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### REDUCING THE RISK FROM COASTAL GEOLOGIC HAZARDS

##### U.S. Geological Survey     Fact Sheet 150-002000

## **Helping Coastal Communities at Risk from Tsunamis–The Role of U.S. Geological Survey Research**

| In 1946, 1960, and 1964, majortsunamis (giant sea wavesusually caused by earthquakesor submarine landslides) struckcoastal areas of the PacificOcean. In the U.S. alone, thesetsunamis killed hundreds ofpeople and caused many tens ofmillions of dollars in damage.Recent events in Papua NewGuinea (1998) and elsewhere arereminders that a catastrophictsunami could strike U.S. coastsat any time. The USGS, workingclosely with NOAA and otherpartners in the National TsunamiHazard Mitigation Program, ishelping to reduce losses fromtsunamis through increased hazardassessment and improvedreal-time warning systems. |
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Aftermath of the 1960Chilean tsunami inHilo, Hawaii, wherethe tsunami caused61 deaths (photocourtesy PacificTsunami Museum).Inset shows damagefrom a local tsunamithat struck OkushiriIsland, Japan, in 1993(photo by DennisSigrist, InternationalTsunami InformationCenter). Floatingdebris, such as seenin these photographs,can causesignificantdestructionduringtsunamirunup.

Population pressures in the coastal zonesof the Pacific Ocean and the Caribbean areresulting in unprecedented shoreline development,putting residents, tourists, andproperty at increased risk from giant seawaves, called tsunamis. Tsunamis, often incorrectlyreferred to as “tidal waves,” can begenerated by distant earthquakes and by localseismic events, submarine landslides,and volcanic eruptions. For ocean-crossingtsunamis, there is often sufficient time toevacuate distant coastal areas, but moretimely and accurate real-time tsunami forecastsare needed to avoid costly falsealarms. Local tsunamis generated byquakes on active seismic zones in Alaska,the Pacific Northwest, the Caribbean,California, and Hawaii can arrive atnearby shorelines in minutes. In thesecases, only better scientific understanding,informed disaster planning, and publiceducation will save lives in future tsunamis.Protecting lives and property from tsunamisdemands a clear understanding of howtsunamis are generated, the identification oflikely areas at risk, and mitigation effortsbased on public education. The NationalTsunami Hazard Mitigation Program(NTHMP), a partnership of the States ofAlaska, California, Hawaii, Oregon, andWashington and the Federal EmergencyManagement Agency, National Oceanic andAtmospheric Administration (NOAA), andU.S. Geological Survey (USGS), is currentlypreparing tsunami inundation mapsand implementing mitigation plans forstates bordering the Pacific Ocean. TheNTHMP is also providing tsunami earlywarnings for these states by means of deepoceantsunami detectors and new seismicstations on land. The cooperation of Federaland state agencies in the NTHMP isfurthering scientific understanding of tsunamihazards and facilitating the developmentof plans to prepare coastal communitiesto better survive future tsunamis.The mission of the USGS includes understandingthe geologic mechanisms, frequency,magnitude, and physical consequencesof natural hazards. In recent years,the USGS has upgraded its seismic networksto provide NOAA's tsunami warningcenters with detailed earthquake informationto aid in timely notification of the public.The USGS and NOAA are currentlyproviding real-time seismic and tsunami informationto state offices of emergency servicesin Alaska, Washington, Oregon, California,and Hawaii.

To ensure that its efforts to reduce the riskfrom tsunamis are focused where they willbe most effective, the USGS convened atwo-day workshop in Seattle, Washington,in January 2000. This workshop was attendedby scientists and managers from theUSGS, NOAA, state agencies, andacademia. Based on needs identified by theworkshop participants, the following recommendationson the role of the USGS in tsunamiresearch were made:

**Seismic networks**—It is importantthat, as advances in technology and scienceallow, the USGS continue to enhanceand improve the quality and quantity ofseismic data supplied to tsunami warningcenters. The USGS should also determinethe need for additional strong-motion seismicstations along U.S. coasts. Further, theUSGS should continue to develop softwarethat supports tsunami warnings for localearthquakes, particularly in Hawaii, Alaska,the Pacific Northwest, and the Caribbean.

A tsunami caused by an earthquake along asubduction zone happens when the leading edgeof the overriding tectonic plate breaks free andsprings seaward, displacing the sea floor and thewater above it. The arrival time and severity of thetsunami can best be predicted using complexearthquake parameters, such as accurate depthdetermination, focal mechanism, and highresolutionsea-floor displacement patterns.

