A case study of the spatial distribution of seismic hazard (El Salvador)

The republic of El Salvador in Central America is an area of high seismic h azard where at least twelve destructive earthquakes have occurred this cent ury alone. The principal sources of seismic hazard are earthquakes associat ed with the subduction of the Cocos plate in the Middle America Trench and upper-crustal earthquakes in the chain of Quaternary volcanoes that runs ac ross the country parallel to the subduction trench. Hazard assessments for Central America have suggested almost uniform distribution of hazard throug hout El Salvador. Seismic zonations for three successive building codes in El Salvador simply divide the country into two regions, with the higher haz ard zone containing the volcanoes and the coastal areas. Historical records suggest that the greatest hazard is posed by the upper-crustal earthquakes concentrated on the volcanic centres which, although of smaller magnitude than the subduction events, are generally of shallow focus and coincide wit h the main population centres, These earthquakes have repeatedly caused int ense damage over small areas in the vicinity of some of the main volcanoes. This study focuses on El Salvador to explore the capability of different a pproaches to hazard assessment to reflect significant variations of seismic hazard within small geographical areas. In the study, three 'zone-free' me thods are employed as well as the Cornell-McGuire approach. The results of the assessments are compared and their implications for seismic zoning for construction and insurance purposes are discussed.