Guam Hazard Mitigation Plan - Western States Seismic Policy Council

SECTIONFIVERisk Assessmentcanyons and stream valleys. Debris flow fronts can travel at high speeds, exceeding 50 miles perhour. In most cases, only large basins designed specifically to trap these flows are capable ofresisting the forces that accompany them.LocationLandslide: An adequate landslide inventory map is not available for Guam. Therefore, the mostappropriate way to identify where landslide hazards occur on Guam is to determine thesusceptibility of an area based on the geologic units mapped at the surface.Susceptibility to landslide hazards on Guam has been determined by the geology and the slopeangle of the various specific areas on the island. Using these two factors, a quantitative rating ofthe potential of an area for a landslide to occur was defined. As shown on Figure D-23, allslopes with an angle of 30 percent or more are considered to have a moderate to high potentialfor a landslide to occur. All slopes that have a slope of less than 5 percent are considered to havea low potential, regardless of the geologic deposits present. As such, Yona, Santa Rita, Piti,Asan/Maina, Talofofo, Inarajan, Merizo, Umatac, and Agat all have proportionally large areaswith a moderate, a moderate to high, or a high potential for landslides to occur. Only therelatively flat areas along the eastern coast of this half of the island and the flat areas aroundApra Harbor have a low potential for landslides.It is important to note that this simplified assessment does not include some potentially importantfactors, such as local geologic structures. Many of the landslides triggered during the August 3,1993, earthquake were associated with faults. When the orientation of potential landslide failureplanes (such as bedding or fault planes) is favorable for failure with respect to the slope,landslides can occur in geologic units not generally prone to fail, particularly during earthquakesor when the ground is saturated.Mudslide: The areas most susceptible to mudslides are steep areas made of clay, areas wheremudslides occurred before but bedrock is not yet exposed, and sloped clay areas wherevegetation has been removed. Steep refers to angles that range from 10 degrees from the verticalto about 40 degrees from the vertical. For angles of less than 10 degrees from the vertical, waterflows over the clay and forms waterfalls. For angles greater than 40 degrees from the vertical,gravity does not normally cause the mud to give way. As such, Nimitz Hill, Santa Rita, the CrossIsland Road, and the southern mountain areas are the most susceptible mudslide locations onGuam.Post-Fire Debris Flow: Land that is adjacent to or downslope of recently burned areas issusceptible to potentially hazardous debris flows. Areas susceptible to recent burn and potentialdebris flow include localities on the west coast, such as Agat and Umatac, and the areas nearTalofofo, Inarajan, and Merizo.Previous OccurrencesLandslide: Available historical records of landslides due to earthquakes document thatlandslides resulted from the earthquakes that occurred on September 22, 1902; October 30, 1936;November 1, 1976; and August 8, 1993. Many of the landslides that occurred during the large1993 earthquake are well documented. For example, a large landslide was observed along aremote sea cliff on the northeast side of Guam near Lajuna Point. This slide was not known toresult in any damage. Many relatively smaller slides were observed on steep cut-slopes andlimestone cliffs throughout the island. Several of these landslides occurred immediately adjacent5-42