Caracas, Venezuela, site effect determination with microtremors

Caracas 1967 earthquake caused heavy damage to multi-story buildings. In 19 95, 184 microtremor measurement points were performed over the city. The me asurement grid was more or less dense and covered the main part of the allu vial basin as well as surrounding rock basement. For each point, the horizo ntal record spectrum was divided by the vertical one (H/V ratio). Subsequen tly, the strongest value (Ao) of this ratio in a given frequency band was k ept, as well as the frequency (Fo) where it occurred. Spatial interpolation s of Ao and Fo were performed among all points of Palos Grandes district, A map was plotted representing a single surface where Ao is relief and: Fo i s represented by grey gradation. An alluvion thickness (H) map can be compa red with this result. Damaged buildings are located on the same map. Fo dec reases until 0.6 Hz when alluvion thickness (H) increases. Fo values fit wi th frequencies previously predicted from computation and with 1967 earthqua ke observations. Moreover, interpolation surfaces show that amplification ( Ao) of H/V on microtremor is quite low above rock but is high on the south part of the basin. The maximum occurs over the non-urbanized zone. However the other area, where Ao is higher than 5, roughly corresponds to the locat ion of the four collapsed buildings, Furthermore, the estimated natural fre quency of these buildings was around Fo. Graphics showing H, Fo and: Ao evo lution through the basin were composed. Links between all these values are noticeable. Fo is claimed to be very similar to the resonance frequency of soil. As for Ao, it could be a fairly relevant sign of damage seriousness. Microtremor technique is an economic tool and it allows measurement grid as dense as desired. H/V ratio processing followed by interpolation of maximu m values provides precise and useful information about expected site effect .