Guam Hazard Mitigation Plan - Western States Seismic Policy Council

SECTIONFIVERisk Assessmenttypical wind-generated swells on the ocean, which might have a period of about 10 seconds anda wavelength of up to 300 feet. Tsunamis cannot be felt aboard ships nor can they be seen fromthe air in the open ocean. In deep water, the waves may reach speeds exceeding 700 miles perhour.Tsunamis reaching heights of more than 100 feet have been recorded. As a tsunami waveapproaches the shallow coastal waters, it appears normal and its speed decreases. Then as thetsunami nears the coastline, it can grow to a great height, smash into the shore, and cause muchdestruction.Tsunamis not only affect beaches that are open to the ocean, but also bay mouths, tidal flats, andthe shores of large coastal rivers. Tsunami waves can also diffract around land masses. Becausetsunamis are not symmetrical, the waves can be much stronger in one direction than another,depending on the nature of the source and the surrounding geography. However, tsunamispropagate outward from their source, so coasts in the shadow of affected land masses are usuallyfairly safe.Tsunamis can originate hundreds or even thousands of miles away from coastal areas. Localgeography may intensify the effect of a tsunami. Areas at greatest risk are less than 50 feet abovesea level and within 1 mile of the shoreline. Tsunamis arrive as a series of successive “crests”(high-water levels) and “troughs” (low-water levels). These successive crests and troughs canoccur anywhere from 5 to 90 minutes apart. They usually occur 10 to 45 minutes apart.Tsunami run-up occurs when a peak in the tsunami wave travels from the near-shore region ontoshore. Run-up is usually expressed in meters above normal high tide. Except for the largesttsunamis, such as the 2004 Indian Ocean event, tsunamis generally do not result in giantbreaking waves (like normal surf waves at the beach that curl over as they approach shore).Rather, they come in much like very strong and fast-moving tides (i.e., strong surges and rapidchanges in sea level). Much of the damage inflicted by tsunamis is caused by strong currents andfloating debris. Tsunamis often travel much farther inland than normal waves. Most deathsduring a tsunami result from drowning. Associated risks often include flooding, polluted watersupplies, and damage to structures and utilities, which can lead to fires.LocationFigure D-26 shows the potential areas for tsunami inundation. These areas include all landmasses below 16.4 feet in mean sea elevation and the inundation areas for the five bays (ApraHarbor, Tumon Bay, Pago Bay, Agana Bay, and Inarajan Bay) listed in the tsunami hazardassessment study completed by the Pacific Risk Management `Ohana, the Pacific MarineEnvironmental Laboratory Center for Tsunami Research, and the NWS-WFO Pacific ServicesCenter (PSC) in October 2009.Previous OccurrencesHistorical data regarding tsunami events on Guam are minimal and likely incomplete. Thegeneral view is that tsunamis occur infrequently on the island and that the band of coral reefssurrounding the island forms a natural barrier against destructive tsunamis. It is likely thattsunami events are underreported because Guam frequently experiences large wave run-upduring typhoon events, and the tsunami run-up on Guam may be less damaging than the waverun-up associated with typhoons.5-50