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by Bruce Jaffe, Bruce Richmond, and Helen Gibbons

The March 11, 2011, magnitude 9.0 Tohoku earthquake off the coast of Japan generated a tsunami that inundated a large area on the northeast coast of Honshu, the country's main island, resulting in widespread devastation (see related Sound Waves article "Japan Lashed by Powerful Earthquake, Devastating Tsunami"). At this writing (June 1), the death toll stands at approximately 15,000 and the number of missing at approximately 8,000. The earthquake was one of the five most powerful earthquakes recorded in the world since instrumental detection and record keeping began around 1900. The resulting tsunami flooded dozens of coastal cities, numerous ports, and the broad coastal plain around Sendai—the nearest major city to the earthquake, at a distance of 130 km (80 mi) from the epicenter. It is now estimated from aerial and satellite photography that an area of almost 500 km² (200 mi²) was inundated by the tsunami.

In May 2011, U.S. Geological Survey (USGS) scientists Bruce Jaffe and Bruce Richmond examined sediment deposited by the tsunami in and around Sendai as part of an international tsunami survey team organized by Japanese scientific cooperators. The 11 members of the team came from Japan, the United States, Australia, Poland, the United Kingdom, and Indonesia.

Survey teams try to enter tsunami-stricken areas as soon as possible after rescue and recovery work to document physical evidence of tsunami flow characteristics—such as debris in trees, high-water stains on buildings, and sedimentary deposits—before it is degraded or destroyed by natural forces or cleanup activities. Japanese researchers began this process during the second week after the earthquake; as the situation in Japan improved, they invited scientists from the international tsunami-research community to assist with gathering data from the large area affected by the tsunami.

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The May reconnaissance survey focused on the characteristics of tsunami sediment deposits in the vicinity of Sendai Airport, with a specific emphasis on how these characteristics varied with tsunami speed and flow depth, topography (including microtopography), distance from the coast, urban and rural settings, land subsidence caused by the earthquake, and other aspects of natural and man-made features of the landscape.

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Among the information the scientists collected were data on water levels, flow directions, topography, sediment thickness, grain size, and sedimentary structures (patterns in the sediment produced by variations in such factors as the speed of the water from which the sediment was deposited and the composition and grain size of the sediment). They also collected sediment samples for microfossil, geochemical, and other analyses. The sedimentary evidence of the tsunami was complicated by liquefaction of some coastal-plain sediment during the earthquake and an extensive canal system that affected the movement of the tsunami waves and the sediment they carried.

This tsunami was particularly well documented in video footage shot by news agencies and citizens. The survey team took advantage of the video data by focusing their measurement and sampling efforts on areas where footage is available. One video they are using shows the tsunami hitting the Sendai Airport. The team collected data along a transect near the airport where sand and mud were deposited more than 4 km inland—the longest post-tsunami transect in which tsunami deposits have been mapped in detail. The scientists will compare data from the sediment deposits with data from the video in hopes of relating characteristics of the deposits to the flow history of the tsunami. For example, they will compare the number and thickness of layers in the tsunami deposits with the number of waves and the flow speeds observed in the video. 

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The data collected in Japan, like data collected in field surveys after other recent tsunamis, will improve scientists' ability to recognize and interpret tsunami deposits from ancient events. Some of the scientists on the survey team will use the data to develop numerical models of tsunami sediment transport that can be applied to tsunami deposits in the geologic record to determine the characteristics—approximate size and speed, for example—of ancient tsunamis. Data from the Japanese study will also help scientists improve criteria for distinguishing tsunami deposits from large-storm deposits.

Additionally, the reconnaissance team will attempt to draw analogies between the March 11, 2011, tsunami deposits and deposits of predecessor events, such as the Jōgan tsunami, which struck the Sendai plain in A.D. 869 and was named after the emperor of the time. Japanese scientist Koji Minoura (Tohoku University) and colleagues published a paper in 2001 describing Jōgan tsunami sand deposits and two older sand deposits interpreted as evidence of earlier large tsunamis (Journal of Natural Disaster Science, v. 23, no. 2, p. 83-88). The ages of the two earlier tsunami deposits are A.D. 140-150 and 910-670 B.C. On the basis of this limited paleotsunami record for the Sendai coastal plain, the authors inferred a tsunami recurrence interval of 800-1,100 years. At the end of the paper they wrote, "More than 1,100 years have passed since the Jōgan tsunami and, given the reoccurrence interval, the possibility of a large tsunami striking the Sendai plain is high. Our numerical findings indicate that a tsunami similar to the Jōgan one would inundate the present coastal plain for about 2.5-3 km inland," a prediction that proved remarkably accurate.

Japan not only has the longest written record of historical tsunamis in the world, but it also has a vigorous paleotsunami research program designed to extend the tsunami record into the past and improve tsunami-hazard assessments. Examining field evidence of recent and historical tsunamis is critical to understanding these infrequent but catastrophic phenomena. The data collected recently in Japan by the international tsunami survey team will be used to develop tools for interpreting the geologic record of tsunamis. These tools can be applied to other areas where the same coastal hazard is present, such as the Pacific Northwest coast of the United States, and used to decrease future loss of life and property from tsunamis.

To view additional photographs of the March 2011 tsunami's impacts, please visit "International Tsunami Survey Team Visits Japan, May 2011," a USGS Pacific Coastal and Marine Science Center field report.