Beneath the Korean Peninsula, earth is rumbling
After the 2011 Tohoku earthquake, tectonic plates across East Asia were never the same
At 8:32 p.m. on Sep. 12, 2016, South Korea experienced the biggest earthquake on the Korean Peninsula since 1978, when the nation started monitoring the activity. The 5.8-magnitude quake’s epicenter was eight kilometers southwest of Gyeongju in North Gyeongsang Province. A 5.1-magnitude quake struck some 50 minutes before, nine kilometers from the city in the same direction.
Security cameras across the city showed street vendors’ windows being shattered and people running for their lives. Twenty-three were injured and there were almost 1,120 damage reports.
It was an unprecedented event that sparked interest nationwide and signaled that South Korea may no longer be an “earthquake haven” as many blindly believed.
It may be hard to discern what is going on underground and harder to admit that seismic peace on the Korean Peninsula, so far undisturbed by major earthquakes, is no longer guaranteed. But the truth is that the earth beneath the region experienced a turning point on Mar. 11, 2011, when a magnitude-9.0 mega-thrust quake rattled the Pacific seabed some 70 kilometers east of the Oshika Peninsula of Tohoku in northern Japan. It immediately caused a tsunami that pummeled nuclear power plants in Okuma in Fukushima Prefecture, leading to the Fukushima nuclear disaster.
Some across the East Sea might have thought the natural occurrence a distant foreign affair, but what they didn’t realize is that the colossal quake also brought consequences for Koreans.
These gained more traction on Sept. 3 this year when North Korea detonated a nuclear device underground near Punggye-ri in North Hamgyung Province, its sixth nuclear test. A magnitude-5 earthquake quickly followed that day 6.6 kilometers southeast of the site. Twenty days later, a magnitude-3 tremor struck six kilometers northwest of the site. It was hard to dismiss the link between the test and the seismic activity. The experiment threat- ened peace not only across the East Asia and Pacific regions but also beneath the two Koreas.
Impact by Tohoku earthquake, N. Korean nuclear test
Korea Seismological Institute head Kim So-gu said the 2011 earthquake near Tohoku was rare in its intensity. Strong enough to “shake the underground axes,” the quake shifted the Korean Peninsula’s position eastward by two centimeters and Ulleungdo Island and Dokdo Island in the peninsula’s eastern waters by four to five centimeters.
“The Tohoku earthquake awakened nearby faults that so far had remained quiet,” said Kim, one of South Korea’s pioneers in earthquake research. “One example is a fault created between Ulsan and Gyeongju. I call it the Gyeongju-Ulsan fault.”
Kim said the 2016 activity at the Gyeongju-Ulsan fault caused the Gyeongju earthquake not the Yangsan fault between Busan and Gyeongju that many news outlets had reported as the quake’s origin. The Tohoku earthquake created many other faults in oceanic plates under Japan and the Korean Peninsula — the Filipino Plate to the south, the North American Plate to the north, the Pacific Plate to the east and the Eurasian Plate to the west — raising earthquake risks to South Korea.
“Although South Korea has been comfortably ‘sandwiched’ between Japan that directly faces plate boundaries and China that constantly experienced seismic activity from the Indian Plate and Eurasian Plate pressing against each other, it now has more chances to see quakes on its own turf following the Tohoku earthquake,” Kim said.
The Tohoku earthquake’s sheer magnitude was likely weaker in its intensity at its onset. For mega-thrust earthquakes like Tohoku, continuous subduction of the mega-thrust causes stress and strain to build up in the overlying plate, developing magnitude-7 earthquakes into magnitude-8s, according to Hiroshi Sato, a geology professor from the Earthquake Research Institute at the University of Tokyo.
“Such tendency is clearly identified in Japan, just sitting on the subduction mega-thrust,” said Sato, adding that the number of earthquakes in South Korea is “overwhelmingly small compared to Japan.”
What was possibly equivalent to the Tohoku earthquake’s magnitude was the North Korean nuclear test. Following the test, the Korea Meteorological Administration (KMA) in Seoul said the blast created seismic activity with a magnitude reaching 5.9. But Kim dismissed the reading, saying it was at least 6.4.
“A similar test was conducted in the Aleutian Islands in 1969, which created a magnitude of about one megaton,” said Kim, referring to the test in part of the islands called Amchitka in southwest Alaska. “The test in Punggye-ri swept with as much power underground as the Tohoku earthquake, shattering all the tectonic bases in proximity. It could have been up to 0.9 megatons, close to the nuclear testing in the Aleutians.”
The earthquakes following the North Korean nuclear test were a semi-natural occurrence, Kim said. The test broke the weakest tectonic layers first, causing natural subduction and likely landslides and destruction of tunnels.
