The 2004 Indian Ocean Tsunami, often called the Boxing Day Tsunami, was one of the most powerful and destructive natural disasters in recorded history. Triggered by a massive undersea earthquake, the resulting waves devastated coastal communities across fourteen countries bordering the Indian Ocean. Estimating the true size of the wave is complex because the water height varied dramatically depending on location and local geography. The event tragically claimed an estimated 227,898 lives.
Understanding Tsunami Run-Up and Inundation
The question of “how high” a tsunami was requires a specific scientific definition, as the measurement changes drastically from the deep ocean to the shoreline. In the open sea, the wave height—the vertical distance from crest to trough—is often less than a meter and barely noticeable to ships. This small amplitude wave travels extremely fast, sometimes over 800 kilometers per hour, carrying immense energy.
The height becomes destructive only as the wave nears the coast, where shallow water forces the energy upward in a process called shoaling. The most relevant measurement for destructive power is the run-up, defined as the maximum vertical height the water reaches above sea level on dry land. This run-up figure is distinct from inundation, which is the horizontal distance the water travels inland from the coastline. Run-up is the definitive metric used by scientists to quantify the maximum vertical reach of the water.
The Maximum Measured Heights
The most extreme run-up heights were recorded near the epicenter along the western coast of Sumatra, Indonesia, particularly in Aceh province. Post-disaster field surveys confirmed the waves reached extraordinary heights along this coastline. Sustained run-up measurements in the hardest-hit areas, such as Banda Aceh, commonly exceeded 15 to 20 meters.
The absolute maximum run-up measurement was recorded at a localized spot between Lhoknga and Leupung, on the northern tip of Sumatra. Unique coastal topography, including a narrow V-shaped bay, channeled the water, forcing the tsunami to a peak height of 51 meters (167 feet) above sea level.
This exceptional measurement represents the highest seismically-generated tsunami run-up ever recorded. These extreme heights resulted from wave energy being intensely focused by the shape of the seabed and shoreline.
Varying Impact Across Affected Regions
The height of the tsunami was not uniform, decreasing significantly with distance from the source and fluctuating based on the local coastal environment. In Sri Lanka, over 1,600 kilometers from the epicenter, run-up heights ranged from 4 to 12 meters. Despite the lower vertical height compared to Sumatra, widespread inundation across Sri Lanka’s coastal plains led to extensive damage and fatalities.
Thailand’s Andaman coast, including Khao Lak and Phuket, experienced run-up heights ranging from 5 to 12 meters, though some locations reached up to 20 meters. The destruction was amplified by the flat coastal topography, which allowed the water to rush far inland.
On the eastern coast of India, run-up heights typically fell within the 4 to 12-meter range, causing localized devastation, especially in the Andaman and Nicobar Islands.
Even the African coast, over 5,000 kilometers away, felt the impact. Run-up heights reached almost 10 meters in Somalia, and a measurable surge of 1.5 meters was noted in South Africa.
The Seismic Event That Generated the Wave
The massive wave was generated by the Sumatra–Andaman earthquake on December 26, 2004, off the west coast of Sumatra. This undersea event registered a moment magnitude between 9.1 and 9.3, making it one of the largest earthquakes ever recorded. The earthquake was a megathrust event, caused by the Indian tectonic plate subducting beneath the Burma microplate along the Sunda Trench.
The geological mechanism responsible for the tsunami’s exceptional size was the massive vertical displacement of the seafloor. The rupture zone extended for 1,200 to 1,300 kilometers, lifting a large section of the ocean floor. This sudden, large-scale thrust faulting mechanism displaced an enormous volume of water, transferring the earthquake’s energy directly into the column of water above it.