Tsunami: One Year Later, Part I

What We Know About The Event Itself

Worldwide patterns of wave propagation triggered by the 2004 Sumatra-Andaman earthquake. The massive tsunami triggered by an undersea earthquake in the Indian Ocean literally rippled around the world. NOAA scientist Vasily Titov, using seismic data, rendered an animation showing how the tsunami waves propagated around the Earth. Some of the waves reached the United States and many other nations outside the Indian Ocean. This cartoon depicts a period of 44 hours and 27 minutes of tsunami propagation. The tsunami reached the Atlantic and Pacific coasts of the United States about the same time, some 28 hours after the earthquake struck on Dec. 26, 2004, at 00.59 UTC or 7:59 p.m. EST. Titov's model also was used to interpret the data gathered by four satellites for determining the tsunami's wave height. [Image credit: NOAA].

One year ago today, the world was stunned to learn that a giant tidal wave, or tsunami, had smashed into the coastlines of twelve countries in southeast Asia, causing unimaginable devastation and killing what turned out to be hundreds of thousands of people and leaving millions homeless. This tsunami was triggered by the largest earthquake to occur on the planet in forty years; the third largest earthquake since these events were formally measured. Various official agencies measured this submarine earthquake to be between 9.0 and 9.3 on the moment magnitude scale (the higher estimate would make this the second largest earthquake to have ever been measured). Authoritative analyses of the data now estimate the magnitude at 9.15.

This earthquake, known in scientific circles as the Sumatra-Andaman earthquake, occurred on the sea floor of the Indian Ocean at a depth of 30 kilometers (18.6 miles) below average sea level. This was a very large earthquake, geographically speaking; occurring over 1200 kilometers of the fault line that lies roughly between the islands of Sumatra and Andaman. It resulted from a tectonic slip that occurred along the fault line where the India Plate dives beneath the Burma Plate. The epicenter was just north of Simeulue Island, which is located 160 kilometers (100 miles) off the western coast of northern Sumatra.

As a result of this earthquake, Simeulue Island, near the southern end of the fault line, gained at least 6 feet in elevation and the Nicobar Island group, at the northern end of the fault line, were similarly thrust upward while several of these islands were broken into two or even three pieces. The earthquake itself was felt as far away as Thailand, Singapore, Bangladesh, India, Malaysia and the Maldive Islands.

The Sumatra-Andaman earthquake was a megathrust earthquake that occurred over a large geographic area over a period of approximately five minutes, perhaps longer. Megathrust earthquakes are unusual, consisting of vertical movement where one tectonic plate slips beneath another, pushing it upward, in a process known as subduction. In addition to generating vertical movement, megathrust earthquakes may also be accompanied by sideways movements of the tectonic plates, as occurred in this case.

Satellite and GPS data are being used to determine the precise nature and extent of the many geological changes that occurred as a result of this earthquake. For example, it is estimated that the seabed along the fault line rose by several meters, and that some of the smaller islands located south of Sumatra moved southwest by as much as 20 meters (66 feet). Other calculations estimate that the northern tip of Sumatra itself may have moved as much as 36 meters (118 feet) to the southwest.

The earthquake's sudden vertical movement caused the seabed to rise by several meters, which displaced a tremendous amount of seawater. This displaced water rippled outward in a series of waves that traveled faster through deeper seas and slowing in shallower waters. These waves raced through the world's oceans at speeds between 500 to 1,000 kilometers/hour (310 to 620 mph), taking anywhere from fifteen minutes (Aceh) to seven hours (Somalia) to hit land. Unfortunately, until they slowed and mounded up in shallow coastal waters, these tsunamis were not easily detected: satellite data revealed that the largest of these waves, recorded in open ocean two hours after the event, was approximately 60 centimeters (2 feet) high.

Even though it was widely reported that there was only one tsunami, the Sumatra-Andaman earthquake actually triggered a succession of four large and very destructive tsunamis whose peaks arrived on land approximately 30 minutes apart. The third of these four waves was the most devastating, with a peak estimated to be as high as 30 meters (100 feet) in some locales in Aceh. In addition to these four major waves, many smaller tsunamis occurred throughout the region for the remainder of that day.

Because the fault line lies lengthwise in a north-south orientation, the strongest waves were triggered in an east-west direction and thus, the greatest damage from these tsunamis occurred along coastlines in the Indian Ocean that were also oriented east/west. As a result, Bangladesh, a low-lying country located north of the epicenter, suffered relatively few deaths and damages when compared to the much more distant Somalia, located directly west on the African continent.

These tsunamis had a global effect: they devastated portions of the shorelines of Indonesia, Sri Lanka, Southern India, Thailand and neighboring countries. Most of these areas are still ruined, being described by recent visitors as resembling “the aftermath of a bomb blast”. Additionally, tsunamis were recorded along coastlines around the world (see cartoon at top). For example, Struisbaai, South Africa, which is 8,500 kilometers (5,300 miles) away from the epicenter, recorded a 1.5 meter (5 foot) high wave approximately 16 hours after the earthquake. Remarkably, the tsunamis also traveled into the Pacific Ocean, hitting the shoreline at Manzanillo, Mexico, with a 2.6 meter (8.5 foot) wave. These tsunamis also caused deaths as far away as the east coast of Africa, where the most distant tsunami-caused death occurred at Port Elizabeth in South Africa -- 8,000 kilometers (5,000 miles) away from the epicenter!

Speaking of deaths and damages, Part II of this series will explore the human toll due to this event.


Sources


The Global Reach of the 26 December 2004 Sumatra Tsunami (2005) by Vasily Titov, Alexander B. Rabinovich, Harold O. Mofjeld, Richard E. Thomson, Frank I. González. Science, 309 (5743): 2045-2048. abstract (free) and PDF (not free, the weasels!).

2004 Indian Ocean earthquake [Wikipedia].

Tsunamis and Mangroves: The Shrimp Connection [Opinion piece that I wrote in the days after the tsunami]

Index of 11 streaming tsunami stories [National Public Radio].





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tags: tsunami, earthquake, Indian Ocean, Southeast Asia, Sumatra, Indonesia