Guam Hazard Mitigation Plan - Western States Seismic Policy Council

SECTIONFIVERisk Assessmentlateral spreading (also shown in Table 5-20) are mainly surrounding Tumon Bay and ApraHarbor, as much of the area surrounding Apra Harbor contains extensive areas of fill. This areahas exhibited liquefaction and lateral spreading in historical earthquakes. In addition, the largerriver valleys and along the coastlines of Merizo, Inarajan, and Agat are also susceptible toliquefaction and lateral spread.The rest of Guam generally has a very low potential for liquefaction or lateral spreading to occurbecause the types of shallow material mapped at the surface are not conducive to liquefying.However, this assessment is limited by the scale and resolution of previous mapping on Guam;small areas of sediments more susceptible to liquefaction may exist in these very low potentialareas.Previous OccurrencesThe most significant recent earthquake on Guam occurred on August 8, 1993. This M 7.8 eventoccurred 31 miles south of Guam at a depth of 37 miles along the Mariana Trench. No consensusexists as to the source of this earthquake, but evidence from a recent study of aftershockssuggests that this earthquake was an interplate event (i.e., it occurred between the Pacific plateand the Philippine Sea plate). Liquefaction and lateral spreading caused major damage tocommercial and naval port facilities. Landslides were predominantly small local slumps androckfalls along limestone bluffs. Buildings were damaged or destroyed throughout the island.The preliminary estimate of damage to commercial buildings was $112 million. No compileddamage cost is available for this earthquake.One Presidentially declared earthquake disaster has occurred on Guam. On October 12, 2001, aM 7.3, maximum MM Intensity VII earthquake struck Guam. This seismic event was believed tohave caused one injury, but no published estimates of total damage are available. According tothe information available, the center of the earthquake was southeast of the initial centerpoint ofthe August 1993 series, and the earthquake had the following effects: the power was lost, a newschool in Piti was affected, several schools in the Santa Rita area were affected, many buildingswere damaged, and utilities were disrupted. No other earthquakes associated with damage havebeen reported. Information on other significant earthquakes since 1975 is provided in Table 5-6.5-19