On March 28, 2025, a Mw7.7 earthquake struck along a section of the Sagaing Fault in Myanmar at a depth of 10 kilometers, causing Modified Mercalli shaking intensities of IX (violent) around the epicenter. But over 1,000 kilometers away in Bangkok, Thailand, a 30-story tower under construction collapsed, resulting in at least 96 fatalities.
How could an earthquake so far away cause such damage? The answer lies in the complex interaction between seismic waves and local geology. Bangkok sits on a deep sedimentary basin that amplifies longperiod surface waves — those most dangerous for tall structures (Figure 1). This event brings to the forefront a well-known seismic hazard: Distant quakes can wreak havoc on mid- and high-rise buildings in cities built on soft soils.
These long-distance effects are not a new phenomenon. In 1985, mid-rise buildings in Mexico City collapsed more than 350 kilometers from the epicenter of a Mw8.0 earthquake, as lakebed sediments amplified long-period waves. And in 2011, the Mw9.0 Tohoku earthquake generated similar long-period waves that traveled over 370 kilometers, causing high-rise buildings in Tokyo and Osaka to sway for minutes and sustain non-structural damage.
Long-period waves are a known hazard, and most catastrophe models do attempt to capture their effects. But for insurers and other risk managers
with exposure in sedimentary basins — such as San Francisco, Seattle and Istanbul — it's worth asking three pointed questions to better understand modeled loss estimates.
01
Are distant seismic sources included — even those beyond the model domain?
Long-period motions can originate from earthquakes far outside the local hazard zone or even national borders. To avoid underestimating risk, it’s important to ensure that distant rupture sources, including those outside a model’s core geographic domain but still capable of causing losses in your region of interest, are properly considered. For places such as Bangkok, accounting for these distant sources is crucial. The 2025 Myanmar earthquake generated ground motions in Bangkok in the 0.8 to 3.4 second period range, which corresponds to the natural resonant frequencies of buildings between 13 and 55 stories tall. The 30-story tower collapsed because its natural vibration period aligned with the earthquake's dominant shaking frequency, which amplified the building’s movement.