**Earthquake source characterization**—In order to provide accurate warnings andhazard assessments, better characterizationsof critical earthquake source parameters areneeded. Modern methods of analyzing seismicdata developed by the USGS can beused by tsunami warning centers in an overalleffort to improve the accuracy and timelinessof tsunami warnings. Further researchis also recommended into using rapid seismicinversion algorithms to estimate seafloordisplacement, an indicator of likelytsunami size, which could be automated aspart of local tsunami warning systems.

**Hazard assessments**—The NTHMP iscoordinating the preparation of tsunami inundationmaps for high-risk coastal communitiesin Alaska, California, Hawaii, Oregon,and Washington. The USGS can providevaluable guidance in the preparation of thesemaps by analyzing and interpreting depositsfrom historic and prehistoric tsunamis to estimateinundation limits, flow velocities, andrecurrence intervals.

**Coastal bathymetry and topography**—Producing reliable and useful inundationmaps for hazard assessment partially dependson accurate bathymetry and topographyof coastal regions. The USGS, alongwith NOAA, NASA, and the Army Corps ofEngineers, have proven capability to surveycoastal and nearshore bathymetry and topographyusing modern techniques. It isrecommended that the USGS coordinatewith other Federal agencies to providesuch information for hazard mitigationplanning in high priority areas of the PacificNorthwest, Alaska, Hawaii, California,and the Caribbean.

**Sedimentary deposits as keys to tsunamicharacter**—Identification and interpretationof sedimentary deposits leftbehind by prehistoric tsunamis will improveour ability to assess the magnitudeof tsunami risk in areas with an insufficienthistorical record. The USGS willprovide expertise and leadership in sedimenttransport modeling to increase understandingof the mechanics of sedimenttransport in tsunamis and of the depositsthey leave behind. Research would occurin field settings by integrating predictivemodeling with laboratory and post-eventstudies.

**Post-event rapid response**—The visibleeffects of tsunamis are short-livedand may be lost after a single subsequentstorm or during clean-up efforts employingearth-moving equipment. To determinethe effects of tsunami inundation onland, the run-up elevation and distance,flow-speed and direction indicators, andpatterns of sedimentary deposition mustbe mapped and quantified immediatelyfollowing an event. USGS scientistsshould join the International TsunamiSurvey Team, when warranted, to gatherinformation about tsunami deposits andto calibrate sediment transport models.

Computer simulation of a major nearshore earthquakealong the Cascadia Subduction Zone off thePacific Northwest shows the advancing wave frontsof the resultant tsunami.

**Tsunamis generated by landslidesand volcanic events**—Landslides andvolcano flank failures in coastal and islandsettings have also initiated large tsunamis.The accumulated knowledge ofUSGS scientists about submarine andcoastal landslides and active volcanicprocesses should be focused on improvingunderstanding of how and where these catastrophicmass failures may occur. This informationthen can be used in regionalhazard assessments by Federal, state, andlocal authorities.

The efforts of USGS and other cooperatorsin the NTHMP are leading to a betterunderstanding of tsunamis and howcoastal populations can be prepared to survivetheir onslaught. The work of USGSscientists in tsunami research is only partof the ongoing efforts of the USGS to protectpeople's lives and property from geologicand environmental hazards in thecoastal zones of the United States.

***By Eric L. Geist, Guy R. Gelfenbaum, Bruce E. Jaffe, and Jane A. Reid***

Graphic design by Susan Mayfield and Sara Boore; Web design by Michael Diggles

For questions about the content of this report, contact Eric Geist

NATIONAL TSUNAMI HAZARD MITIGATION PROGRAMStates of Alaska, California, Hawaii,Oregon, and WashingtonNational Oceanic and Atmospheric AdministrationFederal Emergency Management AgencyU.S. Geological Survey

For more information contact:
U.S. Geological Survey, MS-999345 Middlefield RoadMenlo Park, CA 94025(650) 329-5042http://walrus.wr.usgs.gov/tsunamihttp://www.pmel.noaa.gov/tsunamiSee also Surviving a Tsunami-Lessons from Chile, Hawaii,and Japan (USGS Circular 1187)

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| Department of the Interior   | Privacy Statement | Disclaimer   | Accessibility |   URL of this page: https://pubs.usgs.gov/fs/fs150-00/  Maintained by: Michael Diggles  Created: November 3, 2000  Last modified: January 3, 2005 (mfd)