Liquefaction susceptibility zonation map of San Juan, Puerto Rico

This paper discusses development of a 1:20,000 scale liquefaction susceptibility map for the San Juan Quadrangle of Puerto Rico. San Juan, with an estimated population of 434,000, is the capital of Puerto Rico, and the most important U.S. commercial and industrial center in the Caribbean region. Puerto Rico is located in a seismically active region characterized by the convergence and lateral translation of the North America and Caribbean plates. Large earthquakes in 1670, 1787, 1867, and 1918 caused significant damage to major parts of the island, including the San Juan area. Paleoliquefaction features, possibly caused by at least three different earthquakes since A.D. 1300, have been found in Holocene floodplain sediments at several sites in western Puerto Rico (Tuttle et al., this volume). The historic earthquakes and paleoliquefaction features demonstrate that the opportunity exists for future liquefaction events to occur in Puerto Rico.The liquefaction susceptibility map was developed through a five step process including (1) preparation of a detailed Quaternary geology map delineating deposit age, depositional environment, and texture; (2) evaluation of Quaternary deposit thickness and depth to groundwater; (3) initial evaluation of relative liquefaction susceptibility (decision tree); (4) liquefaction triggering evaluation using geotechnical borehole data and the Seed and Idriss (1971b) “simplified procedure”; and, (5) identification of units of similar susceptibility and the definition of liquefaction susceptibility zones. The map depicts five liquefaction hazard zones for the greater San Juan metropolitan area that range from very low hazard to very high hazard. Areas of very high hazard occur along the edges of Bahia de San Juan and Laguna San Jose. Extensive swamp deposits and artificial fill over swamp deposits containing loose sandy soils occur in these areas. Areas of high hazard occur along the beach and areas in the vicinity of the airport. Most of the area to the south of Bahia de San Juan and Laguna San Jose lies within moderate, low, or very low hazard zones.