Thailand is prone to natural disasters, such as floods, droughts, tropical storms, and forest fires, but weak to moderate (below magnitude 6.0) earthquakes are not uncommon and earthquake risk should not be underestimated. In the last two decades, three notable earthquakes have affected Thailand, resulting in extensive damage and/or loss of life.
In May 16, 2007, an M6.3 earthquake in neighboring country Laos caused damage to public buildings, including a school, a hospital, and historic structures. An M6.9 temblor that struck near the border with Thailand and Laos on March 24, 2011, caused one death in northern Thailand and was felt in the northwest as well as in Bangkok. And most recently the May 5, 2014, M6.1 Mae Lao earthquake in Chiang Rai, northern Thailand, was one of the most significant seismic events ever recorded in the country. This temblor resulted in extensive damage to more than 15,000 buildings and infrastructures, as well as one death and 107 injuries. According to the Department of Disaster Prevention and Mitigation at Chiang Rai, the estimated total economic loss reached USD 300 million.
Seismic Activity and Seismotectonic Setting
Thailand is situated away from sources that can cause large seismic events and is widely perceived to be in a low seismic hazard zone. Recent paleoseismological studies have confirmed, however, that Thailand is affected by 11 seismic fault zones and more than 50 active faults located inside or outside the country’s territory that can cause hazardous seismic ground shaking.
Located on a stable portion of the Eurasian Plate, Thailand is broadly characterized by transtensional stress and thus most significant seismic events are caused by normal or strike-slip motion. In neighboring Myanmar (Burma), the Sagaing fault is a major right-lateral strike-slip fault that was responsible for several earthquakes of M > 7.0 within recent history. Similarly, the Three Pagoda fault demonstrates right-lateral strike-slip motion and straddles the border between Thailand and Myanmar.
Thailand’s capital, Bangkok, is situated far from the country’s main seismic sources and there is no historical record of earthquakes close to the city (Figure 1). But this does not mean that Bangkok is free of seismic risk.
Bangkok is built on deep and soft alluvial soil, which can amplify incoming ground motion even from events occurring hundreds of kilometers distant. The ground motion experienced at locations on deep alluvial basins can be further amplified above and beyond what would be expected using the near-surface soil stiffness parameter only.
A well-known example of this amplification phenomenon was observed in the M8.1 Michoacán event in 1985, when an earthquake on the west coast of Mexico caused extensive damage and more than 5,000 casualties in Mexico City, 350 km from the epicenter. Although the magnitude was large, the quake's epicenter was sufficiently distant from concentrations of exposure inland that seismologists and engineers would not have expected it to cause significant damage. But Mexico City lies on silt and volcanic clay sediments, which resonate with long-period seismic waves and can quite dramatically increase their amplitude.
One important ground motion characteristic in the context of building damage is frequency, or number of complete cycles of vibration per second. A building responds to frequency content in ground motion by vibrating, and its vibration tends to center around a particular frequency—its natural frequency. Tall buildings have lower natural frequencies and when the ground motion is also dominated by low-frequency (long period) waves, the ground motion and building resonate, leading to greater building displacement and therefore damage.
Low-frequency seismic waves can travel great distances, while high-frequency seismic waves tend to attenuate quickly over distance. For that reason, when the seismic waves from the Michoacán earthquake finally reached Mexico City, low-frequency, long-period waves dominated. The frequency of these waves coincided with the natural period of the lakebed soil and thus the waves were amplified. The buildings whose natural frequencies coincided with the frequencies of the waves were mid-rise structures and some high-rise structures (essentially, buildings 6-15 stories tall). Many of these experienced either severe damage or collapse.
The AIR Earthquake Model for Southeast Asia
Because Mexico City has geological conditions particularly well aligned to yield destruction from distant subduction earthquakes it serves as a warning for Bangkok. Due to its surficial geologic setting, the more than 1,400 tall buildings (12 stories or more) in the metropolitan area and surrounding provinces, and the ability of regional sources to cause a major earthquake, Bangkok is susceptible.
The AIR Earthquake Model for Southeast Asia incorporates Thailand’s complex seismicity by generating events along known crustal faults, in subduction zones, and within a number of seismic source zones of Thailand and neighboring countries through smoothed background seismicity. The model implements surficial soil maps for the entire country at 2,500-meter resolution, and a 50-meter resolution microzonation map for Bangkok. In addition, the model supports five different height bands for Thailand, including the high-rise and tall bands that can be found in Bangkok. The loss cost map (Figure 2) confirms that higher losses are expected in areas of high seismicity, such as northern and western parts of the country, but also captures significant losses in the central and eastern parts of Bangkok Province.
Reliably assess and manage seismic risk with AIR earthquake models.
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