Gyeongju earthquakes

By Kim Tai-Jin

Last September, the region of Gyeongju was struck by the biggest earthquake recorded in recent Korean history, and since then, the citizens of Gyeongju have been subject to a series of aftershocks, which affect their daily lives.

North Korea conducted five underground tests at the Punggae-ri nuclear test site, which is made of Cretaceous granite. However, there are 4,380 venting holes in the Hupo Basin. It is assumed that the Yangsan and Ulsan Faults cause earthquakes to converge in Gyeongju to channel seismic energies through the Yangsan Fault until they are released in the Hupo Bank near Pohang through the venting holes in the cold seawater of the Hupo Basin.

During the 2016 Gyeongju earthquake, the foreshock occurred presumably via a shortcut from the Punggae-ri nuclear test site through the East Sea with the North Korea Plateau, South Korea Plateau, Hupo Fault, Hupo Bank, Hupo Basin, Yangsan Fault and Gyeongju until it was blocked by the high mountains. The Hupo Basin near Pohang is spread along the Hupo Bank, which is 85 kilometers long and 2.5-16.5 kilometers wide and where snow crabs live 200-400 meters below in the Hupo Basin at 3 degrees Celsius.

Therefore, seismic energies from Gyeongju are continuously carried along the northern Yangsan Fault and eventually released in the East Sea through the 4,380 geothermal venting holes in the Hupo Basin, which may allow the well-known growth of the snow crab (Chionoecetes opilio) near the Uljin, Hupo and Ganggu estuaries and Youngduk near Pohang.

Although earthquakes of strong magnitude had occurred in Gyeongju a thousand years before, seismic impacts had passed over Gyeongju with the channeling of the northern Yangsan Fault until they were released through the geothermal venting holes in the Hupo Basin to the East Sea.

In the case of the 2016 Gyeongju earthquake induced by the North Korean nuclear test, the direction of the seismic energy flow was totally opposite. That is, while the seismic sources are normally upward in the northern direction from Gyeongju to the East Sea, the source of the 2016 Gyeongju earthquake went downward toward the southern direction from Punggae-ri in North Korea through the short course of the East Sea to the Yangsan Fault and Gyeongju with the foreshock.

The main shock came 48 minutes later through the Baekdu Great Mountains. Due to the two different propagating routes of the East Sea and Baekdu Mountains between Punggae-ri and the city of Gyeongju, most of the 450 faults on the Korean Peninsula might be activated to reserve their seismic energies in individual faults, which could be the reason for the many aftershocks of 611 earthquakes.

Such areas could be local reservoirs of seismic energies induced by the North Korean underground nuclear test, after which the stored energies can be slowly released in the form of aftershocks. A recent geothermal study revealed that the seismic energies that gathered in Gyeongju from the intersection of the Yangsan and Ulsan Faults could endure an earthquake with a magnitude of up to M 7.73 through the channeling of the northern Yangsan Fault and the 4,380 geothermal venting holes in the Hupo Basin. This may be why there had been no disastrous damages, except for 100 deaths, during with the magnitude M 6.7 earthquake a thousand years ago in Gyeongju, when it was the ancient capital of the Silla Kingdom in 779 AD.

Therefore, tourists can safely visit Gyeongju and do not have to worry about earthquakes. I believe seismic energies in the Gyeongju area could pass through the channeling of the northern Yangsan Fault and the 4,380 geothermal venting holes in the Hupo Basin and released in the cold East Sea without causing seismic damages to the Gyeongju region.

The writer is a professor of chemical engineering at The University of Suwon. He can be reached at tjkim@suwon.ac.kr.