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Geological Impacts of the February 2010 Tsunami in Chile
On February 27, 2010, a magnitude 8.8 earthquake occurred in relatively shallow water offshore of the central coast of Chile. With a rupture zone nearly 500 km long, the earthquake generated a series of tsunami waves that inundated the shore along more than 550 km of coastline (see related Sound Waves article "The 2010 Chilean Tsunami and Uncertainty in Tsunami Modeling"). In response to requests for assistance from the Chilean government through UNESCO (United Nations Educational, Scientific and Cultural Organization), the U.S. Geological Survey (USGS) sent a team of scientists to collaborate with Chilean scientists in investigating the geological impacts of the tsunami. The purpose of the 10-day trip (April 24-May 2) was to better understand the 2010 event and to provide information for improved modeling and prediction that can be used to mitigate loss of life and damage from future tsunamis.
The USGS tsunami geology team consisted of (in alphabetical order) Mark Buckley, Guy Gelfenbaum, Bob Morton (team leader), and Bruce Richmond. Chilean scientists, who provided in-country logistical support and assistance in the field, were Adriano Cecioni, a geology professor at the University of Concepción, and students from the university, including Osvaldo Artal, Constanza Hoffmann, and Felipe Perez.
The USGS team selected five sites for comprehensive investigation along a 200-km segment of coast both north and south of the earthquake epicenter that included diverse geological settings (delta plain, deeply embayed alluvial valley, coastal plain near river mouth). All of the sites were selected because their geological settings made them efficient catchments for tsunami deposits and therefore excellent recorders of the 2010 tsunami and potential recorders of past extreme events. At four of the five sites studied, detailed measurements were made of topography, flow depths, flow directions and flow-direction histories, tsunami-inundation distances, vertical erosion, and sediment deposition. The measurements were made in a manner consistent with data collected by USGS scientists during previous post-tsunami surveys, such as those conducted in Papua New Guinea after the 1998 tsunami (http://walrus.wr.usgs.gov/tsunami/itst.html), in Peru after the 2001 tsunami (http://walrus.wr.usgs.gov/peru2/), in Sri Lanka (http://soundwaves.usgs.gov/2005/02/, http://walrus.wr.usgs.gov/tsunami/srilanka05/) and Sumatra (http://soundwaves.usgs.gov/2005/03/, http://walrus.wr.usgs.gov/tsunami/sumatra05/) after the 2004 Indian Ocean tsunami, and in Samoa and American Samoa after the 2009 tsunami (http://soundwaves.usgs.gov/2009/12/, http://walrus.wr.usgs.gov/news/samoaabout.html).
At each of the sites, the team made some important scientific observations that add to our growing understanding of tsunami impacts. For example, the Chilean tsunami caused substantial erosion and deposition that both decreased and increased local coastal-plain elevations by as much as 1 m. The tsunami erosion was concentrated near the shore, where wide areas of land were planed off, soil was scoured from around the bases of trees, and return flow incised channels as deep as 1.6 m. Tsunami deposits, including some boulder-size clasts, were found at all sites, and the sand deposits extended to near the limit of inundation except at one site. The abundance of plane-parallel stratification in some deposits and the presence at one site of large sand waves indicate that at least some of the sediment was transported as bed load (rolled and pushed along the land surface) and not as suspended load (suspended in the water). The team also found evidence that vegetation height and density controlled tsunami-deposit thickness, and measurements were made to characterize and quantify the vegetation density at several sites. At the two open-coast sites, there was clear evidence of multiple strong onshore waves that arrived at different times and from different directions. The multiple onshore flow directions likely resulted from upward vertical displacement along the rupture zone at locations both north and south of the field sites. The flow-direction history that we interpreted in the field at one site was confirmed by an eyewitness account of the sequence of wave directions. The maximum observed tsunami-inundation distance (2.35 km) was up an alluvial valley.
A full report of the team's observations was recently released as USGS Open-File Report 2010-1116, Geological Impacts and Sedimentary Record of the February 27, 2010, Chile Tsunami—La Trinchera to Concepción.
After conducting their fieldwork, the USGS team was honored with a breakfast reception at the Santiago home of the U.S. Ambassador to Chile, Paul Simons. Attending the reception were leading Chilean scientists and ranking government officials, including Sergio Barrientos, Director of the Chilean Seismological Service, University of Chile; Patricio Winckler, School of Marine Sciences and Natural Resources, University of Valparaíso; Juan Díaz, Professor of Marine Geology and Geophysics, School of Marine Sciences, Catholic University of Valparaíso; Ricardo Norambuena, UNESCO Coordinator of Coastal Programs; María José Castañeda, Chief of Staff to the Minister, Ministry of the Environment; Capitán Andrés Enríquez, Chief of Plans and Operations, Hydrographic and Oceanographic Service of the Navy; Vicente Nuñez, Director, National Office of Emergency Ministry of the Interior; Mary Brett Rogers-Springs, Environment, Science and Technology Economic Section of the U.S. Embassy; and Dinah Lee Arnett, Environment, Science and Technology Public Affairs Section of the U.S. Embassy. During the reception the USGS team made several contacts that have already proven beneficial in obtaining important scientific information that supplements our observations.
in this issue:
Geological Impacts of the Feb. 2010 Tsunami in Chile
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