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USGS Scientists Study Sediment Deposited by 2004 Indian Ocean Tsunami
In January, U.S. Geological Survey (USGS) scientists traveled to countries on the Indian Ocean to study sediment deposited by the devastating tsunami of December 26, 2004. They hope to gain knowledge that will help them to identify ancient tsunami deposits in the geologic record—which extends much farther into the past than written records—and so compile a history of tsunamis that can be used to assess a region's future tsunami risk.
Above: USGS scientists took part in post-tsunami field surveys in Sri Lanka and on Indonesia's island of Sumatra. Enlarged map shows some of the sites visited by the scientific team in Sri Lanka. [larger version] |
"The USGS shares a common interest with scientists around the world to reduce the devastating effects of earthquakes and tsunamis on society," said Lisa Robbins, director of the USGS Center for Coastal and Watershed Studies in St. Petersburg, FL. "The goal of the USGS is to help improve the world's scientific knowledge of these events, so that measures can be taken to reduce the effects of future earthquakes and tsunamis."
Above: Damage at Kalmunai, eastern Sri Lanka. The ground is littered with boats that were swept inland, sediment deposits, bricks, and other debris. This view is an excerpt from a panorama movie that was created by Gerry Hatcher (USGS, Santa Cruz, CA) from photographs taken by survey-team members. [larger version] |
Bruce Jaffe (Santa Cruz, CA) and Robert Morton (St. Petersburg, FL) worked in Sri Lanka from January 9-15, 2005, with an international survey team headed by Philip Liu of Cornell University. Guy Gelfenbaum (Menlo Park, CA) worked on Indonesia's island of Sumatra from January 18-31 with an international survey team headed by Yoshinobu Tsuji of Tokyo University's Earthquake Research Institute.
The survey teams gathered data on estimated wave heights, extent of inundation, tsunami sand deposits, erosion by the tsunami, flow directions, and other information related to the physical aspects of the tsunami waves that may disappear quickly. The team in Indonesia, much closer to the epicenter of the earthquake that caused the tsunami, also looked for evidence of coastal subsidence. USGS scientists focused on the sediment deposited by the tsunami, from which they hope to learn not only about the recent tsunami but, ultimately, about past tsunami history and future tsunami risk. By examining the thickness and grain-size distribution of a tsunami deposit, for example, scientists may be able to deduce wave height and flow velocity—two of the most important factors in determining the destructive power of a tsunami. By studying deposits from recent tsunamis, scientists hope to be better able to identify tsunami deposits in the geologic record—to distinguish them from large-storm deposits, for example. Where an ancient tsunami deposit is identified, the characteristics of the deposit may yield information about the tsunami wave that deposited it. Where more than one tsunami deposit is preserved in a sedimentary sequence, the record may help scientists to determine how often tsunamis are likely to occur.
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Above Left: Harindra (Joe) Fernando (second from left), a professor from Arizona State University and also a native Sri Lankan, gathers eyewitness accounts from survivors in Paiyagala, southwestern Sri Lanka, on the property of a Catholic church whose schoolhouse was destroyed by the tsunami. The white bucket at the young women's feet contains toys and wooden blocks that they have collected from the rubble and are washing with water from the well at right—their way of making progress toward the community's long-term goal of rebuilding the school. Third from left is reporter William Hermann of the Arizona Republic, who traveled with the scientific team in Sri Lanka. Photograph by Bob Morton. [larger version]
Above Right: Tsunami deposit at Nilaveli Beach Hotel, east coast of Sri Lanka. Photograph by Bretwood Higman, University of Washington. [larger version] |
"The historical record of tsunamis is too short to accurately define tsunami risk for most of the world," says Jaffe. "Even Japan, which has the longest written record in the world [dating to about A.D. 700], has an active research program to extend the record of tsunamis farther into the past by using tsunami deposits." Some countries, including Sri Lanka, one of the hardest hit by the December tsunami, have only a sparse written record of previous tsunamis. In such cases, says Jaffe, "the sedimentary record of tsunamis may provide the best evidence of a region's risk from tsunamis."
