Attention is focused on how the 7.1-magnitude quake that struck Japan's Kumamoto Prefecture on the 28th might affect seismic activity on the Korean Peninsula. Experts said this quake is unlikely to directly trigger another earthquake in Korea. However, they noted that if it stimulates the Nankai Trough, where the risk of a major earthquake has built up, and leads to an even larger quake, supertall buildings along the southern coast, including in Busan, could shake strongly for a long time, requiring preparedness.
This quake occurred on the Futagawa-Hinagu fault system, where the magnitude-7-range Kumamoto earthquake struck in 2016. At the time, quakes of magnitude 5.0 off Ulsan in July and 5.8 in Gyeongju in September followed, prompting analysis that Japan's earthquakes might have influenced seismic activity on the Korean Peninsula.
However, the sections that actually ruptured in this year's quake and the 2016 quake appear to be different. Hong Tae-kyung, a professor in the Department of Earth System Sciences at Yonsei University, said in a phone call with ChosunBiz on the 29th, "In the 2016 earthquake, the fault ruptured to the northeast from the hypocenter, whereas this time, a segment about 12 kilometers to the southwest broke," adding, "It is possible that a portion of the fault that did not move and remained 10 years ago broke this time."
When an earthquake occurs, seismic waves spread in all directions, changing the balance of forces on nearby faults. The force accumulated in rocks underground is called "stress," and if a fault's stress is already near its limit, it can be affected even by relatively small shocks.
Park Jeong-ho, head of the Earthquake Research Center at the Korea Institute of Geoscience and Mineral Resources (KIGAM), said it is difficult to conclude there is absolutely no impact from this quake, given that the crust around Japan and the Korean Peninsula are connected to each other. However, depending on the state of faults in Korea, he said the impact may not appear at all or could emerge as seismic activity months or years later.
Meanwhile, Hong took a cautious view about interpreting the 2016 Ulsan and Gyeongju earthquakes as a direct result of the Kumamoto quake at the time. While seismic waves transmitted from Kumamoto produced shaking along the south coast strong enough for people to feel, he said the energy was not strong enough to rupture domestic faults. He analyzed that the Ulsan and Gyeongju quakes may have been more heavily influenced by the 2011 Great East Japan Earthquake.
The area Hong focused on more in relation to this quake is the Nankai Trough off southern Japan. The Nankai Trough is a plate boundary that extends from off Shizuoka Prefecture to Hyuganada east of Kyushu. Large earthquakes have recurred there at intervals of about 100 to 150 years. Since the great earthquakes in 1944 and 1946, energy has accumulated for a long time, and the possibility of a megaquake of magnitude 8 to 9 has been raised.
Hong said, "As the vibrations from this Kumamoto earthquake were transmitted to the Nankai Trough, the ground is analyzed to have moved at a speed of about 2 centimeters per second," adding, "Just as a loosened screw becomes looser if you keep shaking it, additional vibrations may have been applied to a fault already under a lot of stress, changing its condition in a direction that makes an earthquake more likely."
If a megaquake occurs in the Nankai Trough, the Korean Peninsula could be affected by "long-period ground motion," in which the ground shakes slowly and for a long time. Long-period ground motion transmits vibrational energy relatively well over long distances, and if the period of shaking is similar to a high-rise building's natural sway, resonance can occur, increasing movement in the upper floors.
Hong said, "In this earthquake, low-frequency energy of about 0.05 Hz appeared strongly. 0.05 Hz means it takes about 20 seconds for a single sway," adding, "Such slow vibrations can be felt more strongly in high-rise buildings along the southern coast, including Busan and South Gyeongsang."
He added, "If a magnitude 9.0-level earthquake occurs in the Nankai Trough, the amplitude of ground motion along the southern coast could grow to as much as 30 centimeters," and "It is necessary to check whether the supertall buildings clustered along the southern coast, such as Haeundae in Busan, can withstand slow shaking that lasts a long time."
Hong also cited as a task the fact that earthquake research in Korea is focused on faults on the peninsula, while analysis of Japanese faults and the Nankai Trough relies on data from Japanese researchers. He emphasized, "While sharing observational data with Japanese research institutions and expanding joint research, long-period ground motion from major earthquakes overseas should be reflected in safety inspections of domestic high-rise buildings and in disaster response systems."