Guam Hazard Mitigation Plan - Western States Seismic Policy Council

SECTIONFIVERisk Assessmentscarps offsetting limestones that are likely Quaternary or even late Quaternary in age, offsets oflate Quaternary marine terraces, and even offsets of young algal pools.Liquefaction and Lateral Spreads: The only readily available information on historicalliquefaction and lateral spread events concerns the earthquake that occurred on August 8, 1993.The areas of historic liquefaction and lateral spread are shown on Figure D-14. The occurrenceswere originally documented in the 1995 EERI report on this event. As shown on Figure D-14,liquefaction and lateral spreading was observed at the following locations:• In the areas surrounding Apra Harbor and Piti Power Plant, liquefaction occurred where coralfill material overlies fine-grained lagoonal and estuarine deposits. The groundwater in theseareas was approximately 7 to 8 feet below ground surface. At the commercial port facility,liquefaction and lateral spreading caused horizontal displacement of up to 24 inches andcrane tracks and bulkheads were warped. Cracks were observed that extended for 200 to 300feet and were up to 8 feet deep. According to information on the website of the Guam PowerAuthority, damage of $8-$10 million occurred at the port• At the Naval Station port facilities, most of the wharves sustained structural damage fromliquefaction and lateral spreading. The Navy allocated $2.8 million toward initial wharfrepairs, but the estimate for the total cost of the damage was $25.15 million.• Extensive evidence of liquefaction was observed at the Piti Power Plant. Ground fissures andsand boils ejected coral sands. Liquefaction caused up to 4 feet of settlement. Althoughliquefaction and lateral spreading caused nonstructural damage, no structural damageoccurred to the plant due to liquefaction or lateral spreading.• Liquefaction was evident near the new courthouse in downtown Agana. This area wasdeveloped with fill material on top of fine-grained Agana marsh sediments. No cost estimateof the damage to this area is available.Probability of Future EventsFault and seismic data for the region in which Guam is located are generally scarce. Guam is in aremote region of the Pacific Ocean, and the historical seismic catalog for moderate-sized eventsis most likely incomplete and the historical record for large events is likely inadequate becausethe recurrence intervals for subduction zone earthquakes may be long. However, based onrecorded occurrences from 1975-2011, an earthquake is likely to be felt on Guam every 4-5years.The 1999 Global Seismic Hazard Assessment Program (GSHAP) conducted a generalized globalprobabilistic seismic hazard analysis that included Guam. GSHAP calculated PGAs with a 10percent chance of being exceeded in 50 years. Guam lies in the PGA contour range of 0.16g to0.24g. The GSHAP analysis only considered regional source zones; the potential hazard from thesubduction zone or crustal faults was not included in the analysis.Surface Fault Rupture: Beyond identifying the general locations of the potentially active faulttraces on Guam, the data available on these faults are not adequate to characterize the faults andanalyze the probability for surface faulting to occur. To fully characterize the hazards fromsurface faulting, the exact locations, ages, sense of motion, and dips associated with the faultsneed to be researched and identified. However, the approximate locations of many potentially5-21