Site effects of the 1997 Cariaco, Venezuela earthquake

During the July 9, 1997 Cariaco earthquake, the small town of Cariaco (located 10 km SW from the epicenter) and Cumaná (capital of the State of Sucre, located about 80 km west from the epicenter) were the most affected towns. The damage in Cariaco was essentially restricted to one-century-old dwellings in the downtown area, but also three rather modern buildings collapsed. A maximum intensity of VIII (MMI) was determined for the epicentral area with a clear orientation of the major damage along the strike of the El Pilar fault in east–west direction. The induced effects associated with this event are dominated by liquefaction phenomena and lateral spreading on soft sedimentary lowlands (along the shoreline of the Cariaco Gulf and riverbeds), as well as sliding at unstable slopes. Site studies were carried out in Cariaco, involving the geotechnical analysis of boreholes, seismic refraction studies and microtremor measurements to determine the characteristics of the Quaternary sediment fill in the area. From seismic refraction surveys, an interface separating sediments with S-wave velocity lower than 700 m/s from stiffer ones was located at 60–90 m in depth in the southern part of Cariaco. Further north it is supposed to exceed 90 m. Predominant periods of soil, derived from microtremor observations in Cariaco, vary between 0.6 and 1.2 s. The high percentage of damage in the center of Cariaco can be attributed to the poor quality of the dwellings combined with the presence of thick, poorly consolidated soils, and, in some particular cases, to liquefaction phenomena.