The 2004 Indian Ocean event, commonly referred to as the Boxing Day Tsunami, remains a tragedy and a pivotal moment in modern seismology. This disaster stemmed from one of the most powerful earthquakes ever recorded, which unleashed a massive wave that caused widespread destruction across multiple continents. The scale of the seismic rupture and the subsequent tsunami drew global attention to the destructive power held by the Earth’s tectonic forces. Understanding the size and mechanism of this earthquake is essential for comprehending the resulting catastrophe and mitigating future risks.
Magnitude and Epicenter Details
The earthquake that caused the 2004 tsunami was a seismic event ultimately assigned a moment magnitude (\(M_w\)) of 9.1 to 9.3, making it the third-largest earthquake recorded globally since 1900. Initial estimates were significantly lower, sometimes reported as low as \(M_w\) 8.1, because the length and duration of the rupture were not immediately apparent from standard seismic measurements. The Moment Magnitude Scale is a more accurate measure for mega-earthquakes because it accounts for the total energy released over the entire rupture area. The earthquake occurred on December 26, 2004, at 00:58:53 UTC.
The epicenter was located in the Indian Ocean, approximately 160 kilometers west of the northern tip of Sumatra, Indonesia, near the island of Simeulue. The hypocenter, the point where the rupture began, was at a depth of about 30 kilometers beneath the seafloor. The initial underestimation resulted from the earthquake’s slow, lengthy rupture, which released energy over a vast area and a long period. The full extent of the energy release only became clear after detailed analysis measuring the long-period seismic waves generated by the extended rupture process.
Tectonic Setting and Rupture Dynamics
The 2004 event originated at the Sunda Megathrust, a fault line that marks a destructive plate boundary. Here, the denser Indian Plate is subducting beneath the lighter Burma Plate, which carries northern Sumatra and the Andaman and Nicobar Islands. This boundary is a subduction zone where stress accumulates over decades as the plates lock together and resist smooth movement.
The earthquake occurred when the accumulated stress overcame the frictional lock between the two plates, resulting in a sudden slip. The rupture zone extended between 1,300 and 1,600 kilometers, comparable to the length of California. The fault surface slipped by an average of about 15 meters, with some sections displacing by as much as 20 meters.
This extended slip unfolded in phases over a period of about eight to ten minutes, an extremely long duration for an earthquake. The long rupture time and the large area of the fault that moved were the primary reasons for the earthquake’s megathrust classification and high magnitude. The movement of the oceanic plate beneath the overriding plate caused the seafloor to spring upward, setting the stage for the tsunami.
Tsunami Generation Mechanism
The earthquake generated the tsunami through the sudden vertical displacement of the seafloor, a process known as co-seismic uplift. As the overriding Burma Plate sprung up, it lifted the entire column of water above the rupture zone by several meters. This instantaneous movement of the water column created the initial energy pulse for the tsunami.
This process is fundamentally different from wind-generated ocean waves, which only affect the surface layer of the water. The vertical movement of the ocean floor across the 1,300-kilometer fault line displaced an estimated 30 cubic kilometers of water. This volume of displaced water then radiated outward in all directions from the fault line, acting as a line source rather than a point source.
In the deep ocean, the tsunami waves traveled at speeds reaching up to 800 kilometers per hour, similar to a jet plane. The waves had an extremely long wavelength, sometimes hundreds of kilometers long, making them nearly imperceptible in the open ocean with heights of only a few feet. As the waves approached coastlines and entered shallower water, their speed decreased, but their height increased due to the conservation of energy, building into destructive walls of water.
Geographical Scope of the Tsunami
The energy radiated by the earthquake’s rupture was widely distributed, affecting coastal regions across the entire Indian Ocean basin. Waves reached the closest coastlines of Sumatra and the Nicobar Islands within 15 to 30 minutes. The tsunami arrived in Thailand, Sri Lanka, and India approximately 90 minutes to two hours later.
The waves maintained their destructive power over vast distances, striking the coast of Somalia in East Africa, over 5,000 kilometers away from the epicenter, more than seven hours after the earthquake. Even the distant South African coast recorded a noticeable surge about 16 hours later. The maximum measured wave run-up—the vertical height the water reached above sea level—was 51 meters in isolated spots near Banda Aceh, Indonesia.
In Indonesia’s Aceh province, wave heights routinely exceeded 15 to 30 meters. Run-up heights of 4 to 12 meters were recorded in Sri Lanka and India, and between 5 and 20 meters along the coast of Thailand. The destruction across 14 countries demonstrated the tsunami’s geographical reach.