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The Sumatra Earthquake of 26 December 2004

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normal mode 0S3

Normal mode 0S3 (35.5 min) raised the ground 0.1 mm worldwide

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A minute before 8 o'clock in the morning local time, a colossal earthquake began to shake the northern part of Sumatra and the Andaman Sea to its north. Seven minutes later a stretch of the Indonesian subduction zone 1200 kilometers long had slipped by an average distance of 15 meters. The moment magnitude of the event was eventually estimated as 9.3, making it the second-largest earthquake since seismographs were invented around 1900. (See a location map and focal mechanisms on the Sumatra earthquake figures page.)

The shaking was felt throughout southeastern Asia and caused devastation in northern Sumatra and in the Nicobar and Andaman Islands. The local intensity reached IX on the 12-point Mercalli scale in the Sumatran capital of Banda Aceh, a level that causes universal damage and widespread collapse of structures. Though the intensity of shaking did not reach the maximum on the scale, the motion lasted for several minutes—the duration of shaking is the main difference between magnitude 8 and 9 events.

A large tsunami triggered by the earthquake spread outward from off the Sumatran coast. The worst part of it washed away whole cities in Indonesia, but every country on the shore of the Indian Ocean was also affected. In Indonesia, some 240,000 people died from the quake and tsunami combined. About 47,000 more people died, from Thailand to Tanzania, when the tsunami struck without warning during the next few hours.

This earthquake was the first magnitude-9 event to be recorded by the Global Seismographic Network (GSN), a worldwide set of 137 top-grade instruments. The nearest GSN station, in Sri Lanka, recorded 9.2 cm of vertical motion without distortion. Compare this to 1964, when the machines of the World Wide Standardized Seismic Network were knocked off scale for hours by the 27 March Alaskan quake. The Sumatra earthquake proves that the GSN network is robust and sensitive enough to use for expanded tsunami detection and warnings, if the right resources can be spent on supporting instrumentation and facilities.

The GSN data includes some eyepopping facts. At every spot on Earth, the ground was raised and lowered at least a full centimeter by the seismic waves from Sumatra. The Rayleigh surface waves traveled around the planet several times before dissipating (see this on the figures page). Seismic energy was released at such long wavelengths that they were a substantial fraction of the Earth's circumference. Their interference patterns formed standing waves, like the rhythmic oscillations in a large soap bubble. In effect, the Sumatra earthquake made the Earth ring with these free oscillations like a hammer rings a bell.

The "notes" of the bell, or normal vibrational modes, are at extremely low frequencies: the two strongest modes have periods of about 35.5 and 54 minutes. These oscillations died out within a few weeks. Another mode, the so-called breathing mode, consists of the whole Earth rising and falling at once with a period of 20.5 minutes. This pulse was detectable for several months afterward. (A startling paper by Cinna Lomnitz and Sara Nilsen-Hopseth suggests that the tsunami was actually powered by these normal modes.)

IRIS, the Incorporated Research Institutions for Seismology, has compiled scientific results from the Sumatra earthquake on a special page with plenty of background. And the U.S. Geological Survey's main page for the quake has much material at a less advanced level.

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