The 2004 Indian Ocean Tsunami remains one of the most devastating natural disasters in modern history, causing widespread destruction across the Indian Ocean basin. The Thai coastline, particularly popular tourist destinations along the Andaman Sea, was severely impacted by the force of the arriving waves. Understanding the destructive power of this event requires quantifying the magnitude of the waves that struck the Thai shoreline. This analysis focuses on the geological event that generated the tsunami and the physical measurements used to define the wave’s height upon impact.
The Seismic Event That Generated the Tsunami
The waves were generated by the 2004 Sumatra-Andaman earthquake, which occurred on December 26, 2004, off the west coast of Sumatra, Indonesia. This megathrust earthquake registered a moment magnitude of 9.1 to 9.3, making it one of the largest earthquakes recorded globally since 1900. The event originated in the Sunda Trench, a subduction zone where the Indian plate slides beneath the Burma plate.
The rupture involved a fault section estimated to be around 1,600 kilometers long, slipping by several meters. This uplift of the seafloor displaced an estimated 30 cubic kilometers of ocean water. This displacement acted as the engine for the tsunami, radiating energy across the entire Indian Ocean basin.
How Tsunami Height is Measured
The height of a tsunami in the deep ocean is often negligible, but the waves grow dramatically as they approach the coast. Scientists rely on two primary measurements to quantify the destructive potential of a tsunami upon impact on land. The first is “run-up,” which defines the maximum vertical height the water reached above the normal sea level during the event.
Run-up is measured from the highest watermarks or debris lines left on structures or the ground at the farthest point of inland penetration. The second measurement is “inundation,” which is the horizontal distance the tsunami traveled inland from the shoreline. Run-up height provides the clearest indication of the wave’s vertical magnitude when it strikes the land.
Maximum Run-Up Heights Recorded in Thailand
Run-up heights in Thailand varied significantly across the Andaman Sea coastline, generally ranging from 5 to 20 meters. The highest measurements were concentrated in the province of Phang Nga, specifically in the Khao Lak area. Although the maximum run-up across the Indian Ocean was recorded in Aceh, Indonesia, Khao Lak experienced the most extreme heights outside of Sumatra.
The maximum measured run-up height in Thailand reached approximately 19.6 meters (nearly 64 feet) at Ban Thung Dap on Phra Thong Island, located in Phang Nga. The Khao Lak coastal area, a popular tourist destination, generally saw run-up heights between 5 and 15 meters. In contrast, Phuket Island recorded lower average heights, with some locations on the east coast seeing heights of about 2 meters.
This variability was a direct result of local geography. The widespread devastation in the Khao Lak region was directly related to the high run-up measurements. Significant destruction and fatalities occurred across all six affected Thai provinces, including Krabi and Satun, even in areas with lower run-up measurements due to the sheer volume of water involved.
Local Factors Affecting Wave Magnitude
The difference in run-up heights between locations like Khao Lak and Phuket was primarily due to local coastal geography and underwater features, known as bathymetry. The Khao Lak coastline features a shallow, gently sloping seafloor extending a considerable distance offshore. As the tsunami approached this shallow area, friction caused the wave to slow down, but its energy was conserved by compressing the wave’s volume into a greater height.
This effect, known as shoaling, amplified the wave’s vertical height before it reached the shore, resulting in high run-up measurements. Furthermore, the absence of protective features in some areas, such as fringing coral reefs, allowed the wave to strike the land unimpeded.
In contrast, some coastal areas were protected by deeper bays or the presence of offshore coral reefs. These reefs acted as a natural barrier, dissipating wave energy and reducing the final run-up height measured behind them. The orientation of the coastline relative to the incoming wave front also played a role. Areas directly perpendicular to the wave’s path experienced a more concentrated impact. This complex interaction explains why the destructive power was intensely localized along the Thai coast.