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USGS Scientists Respond to Deadly Samoa Tsunami
U.S. Geological Survey (USGS) tsunami scientists responded quickly after a magnitude 8.0 submarine earthquake occurred at 6:48 a.m. Samoa Standard Time on September 29, 2009, approximately 190 km (120 mi) south of Samoa. The earthquake triggered a tsunami that caused deaths and widespread damage in Samoa, American Samoa, and Tonga.
Eyewitnesses reported three to seven tsunami waves; the largest were higher than 5 m and reached more than 500 m inland. Thanks to educational efforts, residents of the affected islands evacuated to high ground after the earthquake. Had they not, the death toll would have been in the thousands. Nevertheless, some people were overtaken by the tsunami waves, which traveled very quickly through the deep water between the epicenter and the nearest islands—a circumstance that one USGS scientist called "heart-wrenching." The death toll was 148 on Samoa, 34 in American Samoa, and 9 in Tonga.
The earthquake occurred near the boundary between the Pacific and Australian tectonic plates, one of the most active earthquake regions in the world. At the latitude of the earthquake, the Pacific plate is subducting westward beneath the Australian plate. The earthquake was centered near the north end of a 3,000-km-long segment of the Pacific/Australia plate boundary that trends north-northeast and is marked by the Tonga trench. On the basis of currently available information, USGS seismologists infer that the September 29 earthquake occurred as a normal-fault rupture within the outer rise of the subducting Pacific plate, where the plate bends downward toward the Earth's mantle (see subduction-zone diagram, below left). Most fault ruptures in subduction zones occur on thrust or reverse faults between the two plates, where compression pushes rock on one side of the fault up and over rock on the other side (see diagram of fault types, above). The earthquake that triggered the 2004 Indian Ocean tsunami was a thrust-fault rupture. Normal-fault ruptures, in which rock on one side of the fault slips down and away from rock on the other side, are less common in subduction zones but do occur in such places as the outer rise, where bending of the subducting plate exerts extensional forces on the rock.
Thrust faulting, reverse faulting, and normal faulting all produce vertical displacement of the seafloor, which in turn displaces water above, giving rise to the tsunami (learn more at Life of a Tsunami). Relative to many subduction-zone earthquakes, the September 29 earthquake was "shallow"—it occurred only about 12 km (7 mi) below the seafloor—and so most of its energy reached the seafloor and was transferred to the overlying water. What's more, the September 29 earthquake occurred beneath relatively deep water, currently estimated at about 5 to 7 km (scientists are still analyzing data to determine more precisely where the fault rupture occurred). The deeper the water above the earthquake, the more the tsunami amplifies when it travels into shallow water. Finally, the speed of a tsunami is related to the depth of the water through which it travels: the deeper the water, the faster the tsunami. The deep water between the earthquake epicenter and the islands in the Samoa region allowed the tsunami waves to speed toward the islands at about 500 mph in the open ocean, or as fast as a 747 jumbo jet. Thus the first tsunami wave reached some coasts facing the epicenter as soon as 14 minutes after the earthquake. Shallow water close to the islands slowed the tsunami to approximately 30 mph by the time it hit the shore, but even at that speed the huge mass of moving water exerted tremendous force. As news of the earthquake and tsunami broke, private citizens as well as numerous reporters from news agencies and radio and television stations contacted the USGS for additional information. Among the scientists who answered their questions were tsunami experts Eric Geist, a geophysicist who models the generation and propagation of tsunami waves; Bruce Jaffe, an oceanographer who studies sediment deposited by tsunamis for clues to tsunami history; and Uri ten Brink, a geophysicist who studies tsunami and earthquake hazards in the Caribbean region.
Plans were soon set in motion for a USGS rapid-response team from Menlo Park and Santa Cruz, California, to travel to American Samoa to collect geologic data expected to be quickly degraded or destroyed by recovery activities and natural processes. Jaffe arrived in Pago Pago, on the island of Tutuila, American Samoa, on October 4 and began working with an International Tsunami Survey Team. He was joined later by USGS colleagues Bruce Richmond, Mark Buckley, Guy Gelfenbaum, Steve Watt, and Alex Apotsos, as well as oceanographer Walter Dudley of the University of Hawai‘i, Hilo, and geologist Brian McAdoo of Vassar College. The researchers collected time-sensitive data to help them determine the height of tsunami waves at various sites, flow directions, and the distances the waves traveled inland. They also studied the transport of sediment and other debris, looked for evidence of subsidence or uplift caused by the earthquake, documented erosion caused by the tsunami waves, and made other observations critical to the better understanding of tsunami impacts and processes. For news and photographs from this team, visit Notes From the Field: USGS Scientists in Samoa and American Samoa Studying Impacts of Recent Tsunami, October 2009. A second USGS team—Dan McNamara and Jeff Fox of the USGS National Earthquake Information Center in Golden, Colorado—deployed six seismic stations in American Samoa to detect aftershocks and collect data for determining ground-motion attenuation, a key parameter for assessing seismic hazard. Both teams coordinated with other Federal and local agencies to ensure that scientific activities did not interfere with rescue and recovery activities. After their fieldwork on American Samoa, Richmond, Buckley, Dudley, and McAdoo traveled to Apia on the island of Upolu, Samoa, to join an international rapid-response effort to map tsunami impacts in Samoa. For Richmond's reports on this effort, visit Notes From the Field: USGS Scientists in Samoa and American Samoa Studying Impacts of Recent Tsunami.
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in this issue:
Scientists Respond to Samoa Tsunami Scientist in American Samoa Helps Calm Tsunami Fears Groundwater Studies on U.S. West Coast and Hawai'i
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U. S. Department of the Interior | U.S. Geological Survey
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