Arrival First Wave Stretched
Arrival First Wave Stretched. Part of A Century of Earthquakes poster THE KILLER WAVE
On December 26, 2004 the world saw yet again how the stress built up over hundreds of years by slow and almost imperceptible motions of tectonic plates can be released with devastating effect. A great earthquake beneath the Indonesian island of Sumatra generated a massive sea wave, or tsunami, that raced across the Indian Ocean at the speed of a jet plane, wreaking destruction along seacoasts and causing at least 300,000 deaths. Tsunami heights grow substantially as they approach the shallow ocean near the coast, and they can be enormously destructive.
The cause of this event can be traced back over 120 million years ago, when the subcontinent of India separated from Antarctica and started its steady motion northward. 50 million years ago it collided with Asia, raising the Himalayas and forming the Tibetan plateau. The plate collision continues today as the Indian plate moves northward. Part of the plate boundary extends along the subduction zone on the west coast of Sumatra, where an oceanic part of the Indian plate subducts beneath the Burma plate, a small sliver or microplate between the Indian plate and the Sunda plate that includes much of southeast Asia. Every year, about 35 mm of convergence occurs between the Indian and Burma plates. However, the fault is locked, so stress builds up on it. Eventually the accumulated stress exceeds the strength of the fault, and it slips. During the Sumatra earthquake a huge area of this plate interface slipped, generating seismic waves that inflicted major damage near the earthquake. Moreover, because this plate boundary occurs at an underwater trench, the overriding Burma plate that had been dragged down since the last major earthquake rebounded and displaced many cubic kilometers of ocean, causing the devastating tsunami.