SEOUL -- A 5.8-magnitude earthquake jolted southeastern South Korea on Monday, the biggest-ever quake striking the country, injuring at least two people and making people feel the tremor nationwide, Seoul's weather agency said.

The biggest tremor came less than an hour after a 5.1-magnitude earthquake struck on the outskirts of Gyeongju city in North Gyeongsang province at about 7:44 p.m. local time (1044 GMT).

The epicenter of the first quake, which recorded the country's fifth-biggest seismic intensity, was some 9 km southwest of Gyeongju city, an official at the Korea Meteorological Administration (KMA) said in a televised press briefing.

The second tremor with the highest-ever magnitude of 5.8 occurred at 8:32 p.m. at a nearby location, just 1.4 km away from the first epicenter.

Concerns emerge here that South Korea is no longer a safety zone from seismic activity as earthquakes of 5.0 magnitudes or higher have happened three times in 2016 alone.

A 5.0-magnitude quake struck in waters some 52 km east of the country's southeastern city of Ulsan on July 5. The North Gyeongsang province has been hit by quakes 62 times in the past 10 years, including Monday's two tremors.

At least 22 aftershocks with two to four magnitudes followed the big quakes. The weather agency sees no possibility for tsunami and little chance for bigger quakes.

Two people were injured in Gyeongju city, but no more injuries have been reported yet.

According to TV footage, cracks were seen in roads and in the walls of buildings and apartments. One house saw its roof collapsed, with bottles and items broken and strewn across the floor after falling from shelves at a discount outlet.

The big quakes were felt nationwide from the epicenter's adjacent cities of Ulsan and Busan to as far north as capital Seoul and far south as the southern resort island of Jeju.

A combined cycle power plant, fired by liquefied natural gas (LNG), in Ulsan city halted operation after the strong earthquakes hit the southeastern region.

Nuclear power plants, the majority of which are located in southeast South Korea, are being normally operated despite the big quakes.