Guam Hazard Mitigation Plan - Western States Seismic Policy Council

SECTIONFIVERisk AssessmentTable 5-13Monthly Frequency of Cloud-to-Ground Lightning on GuamJan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual0.7 0.2 0.1 0.6 1.2 3.0 6.0 7.2 7.9 4.2 2.7 0.3 33.9Source: NCDC, 2000.5.3.9 Non-Seismic Ground Failure (Sinkholes)NatureSinkholes are a characteristic of karst topography; karst geology occurs when rainwater dissolvescarbonate rocks, such as limestone, and causes voids, including epikarst, sinkholes, and caves, inthe surface and subsurface. Sinkholes are defined as a natural depression or hole in the landformed by the erosion and sometimes the collapse of the underlying rock or soil. Sinkholes aretypically caused by the chemical dissolution of underlying carbonate rocks; often, groundwaterdissolves the carbonate cement that holds together sandstone particles and then carries away theuncemented particles; this process can form a void. Other formation processes for sinkholesinclude the collapse of a cave roof and the lowering of the water table. The formation ofsinkholes can be facilitated by high groundwater flow, which is often caused by high rainfall.LocationIn 2004, WERI completed a study of the karst features of Guam (WERI 2004). The studyshowed that northern Guam is almost entirely karst terrain, and the south is mostly volcanicterrain with karst on some outlying limestone units. Accordingly, the vast majority of sinkholesare found in northern Guam. Sinkholes in northern Guam are scattered across the plateau surface,many of which can be seen in the field at Radio Barrigada and off the road toward RitidianBeach. Of the sinkholes found in southern Guam, the largest concentration is northeast of FenaReservoir, though significant sinkholes are also found in the southern mountain ridge and thesouthern part of the eastern coast.Previous OccurrencesThe 2004 WERI study identified 1,252 sinkholes in northern Guam and 197 sinkholes insouthern Guam. The sinkholes in northern Guam have depths of over 98 feet and have recordedlengths of hundreds of feet. The largest and deepest sinkholes are found on the Naval BaseOrdnance Annex. Since 2004, no further comprehensive studies have been completed. Therefore,no information is available about sinkhole occurrences from 2004 to the present.Probability of Future EventsRecent history and the karst land geology found throughout Guam indicate that sinkholes willcontinue to occur on Guam. However, due to lack of data, the rate at which sinkholes willdevelop is unknown.5-36