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Fieldwork

Indian Ocean Earthquake Triggers Deadly Tsunami


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Tsunami warning signOne lesson that everyone around the world can learn from this devastating event is to heed the natural warning signs of an approaching tsunami: if you feel the ground shaking near the coast or if you see an unusual disturbance of the ocean, such as the water withdrawing far from shore, it is important to move to high ground. Because of the complex behavior of tsunami waves near the coast, the first wave of a tsunami is generally not the largest, emphasizing the importance of staying away from the coast until wave activity has subsided (commonly several hours or even days).

For a discussion of tsunamis that have hit the United States, plus links to Web sites with general information about tsunamis and what to do during a tsunami warning, please see the article "Could It Happen Here?" in this issue. For lessons learned from the stories of people who have survived tsunamis, please see USGS Circular 1187, "Surviving a Tsunami—Lessons from Chile, Hawaii, and Japan," (PDF files in English and Spanish can be accessed from this site).

U.S. Geological Survey (USGS) scientists are assessing preliminary data and beginning more detailed studies of the devastating tsunami that lashed coasts around the Indian Ocean on December 26, 2004. The large tsunami waves were generated by a magnitude 9.0 earthquake off the northwest coast of Indonesia's island of Sumatra. The earthquake occurred on the interface between the India and Burma tectonic plates where the India plate subducts beneath the overriding Burma plate. USGS scientists estimate that the sea floor in the vicinity of the earthquake was uplifted by several meters. Displacement of water above the sea floor triggered the tsunami, which caused catastrophic levels of destruction in countries around the Indian Ocean basin—even as far as the east coast of Africa—with Indonesia, Sri Lanka, India, Thailand, Somalia, Maldives, Malaysia, Myanmar, Tanzania, Bangladesh, and Kenya among the countries hardest hit. The death toll reported by the Associated Press on January 10, 2005, was more than 150,000 and expected to rise.

The tsunami arrived in northern Sumatra approximately 1/2 hour after the earthquake, in Thailand approximately 1 1/2 to 2 hours after the earthquake, and in Sri Lanka approximately 2 to 3 hours after the earthquake. According to initial modeling and eyewitness accounts, areas east of the earthquake rupture, or "generation area," were first affected by a negative wave (drawdown of water and retreat from shore before a rise in water), whereas areas west of the generation area were first affected by a positive wave (no drawdown or retreat of water before the first tsunami wave hit). Maximum wave heights estimated from media reports are Sumatra, 10 to 15 m; Sri Lanka, 5 to 10 m; India, 5 to 6 m; Andaman Islands, 5 m; Thailand, 3 to 5 m; and Kenya, 2 to 3 m. Some energy from the tsunami "leaked" into adjoining oceans, producing sea-level fluctuations at many places around the world (see West Coast/Alaska Tsunami Warning Center).

computer model of the December 26, 2004, Indian Ocean tsunami
Above: Waves heights are greatly exaggerated relative to water depth in this computer model of the December 26, 2004, Indian Ocean tsunami (frame from an animation that will soon be posted at URL USGS Tsunami and Earthquakes Web page). Image shows the tsunami 30 minutes after it was triggered by the earthquake. Land areas are green, with Sumatra to the right of the tsunami and India and Sri Lanka at the top of the image. The ocean floor is gray (blue in the cutaway at bottom left), with light shading to show its bathymetry. Heights of waves and sea-floor features have been vertically exaggerated to make details easier to see. [larger version]

Map showing areas affected by the 2004 Indian Ocean tsunami. Frame from a computer animation of the December 26, 2004, Indian Ocean tsunami.
Above Left: Areas affected by the 2004 Indian Ocean tsunami (excerpt from map produced by the ReliefWeb Map Centre, Office for the Coordination of Humanitarian Affairs, United Nations, Jan. 5, 2005). [larger version]

Above Right: Frame from a computer animation of the December 26, 2004, Indian Ocean tsunami (animation can be viewed at URL http://staff.aist.go.jp/kenji.satake/animation.gif). Frame shows the tsunami 10 minutes after it was triggered by the earthquake. Red represents a positive wave (crest arrives first), and blue represents a negative wave (trough arrives first—drawdown warns of approaching crest of tsunami wave). Deeper colors represent larger wave heights. (Note: This model shows a longer wave front than the oblique-perspective model, because the modeler assumed a longer fault rupture as the tsunami trigger. Seismologists are still sifting through the evidence to determine the length of the deep rupture that caused the earthquake and subsequent tsunami.) [larger version]

Map showing the tectonic setting of the magnitude 9.0 earthquake that generated the tsunami.
Above: Tectonic setting of the magnitude 9.0 earthquake that generated the tsunami (from the USGS Earthquake Hazards Program's Sumatra-Andman Islands Earthquake Web page). The earthquake occurred at the interface between the India and Burma tectonic plates and was caused by the release of stresses that develop as the India plate subducts beneath the overriding Burma plate. USGS scientists estimate that during the deep thrust faulting that generated the earthquake, in which rock on one side of the fault moved up and over rock on the other side, the sea floor above the fault was uplifted by several meters. Displacement of overlying seawater triggered the tsunami. The world's largest recorded earthquakes have all been megathrust events, occurring where one tectonic plate subducts beneath another.