“That happened, and that can happen again,” Kim said.
Alert on Ryukyu Trench
Ryukyu Trench, a 1,398 kilometer-long oceanic trench along the southeastern edge of Japan’s Ryukyu Islands in the Philippine Sea, is on Sato’s watch list. The trench’s movement, mixed with other tectonic shifts, has caused seismic activity leading to major earthquakes and can likely produce similar results again.
The trench’s southeastward movement and an expansion of sticking in the Nankai Trough, a submarine trough south of the Nankaido region of Japan’s island of Honshu, caused a magnitude-7 earthquake on the western coast of Kagoshima prefecture in November 2015 and a magnitude-7.1 near Kumamoto in April 2016. It also influenced the Gyeongju earthquake.
“I think the rollback and retreat along the Ryukyu Trench produced the series of earthquakes, including the ones at Kumamoto and Gyeongju,” Sato said. “The Gyeongju earthquake especially showed that the trench’s influence is great. The force of the influence is brought about by the expansion of the Okinawa trough and the retreat of the Ryukyu Trench toward the sea side. In recent years, the influence may have become stronger.”
Kim agreed with Sato on the trench’s role in the Gyeongju earthquake. The trench is at a plate boundary — where major inter plate earthquakes mostly occur — between the Eurasian Plate moving southeastward and the Filipino Plate moving northward.
“The Gyeongju earthquake was indirectly kicked off by the trench,” Kim said. “South Korea, where most of the faults are of the strike-slip type, is influenced more by the Filipino Plate than the Pacific Plate that is much bigger, faster and moving northwestward.”
Earthquakes difficult to predict
At the KMA, issuing earthquake alerts comes down to a matter of seconds. In 2010, the weather watchdog started developing an early warning system for earthquakes of magnitude-5 or higher that can issue alerts within 50 seconds. It is due for completion in 2020.
In 2015, the KMA put the system’s first phase into practice. In July 2016, when a magnitude-5 earthquake hit Ulsan, the system issued a warning to TV broadcasters, the government and municipal governments in 27 seconds. On Sep. 12, when two Gyeongju earthquakes occurred, warnings took 27 and 26 seconds. While some admitted the system was successful, the National Assembly demanded that the KMA issue quicker warnings — under 15 seconds — before the end of 2018.
“The lawmakers emphasized that although the warning is inaccurate in terms of information, it must be issued fast,” said Woo Nam-chul, an earthquake analyst from the administration’s Center for Earthquake and Volcano. “But whenever we issued warnings with the wrong information, it raised the ire of Koreans who are keen on getting a disaster forecast. Keeping accuracy and prompt- ness simultaneously is very difficult.”
While people want quick earthquake alerts, predicting quakes on the Korean Peninsula is hard — more so with major earthquakes — because of a lack of data and nature’s unpredictable ways.
“There has never been any major earthquake on the Korean Peninsula throughout history,” Kim said. “So we have no accumulated data to set up standards for earthquake-proof building codes suitable for our geographical environment. Much of the coastal regions surrounding South Korea are filled grounds with weak tectonic bases, so they are vulnerable to the disasters.”
For geologists, having a database containing an extensive history of earthquakes is important to forecasting.
“Earthquakes exceeding the magnitude-7 class have not occurred on the Korean Peninsula for about 300 years,” Sato said. “But earthquakes similar to Tohoku occurred in 869. It is not prudent to evaluate the risk by only checking the seismic activity of the most-recent 300 years.”
The possibility of a major earthquake in South Korea exists in the Yangsan fault. A 1994 survey by a research group led by Prof. Okada from Kyoto University on a 15-kilometer-deep cross-section of the fault’s earthquake occurrence layer suggested the country’s southwestern region could experience magnitude 7-class earthquakes.
“The average vertical slip rate of the Yangsan fault is 0.03 millimeters per year and the right-lateral displacement rate 0.1 millimeters per year,” said Sato, who was a member of the research group. “Active faults showing such a slip rate commonly have activity once in several thousand years, meaning that the chance of an earthquake by the large movement of the fault is very low. However, the risk exists.”
One way to prepare for a disaster is to understand how it happens. Tokyo University is constructing a numerical model to study the occurrences of major earthquakes in overlying plates in relation to the subduction of tectonic plates. Records show a strong interaction between them, according to the professor.
“In the case of the Korean Peninsula, the distance to the plate boundary is longer than Japan and the effect will be weaker, thus the problems are more complicated,” Sato said. “At this level of science, to predict an earthquake is almost impossible. The only way is to improve the building code and make the buildings stronger.”