Seismic microzonation of Carora, Venezuela

Carora is located in western Venezuela on the edge of the Carora Basin, on Quaternary sediments over Cretaceous and Tertiary rocks in a region with intermediate seismicity. Carora has experienced medium seismic damage (MCS= VII), such as in the M6.9 El Tocuyo earthquake (1950). Some potential seismic sources affecting Carora are Burbusay fault and the Mesa de Aregue fault, this one 20 km to the north.

Geophysical data obtained in Carora include ambient noise measurements which were analyzed using Rayleigh wave ellipticity curve inversion. Three seismic microzones were defined based on results from ReMi (Refraction Microtremor) and topographic analysis for Vs30 and depth determination (H) to a seismic basement with Vs > 1200 m/s. Microzone 1 corresponds to formations with Vs30 > 400 m/s (a Class C site) and H < 80 m. Microzone 2 comprises sediments with Vs30 of 200-400 m/s (a Class D site) and H < 110 m, and microzone 3 comprises sediments with Vs30 of 200-400 m/s (a Class D site) and H of 110-230 m.

Probabilistic Seismic Hazard Assessment (PHSA) was conducted based on the geographic location, Vs30, and basement depths. Faults and sectors of intermediate seismicity within a 2º radius (222 km) of the center of Carora were considered. Three NGA-2013-2014 attenuation relationships were used to calculate rock and site spectral responses at 21 structural periods, for 4 seismic return periods. Then site spectra were smoothed for engineering purpose. Results lead to specified spectra limited by 85% of building code values.