Geophysical investigations and modeling of seismic response for a seismic microzoning study in Caracas, Venezuela

One of the most important natural hazards that threatens Caracas, the capital city of Venezuela, is the seismic hazard, due to its location in the plate boundary zone between the South America and the Caribbean plates. There is historic evidence for several destructive earthquakes in Caracas since 1641, being the most recent one the 1967 Caracas earthquake. This magnitude 6.5 earthquake which occurred some 25 km northwest of Caracas generated heavy damage in distinct zones of the city, thus being the most intensely studied earthquake in Venezuela. Damage investigations of buildings , including soil and building dynamical characteristics, and the earthquake engineering characteristics of the deposits were performed in detail following the earthquake. Sediment thickness in excess of 300 m and related soil-structure interactions were identified as the principal factors for the damage distribution. Recent research projects reveal details of the geological and geophysical characteristics of the valley fill. In that respect, H/V ratios from microtremor measurements show values between 1 and 2 s for the deep sedimentary basins, whereas rock sites from outside the basin show a predominant period down to 0.2 s. The bedrock topography as well as seismic velocities and densities of the sediments are used as input information for the numerical modeling of the seismic response, which was done so far on detailed 2-D profiles across the valley. The results indicate that the depth of the distinct sedimentary layers is not the only crucial parameter for high amplification factors, but a combination of the characteristics of the sedimentary fill, the shape of the bedrock and resonance effects between adjacent sub-basins to the amplification of seismic waves. In addi