Post-Tsunami Field Surveys

By December 31, 2004, six international teams (including Japanese and American teams) had been formed to document the magnitude and effects of the tsunami before the evidence is destroyed. Typically, such teams arrive in the affected areas about one to three weeks after the tsunami occurs. Because this was the largest tsunami in more than 40 years and the area affected is very large, there could be as many as a dozen international teams investigating the tsunami. USGS oceanographer Bruce Jaffe and USGS geologist Bob Morton traveled to Sri Lanka from January 7 to 16 with an international team funded by the National Science Foundation and the USGS to examine inundation areas, estimate wave heights, determine the tsunami's precise arrival time, scour the area for geologic evidence and sedimentary deposits, and examine structural damage. As of this writing, the USGS had also been invited to have scientists participate in post-tsunami surveys in India, Thailand, and Sumatra.

Ideally, post-tsunami surveys will include both a quick response focusing on ephemeral evidence and a later response (possibly in February or March) focusing on tsunami sedimentation and erosion. The quick response will include measurements of water levels, inundation distances (horizontal distance from the shoreline to the farthest inland reach of the tsunami), and indicators of the tsunami's flow direction and flow velocity. The later response will focus on the sediment deposited by the tsunami: whether it has characteristics that reflect those of the tsunami itself, such as its height, power, and extent; how much of the sediment is likely to be preserved in the geologic record; and how much is likely to be eroded away. The more we learn about sedimentary deposits from modern tsunamis, the more accurately we can identify and decipher sedimentary deposits from ancient tsunamis. Because scientists cannot yet predict when a tsunami will occur, learning to read a geologic record of past tsunamis may be one of the only ways to assess future risk.

USGS scientists have conducted such studies of sediment deposited by recent tsunamis in Papua New Guinea (tsunami of 1998, see Preliminary Analysis of Sedimentary Deposits from the 1998 PNG Tsunami) and Peru (tsunami of 2001, see Preliminary Analysis of Sedimentary Deposits from the June 23, 2001 Peru Tsunami). They are working to determine how sediment layers deposited by tsunamis differ from those deposited by large storms, such as hurricanes, to aid identification of tsunami deposits in the geologic record (see Sound Waves article "Group Aims to Distinguish Tsunami Deposits from Large-Storm Deposits in the Geologic Record").

Tsunami Information on the Web

Many Web sites have information about the Indian Ocean tsunami and tsunamis in general. Here are a few particularly useful ones:


Related Sound Waves Stories
Could It Happen Here?
Dec. 2004 / Jan. 2005
Group Aims to Distinguish Tsunami Deposits from Large-Storm Deposits in the Geologic Record
October 2002

Related Web Sites
Preliminary Analysis of Sedimentary Deposits from the 1998 PNG Tsunami
U.S. Geological Survey (USGS)
Preliminary Analysis of Sedimentary Deposits from the June 23, 2001 Peru Tsunami
U.S. Geological Survey (USGS)
West Coast/Alaska Tsunami Warning Center
National Oceanic and Atmospheric Administration (NOAA)
USGS Northern Sumatra Earthquake event page
U.S. Geological Survey (USGS)
Life of a Tsunami
U.S. Geological Survey (USGS)
Pacific Marine Environmental Lab
National Oceanic and Atmospheric Administration (NOAA)
Russian Tsunami Laboratory
Institute of Computational Mathematics and Mathematical Geophysics
UNESCO Tsunami site
Intergovernmental Oceanographic Commission of UNESCO
Scientific Background on the Indian Ocean Earthquake and Tsunami
International Research Institute for Climate Prediction
Surviving a Tsunami—Lessons from Chile, Hawaii, and Japan - USGS Circular 1187
U.S. Geological Survey (USGS)
Can it happen here in the United States?
U.S. Geological Survey (USGS)

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in this issue: Fieldwork cover story:
Indian Ocean Tsunami

Could a Tsunami Happen Here?

Deltaic Habitats in Puget Sound

Invasive Sea Squirt Flourishing

How Sea Floor Sediment Moves

Research Submarine Canyons Named for Marine Geologists

Outreach Appreciation Day for Congressman Young

Students Learn About Coastal and Marine Science

Hurricanes Focus Attention on USGS Research

College Students Visit USGS Center in St. Petersburg

Scientists Participate in Great-American Teach-In

Scientists Interviewed About Invasive Sea Squirt

GIS Day

CCWS Open House

Scientists Interviewed for HBO Program

Meetings International Symposium on Coastal Issues

Jeff Williams Reviews Storm Surge Model

Gulf and Caribbean Fisheries Institutes Conference

Suwannee River Basin and Estuary Integrated Science Workshop

Staff & Center News Regional Executive Visits FISC Office

Jingping Xu Joins Western Coastal and Marine Geology Team

Publications Special Oceanography Issue Includes Sediment Dynamics Article

Dec. / Jan. Publications List


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