The survey in Sri Lanka was the third survey of a tsunami-ravaged coast for Jaffe, who is principal investigator for the USGS Tsunami Hazard Assessment Project. He was a member of an international team that traveled to Papua New Guinea to document the 1998 tsunami (see Preliminary Analysis of Sedimentary Deposits from the 1998 Papua New Guinea (PNG) Tsunami), and later led an international team that documented tsunami sedimentation from the 2001 Peru tsunami (see Preliminary Analysis of Sedimentary Deposits from the June 23, 2001 Peru Tsunami). He has also studied sedimentary evidence of catastrophic tsunami events in Java (1974 tsunami) and the U.S. Pacific Northwest (1700 tsunami). Jaffe's tsunami research focuses on (1) inverse sediment-transport modeling of tsunami deposits to estimate tsunami flow velocity, (2) identification of past tsunami deposits, and (3) developing methods to use tsunami deposits to extend the record of tsunamis beyond the historical record.
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Above Left: Debris left in the trees, like this door in Yala, Sri Lanka, helped the scientists measure the tsunami wave heights. Photograph by Bob Morton. [larger version]
Above Right: Car transported by the tsunami in Yala, Sri Lanka. Note airbag on driver's side. Photograph by James Goff, Geoenvironmental Consultants, New Zealand. [larger version] |
Bob Morton has particular expertise in large-storm deposits, and his recent tsunami research has focused on distinguishing tsunami deposits from storm deposits (see related Sound Waves article, Group Aims to Distinguish Tsunami Deposits from Large-Storm Deposits in the Geologic Record). Both tsunamis and large storms, particularly hurricanes, are capable of inundating coastal regions and depositing sandy sediment over broad areas landward of the beach. Correctly identifying a sandy bed in the geologic record as either a tsunami or a storm deposit is important for an accurate assessment of a region's risk from tsunamis or large storms. Morton hopes to understand better how the recent tsunami affected the Sri Lankan coast so that he can compare tsunami coastal-change data with hurricane coastal-change data collected after the busy 2004 hurricane season in Florida. A member of the USGS Tsunami Hazards Assessment project, Morton is also the principal investigator for the USGS National Assessment of Shoreline Change project, which addresses the problem of beach erosion and shoreline change threatening coastal populations and community infrastructures in the United States.
Guy Gelfenbaum, who also is a member of the USGS Tsunami Hazards Assessment project, is working with Jaffe and Morton on efforts to distinguish large-storm deposits from tsunami deposits. Gelfenbaum examined tsunami deposits in Papua New Guinea in 1998 and in Peru in 2001. His recent work includes collaborating with Giles Lesser (USGS, Menlo Park, CA) to model sediment transport and coastal evolution of Willapa Bay, WA, in response to large Cascadia subduction-zone earthquakes and associated tsunamis.
As of this writing, Bruce Jaffe and Bob Morton are back in the United States and have graciously shared some of their initial findings and photographs. Guy Gelfenbaum is due back soon. Stay tuned for future articles about the work of these three scientists.
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Above: Stranded boat rests atop a sand dune that tsunami waves barely topped on a beach at Yala in southern Sri Lanka. Similar dunes can be seen down the coast. In the middle ground, the dune had been removed to improve the ocean view from the Yala Safari Game Lodge, leaving it vulnerable to the tsunami's full force. Photograph by Bob Morton. [larger version]
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Topography—Natural and Altered—Affects Tsunami's Severity
Uneven patterns of coastal destruction caused by the Indian Ocean tsunami—some areas devastated, others nearby only slightly damaged—illustrate what scientists who make mathematical models of tsunamis have learned over time: tsunami waves are extremely sensitive to details of nearshore and coastal topography. Data like those gathered in Sri Lanka—plus higher-resolution maps of the nearshore sea floor, where available—will help scientists fine-tune their mathematical models and more accurately predict the behavior of future tsunamis.
