday afternoon.

The second-strongest quake to hit Korea this year struck the historical city of Gyeongju, located 277 kilometers southeast of Seoul in the wee hours of the morning. The quake's depth was measured at a shallow 12 kilometers and it was followed by five aftershocks which continued until 5:50 a.m., the Korea Meteorological Administration said.

The National Fire Agency said it had received a total of 107 reports from people nationwide within a few hours of the quake. A total of 49 reports were from North Gyeongsang Province, where the city of Gyeongju is located. Another 40 came from the industrial city of Ulsan, located some 30 kilometers from Gyeongju, with the rest filed from Daegu and Busan.

At around 7 a.m., Prime Minister Han Duck-soo ordered officials to raise the alert level of the Central Disaster and Safety Countermeasures Headquarters to Level 1. The level is the lowest in the government’s three-tier emergency alert system. A separate earthquake alert at the "vigilance” level, the third-highest in the five-tier system, has been issued.

Han urged ranking government officials to “have vigilance” and inspect the emergency systems needed to provide Korea's population with safety.

No damage has been reported at any of the nuclear or hydroelectric plants across the nation, said their operator, Korea Hydro and Nuclear Power Co. Wolseong Nuclear Power Plant, located in Gyeongju, was checked and was operating without any issues, the state-affiliated nuclear power company added.

Thursday’s earthquake is the second-strongest to hit Korea so far this year. The strongest was a 4.5-magnitude tremor that occurred in waters about 59 kilometers northeast of the eastern coastal city of Donghae, 182 kilometers east of Seoul, on May 15, according to the KMA.

This year, Korea has so far detected a total of 99 earthquakes over a magnitude of 2.0 on the peninsula and in its surrounding waters.

Thursday's seismic activity took place in the city where the country's strongest-ever earthquake of 5.8 magnitude hit on Sept. 12, 2016. A total of 23 people were injured and several historical sites were damaged. No deaths were reported.

The Cultural Heritage Administration, which oversees Korea’s historical sites, announced there were no damages in Gyeongju from Thursday’s quake. They vowed to further inspect the artifacts in the area closely for protection and safety.

Meanwhile, reports from people who were startled while sleeping due to the blare of the emergency text alert have been growing. Keywords “earthquake,” “sirens,” “trauma” and “forcefully woken up” were trending in Korean on X, formerly Twitter, at around 9 a.m.