The Trinidad and Tobago microzonation project: San Fernando

Abstract

SUMMARYThe city of San Fernando is the industrial capital of the twin island nation of Trinidad and Tobago in the West Indies. Situated at the west coast, is in close proximity with the Central Range Fault, a major fault crossing the whole island and continuing offshore. Due to this proximity, but also because of the complex tectonics in the area with deep and shallow earthquakes, a detailed seismic hazard map deemed important. A microzonation study was carried throughout the boundaries of the city, using ambient noise techniques. 1200 single-site records of ambient noise, along with 16 microtremor array measurements were obtained. The results indicate that the alluvium deposits in the river basins along with a small part of reclaimed land appear to have velocities in the range 220–500 m s–1 and thickness from shallow to about 60 m. The marls, which cover the majority of the study area, exhibit shear wave velocities in the range of 700–1600 m s–1, depending on the level of compaction, with thickness that can reach almost 100 m. The hardest formations, Naparima Hill and San Fernando formations, in weathered state they appear with velocities above 1000 m s–1, while their unweathered state reaches 3000 m s–1. A parametric investigation which was conducted as an independent investigation to validate the experimental results, is in good agreement. An empirical susceptibility method to calculate the most vulnerable areas, reveals that areas close to coast and the river banks might experience much higher shaking values.