22.5 Earthquakes

It happened on December 26, 2004, at 7:59 a.m. About 35 kilometers beneath the northern tip of the Indonesian island of Sumatra, the ground began to tear as part of the Indian Plate slipped a few meters down into mantle. The tearing continued northward for more than five minutes. When it was over, the process of subduction had unleashed the deadliest earthquake the world had seen in almost 30 years.

An earthquake is a movement of Earth's lithosphere that occurs when rocks in the lithosphere suddenly shift, releasing stored energy. A small portion of the energy released during an earthquake is carried by vibrations called seismic waves. During the Sumatra earthquake, seismic waves began spreading away from the rupture at about 8 kilometers per second. Within a few seconds, buildings in Sumatra began to crack and collapse.

Figure 32 The tsunami triggered by the 2004 Sumatra earthquake caused extensive damage to coastal areas in Southeast Asia. Earthquakes of similar magnitudes have occurred in the Pacific Northwest in the past and could still occur today.

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