Some variations in the effects of December's tsunami resulted from natural variations in topography. The tsunami waves were higher, as might be expected, along shorelines that directly faced the tsunami's line of approach. High waves also struck shorelines facing away from the tsunami's origin, for example, on the southwest coast of Sri Lanka. This phenomenon is predicted by models that show tsunami energy wrapping around islands in shore-parallel "edge waves." In Galle, Sri Lanka, unusually high runup heights, nearly 40 ft in some areas, are believed to have been caused by focusing of the tsunami's energy by the sides of a nearby submarine canyon. In other areas, the tsunami's energy was reduced by offshore topography—for example, buffered by offshore reefs.
Some variations in the tsunami's effects resulted, tragically, from human alterations of the landscape. At the Yala Safari Game Lodge, a resort on the edge of Sri Lanka's Yala National Park, a high sand dune had been removed to improve the ocean view. On adjacent stretches of beach, where the dune was intact, the tsunami waves barely crested the dune. "Where the dune was gone," says Bob Morton, "the tsunami roared right through," demolishing the hotel and killing more than 175 people. Morton called the tragic loss of life and property at the resort "one of the most striking examples we were able to bring back from the field—where human activities increased the coastal hazards."
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Above Left - Before: Rooms at the Yala Safari Game Lodge, scanned from an advertisement given to the scientific team by a Sri Lankan resident. [larger version]
Above Right - After: Site of the demolished Yala Safari Game Lodge. Note the light vertical tiles that can also be seen in the "before" shot. Photograph by Bob Morton. [larger version] |
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Journalists Accompanied the Scientific Team in Sri Lanka
The survey team in Sri Lanka was accompanied by journalists, including Tom Paulson of the Seattle Post-Intelligencer, William Hermann of the Arizona Republic, and Quirin Schiermeier of the scientific journal Nature. The journalists' articles not only describe the scientists' activities and findings but also paint a vivid picture of the tsunami-torn areas and the tsunami survivors. To read them online, go to the search engines on the publications' Web sites (at URLs http://seattlepi.nwsource.com/search/, http://www.azcentral.com/arizonarepublic/, and http://www.nature.com/news/) and type in the keywords "tsunami," "Sri Lanka," and the journalist's last name.
Additional insights are provided by informal online journals—Web logs, or "blogs"—kept by Paulson and Schiermeier. Paulson's entry for January 17, 2005, offers some thoughts on the fast pace set by the scientists: "I don't remember ever being so tired for so long on assignment. I've traveled in Africa, India and Asia—often under difficult circumstances in remote areas—but I'd have to say nothing was quite as exhausting as covering this story. The scientists were determined to study as many sites as possible within the week, before the data disappeared and while eyewitness memories were fresh. Every day was a new region, with a 12-hour sprint from site to site, followed by a search for a place to stay the night...." And on the scientists themselves: "As frenzied and nerdy as that bunch was, I'll miss them." Schiermeier's entry for the same day touches on some emotional aspects of the trip: "I'm still struggling to cope with the things I have seen and heard over the last few days. I guess all the team members feel the same. When we said our goodbyes in the lobby of the Trans Asian hotel in Colombo, we confessed what a conflicting experience this has been for us: doing science, even having some good fun, amidst all the grief and the destruction." Read more at Paulson's blog and Schiermeier's blog.
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 February 2005
in this issue:
 cover story:
Tsunami Deposits
Mapping the San Pedro Shelf
Nearshore Impacts of Dam Removal
Pulley Ridge Deepest Known US Coral Reef
Raising Crane
Climate Features Influenced 2004 Hurricane Landfall Count
USGS Employees Donate Toys
Carl Goodwin Honored at National Ecosystem Conference
Oceanographer Joins Western Coastal & Marine Geology Team
Blood Donor
Netherlands Students Contribute to USGS Studies
Florida Integrated Science Center, Gainesville, FL
NMFS Employees Visit CCWS
February Publications List
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U. S. Department of the Interior | U.S. Geological Survey
