Guam Hazard Mitigation Plan - Western States Seismic Policy Council

SECTIONFIVERisk AssessmentModel was developed in 1987 for the NWS-WFO Tropical Prediction Center at Miami, Florida.The model uses multiple inputs (including the historical tracks of tropical cyclones, the radius oftheir maximum winds, the time and location of their landfall, and the rate of storm decay afterlandfall). HURISK has been modified for the western North Pacific Ocean and is currently themost developed and comprehensive model for tropical cyclone risk for the area. WERI used arelatively comprehensive dataset of 1,469 storms that occurred near Guam during the period1945 through 1997.WERI (1999) determined that a 73.8 percent chance existed that a tropical storm or typhoon willcome within 86 miles of Guam with sustained winds of readily 40 mph for any year, and a 46.3percent chance of a typhoon for any given year. Within any 5 years, a 99.9 percent probabilityexists that a tropical cyclone will come within 86 miles of Guam with at least sustained winds of40 mph, and a 95.6 percent probability of a typhoon occurrence for any given year.In expressing typhoon recurrence through wind speeds, the average return period for minimallystrong typhoon-induced sustained winds to be experienced on Guam (i.e., approximate sustainedwind speeds of 75 mph) will be 4.4 years. The approximate sustained wind speed of a 100-yearstorm on Guam was calculated to be approximately 160 mph, a 50-year storm was calculated tohave approximate sustained wind speeds of 150 mph, and a 20-year storm was calculated to havea sustained wind speed of approximately120 mph. Therefore, a 20-year storm would carry theintensity of Typhoon Omar in 1992 and a 50-year storm would roughly carry the intensity ofSuper Typhoon Paka.It should be noted that the risk assessment performed by WERI did not include storm eventsfrom more recent years, such as Typhoon Chaba, Typhoon Tingting, Typhoon Chata’an,Typhoon Halong, and Super Typhoon Pongsona. It is likely that the inclusion of these morerecent intense typhoon events would change the probability and magnitude calculationsperformed by WERI. However, currently, the risk assessment performed by WERI is the bestand most comprehensive forecast performed to date.5.3.16 TsunamiNatureA tsunami is a series of traveling ocean waves of extremely long length generated bydisturbances associated primarily with earthquakes occurring below or near the ocean floor.Subduction zone earthquakes at plate boundaries often cause tsunamis. However, tsunamis canalso be generated by submarine landslides, submarine volcanic eruptions, the collapse ofvolcanic edifices, and in very rare instances, large meteorite impacts in the ocean.As an oceanic plate is subducted beneath a continental plate, it sometimes brings down the lip ofthe Continental Plate with it. Eventually, too much stress is put on the lip and it snaps back,sending shockwaves through the earth’s crust; these shockwaves cause a tremor under the sea,known as an undersea earthquake. Factors that affect tsunami generation from an earthquakeevent include magnitude (generally, a 7.5 M and above), depth of event (a shallow marine eventthat displaces the seafloor), and type of earthquake (thrust as opposed to strike-slip).In the deep ocean, the length of a tsunami from wave crest to wave crest may be a hundred milesor more but have a wave height of only a few feet or less. Thus, the wave period can be up toseveral hours and wavelengths can exceed several hundred miles. Thus, tsunamis are unlike5-49