[Contribution] What the Nankai Trough megaquake threat means for South Korea

By Lee Mi-seon, administrator of the Korea Meteorological Administration

Could an earthquake in Japan cause high-rise buildings in South Korea, hundreds of kilometers away, to sway?

When an earthquake occurs, we first check its epicenter and magnitude. However, in modern society, where high-rise buildings are common, assessing our safety requires us to ask another question: How will the building I am in respond to the ground motion?

Even within the same area, levels of shaking can vary depending on local site conditions and a building’s height and structural characteristics. Earthquakes can also affect high-rise buildings far from the source.

In 2025, threats of a major Nankai Trough earthquake drew considerable attention in Japan. The Nankai Trough is a submarine trough extending from Suruga Bay, along the southern coast of Shikoku, to the Hyuga-nada Sea off eastern Kyushu.

Major earthquakes have repeatedly occurred in this region throughout history.

In the same year, the Japanese government released damage estimates indicating that a maximum-class Nankai Trough earthquake could cause approximately 298,000 fatalities and leave around 2.35 million buildings completely destroyed or burned down. In July 2025, Japan also revised its Basic Plan for the Promotion of Nankai Trough Earthquake Disaster Management Countermeasures, further strengthening its preparedness for a major earthquake.

When a magnitude 7.1 earthquake struck Kumamoto prefecture, Japan, on July 28, 789 reports of shaking were received in Korea. A maximum instrumental intensity of III was recorded in Busan, South Gyeongsang Province, Jeonnam-Gwangju and Jeju Island. In some areas, residents reported that buildings shook or that they felt a prolonged swaying motion, as if they were on a boat.

The event underscored that an earthquake occurring across the sea in Japan can also affect Korea.

How, then, can a distant earthquake affect high-rise buildings? To understand this phenomenon, we need to focus on long-period ground motion. This refers to seismic shaking with relatively long periods of about 2 to 20 seconds. It is particularly prominent during large earthquakes and can propagate over long distances, as long-period components of ground motion may undergo less attenuation than short-period components.

Buildings have their own natural periods of vibration, determined by their height and structural characteristics.

In general, the taller the building, the longer its natural period. Just as gentle pushes timed to a swing’s natural rhythm can make it swing higher, a building can experience amplified structural response when the period of ground motion approaches its natural period. This phenomenon is known as resonance.

Consequently, a building may undergo significant, prolonged oscillations, with upper floors potentially experiencing greater displacement. Furniture, fixtures and other objects inside the building may shift or topple, posing safety risks.

Past earthquakes illustrate this phenomenon. During the 2011 Great East Japan Earthquake, high-rise buildings in Osaka, approximately 700 kilometers from the epicenter, experienced prolonged oscillations.

Similarly, during the 2025 Myanmar earthquake, high-rise buildings in Bangkok, approximately 1,000 kilometers away, experienced significant shaking and damage.

These cases demonstrate how distant earthquakes can have substantial effects on tall buildings, depending on seismic wave characteristics, local site conditions, and building height and structures.

As major cities continue to grow vertically, understanding and preparing for the effects of long-period ground motion on buildings is becoming increasingly important. Protecting the public from earthquake hazards requires information not only on where and how strongly the ground shook, but also on how much shaking can be expected at different heights within different types of buildings.

To assess the domestic impacts of major overseas earthquakes more rapidly, the Korea Meteorological Administration plans to expand the area covered by its overseas earthquake analysis, previously limited to the vicinity of Kyushu, Japan, to include the Nankai Trough. The KMA is also developing technologies to estimate structural responses to long-period ground motion.

Building on these efforts, it plans to begin providing a pilot long-period ground motion service in 2027. The service aims to provide more detailed information not only on ground-level shaking but also on the shaking expected in buildings of different types and heights.

The timing of a major earthquake along the Nankai Trough cannot be predicted precisely. Nor can we prevent earthquakes or control a major earthquake occurring in another country. However, we can scientifically assess how such an event may affect the places where we live and work, and prepare for those effects in advance.

We must stay one step ahead, preparing before an earthquake occurring far beyond our sight makes itself felt in our daily lives. That is the challenge posed by the potential threat of a major Nankai Trough earthquake.

---

Lee Mi-seon is the administrator of the Korea Meteorological Administration. The views expressed here are the writer’s own. — Ed.