The devastating Boxing Day Tsunami struck on December 26, 2004, originating from a massive undersea earthquake in the Indian Ocean. This catastrophic disaster affected coastal regions across 14 countries, including Indonesia, Sri Lanka, Thailand, and India. The resulting ocean waves caused widespread destruction and an estimated loss of life exceeding 227,000 people, making it one of the deadliest tsunamis in recorded history. The tsunami’s height was complex because the water did not crest at a uniform level, varying significantly depending on the location it reached.
The Underlying Geological Event
The tsunami was triggered by the Sumatra-Andaman Earthquake, one of the largest ever recorded, with a magnitude estimated between 9.1 and 9.3. This seismic event occurred along a subduction zone where the Indian Plate slides beneath the Burma Microplate. The plates had been locked for centuries, accumulating stress until the fault ruptured along 1,300 kilometers off the west coast of Sumatra, Indonesia.
The sudden rupture caused a rapid, vertical displacement of the seafloor by several meters, violently pushing the overlying water column upward. This movement generated the tsunami waves that propagated across the entire Indian Ocean basin. The energy released was so vast that it caused the entire planet to vibrate slightly and even altered the Earth’s rotation.
Understanding Tsunami Measurement
To accurately describe the tsunami’s size, it is necessary to understand the specific measurements used in tsunami science, as the term “height” can be misleading. In the deep ocean, the wave height is often less than a meter, making it nearly undetectable to ships. These waves travel rapidly, but their energy extends through the entire water column, not just the surface.
As the waves approach the coast and enter shallower water, they slow down, and their height dramatically increases through a process known as shoaling. The most destructive and commonly cited measurement is the run-up, which is the maximum vertical height the water reaches above sea level on the land. Other measurements include the inundation distance (how far inland the water travels horizontally) and the flow depth (the height of the water above the ground at a specific point).
Maximum Measured Run-Up Heights
The water height varied drastically across the Indian Ocean, but the maximum recorded run-up heights were catastrophic. The highest measurements were concentrated near the epicenter along the coast of Aceh Province in Sumatra, Indonesia. The maximum surveyed run-up height for the entire event was an extraordinary 51 meters, recorded on a hill near Banda Aceh.
Along the west coast of Aceh, run-up heights commonly ranged between 15 and 30 meters, devastating coastal communities. Distant regions also experienced significant heights; the coasts of Thailand recorded run-up heights between 5 and 20 meters.
Further across the ocean, Sri Lanka and India saw maximum run-up values ranging from 4 to 12 meters, causing immense damage. Even the distant coast of Somalia, over 5,000 kilometers away, recorded run-up heights of nearly 10 meters.
Factors Influencing Local Impact
The significant variation in run-up height was largely due to local geographical features and bathymetry (underwater topography). The slope of the continental shelf plays a primary role in how the tsunami energy is transformed. Where the shelf drops steeply, waves often break closer to shore, resulting in a lower run-up, while a moderately sloped or narrow shelf can focus the wave energy and lead to higher run-up heights.
Local coastal geometry, such as bays and river mouths, also created a funneling effect, concentrating the water and causing higher run-up. Conversely, natural barriers like healthy coral reefs and mangrove forests helped dissipate the tsunami’s energy. Coral reefs acted as a natural breakwater, and dense mangrove root systems created friction that slowed the incoming water.