Pohang earthquake likely connected to Gyeogju, Tohoku disasters
A tsunami floods over the breakwater protecting the coastal city of Miyako at Heigawa estuary area after northeastern Japan was hit by a powerful earthquake on Mar. 11, 2011. / Korea Times file
By Ko Dong-hwan
The magnitude-5.4 earthquake that hit Pohang on Wednesday is likely connected to earthquakes in Gyeongju last year and one under the Pacific Ocean seabed near Tohoku in Japan, South Korean experts said.
The Pohang earthquake happened about 2:30 p.m. nine kilometers north of the port city in North Gyeongsang Province and nine kilometers deep. There were 41 less-intense tremors that followed, including 38 of magnitude-2 to 3 and one of 4.6 about two hours after the main quake.
The quake damaged buildings, roads and cracked city streets. The main shock could be felt in Seoul over 330 kilometers to the north.
The earthquake occurred 14 months after a strike-slip quake (where two sides of a fault slip horizontally past each other) with a magnitude of 5.8 struck Gyeongju, 26 kilometers south of Pohang.
Prof. Hong Tae-kyung from Yonsei University’s Department of Earth System Sciences said the Pohang earthquake is the result of the stored strain energy from the Gyeongju earthquake being released at once.
“The strain from the Gyeongju earthquake has been stored along the northeast and southwest directions,” Hong said. “And the Pohang earthquake has begun accumulating the same kind of strain energy. It may go off in the form of a major earthquake near Gyeongju or Pohang in the future.”
The latest earthquake was influenced by the Gyeongju earthquake just as the latter was affected by the Tohoku earthquake in 2011 and the Kumamoto earthquakes in April 2016, Pukyong National University Professor Kim Yong-seok said.
A magnitude-9.0 mega-thrust quake rattled the Pacific seabed some 70 kilometers east of the Oshika Peninsula of Tohoku in northern Japan, causing a tsunami that pummeled nuclear power plants in Okuma in Fukushima Prefecture, leading to the Fukushima nuclear disaster.
Earthquakes with a magnitude-7 main shock that struck beneath Kumamoto City in Kumamoto Prefecture in the Kyushu Region, and a foreshock with a magnitude of 6.2, killed at least 50 people and injured about 3,000.
“Just as the Gyeongju earthquake is attributable to the stored strain energy beneath the Korean Peninsula from the two large-scale Japanese earthquakes being released, the same causal relation between the Gyeongju and Pohang earthquakes can be responsible for the latest disaster,” Kim said. “Statistics so far hint that South Korea will experience minor aftershocks for the coming three years at least.”
Stranded fishing boats and damage from the earthquake and tsunami on Mar. 11, 2011, are seen in Matsukawaura fishing port in Soma, Fukushima Prefecture, in northeast Japan, Mar. 13, 2011. / Korea Times file
The Korea Meteorological Administration’s earthquake and volcano research center held an emergency debriefing Wednesday following the Pohang earthquake. Center head Lee Mi-sun said the latest earthquake appears to have happened from an unknown fault developing from the Yangsan fault between Busan and Gyeongju.
“The link between the Pohang earthquake and the unknown fault remains to be studied further,” Lee said. “And so do possible connections between earthquakes from Pohang, Gyeongju and Tohoku.”
A theory that the latest earthquake is a manmade disaster also grabbed national attention. Korea University’s geology professor Lee Jin-han said on “Newsroom” on TV network JTBC Wednesday that a geothermal power station under construction in Pohang caused the earthquake.
Two holes were drilled 4.5 kilometers deep to tap steam to run a turbine. But the deeper the holes are drilled, the stronger the water pressure, increasing the risk of breaking underground rocks and causing seismic shifts.
Lee said geothermal stations in other countries drilled only tens to hundreds of meters down. He said unnoticeably small seismic activity has consistently occurred around the thermal station ― just two kilometers east of the Pohang earthquake’s epicenter.
“It’s not 100 percent for sure, but the earthquake could have been manmade,” Lee said.