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SUMMARY In this study, we present the results from the microzonation study conducted in Port of Spain (PoS), capital of the Republic of Trinidad & Tobago. A dense grid of single-site recordings was used to determine the fundamental frequency of soil above bedrock, while a grid of 26 array recordings comprised the database for finding the 1-D shear wave velocity, with depth. The resonant frequency was found to range from <1.0 Hz, for the deeper sediments to the south, near the coast, to above 4.0 Hz, on the northern outskirts of the city, closer to the rock formations. The array data processing revealed a shear wave velocity less than 360 m s–1, for the alluvial deposits, whilst for the harder formations, the velocity was at least 1000 m s–1. To validate the results, a parametric investigation, using synthetic seismograms of ambient noise for simplified 1-D models of the PoS basin sediments, was conducted. A 3-D geological model of the basin was developed, by integrating the experimental results with the simulated data. The model suggests a gradual increase, from north to south, in sediment depth down to ∼160 m. In order to understand and explain the variation of the resonance frequency, a review of the historical development of the area, for the past 250 yr, revealed large-scale, non-engineered land reclamation in the 19th and 20th centuries, resulting in areas with anomalously high amplification of seismic motion.

This book chapter explains the methodology to determine the shear wave velocity V S profile employing microtremors array data at Port of Spain, Trinidad, and its implication in the seismic amplification and liquefaction hazard in the city. We divide this study into five sections; firstly, we introduce a description of the spectral autocorrelation method and the genetic algorithm schemes to retrieve the Vs and thickness of soil layers. Secondly, we validate the soil profiles via inspection of the ellipticity pattern at such sites; we also compared the observed horizontal-tovertical spectral ratios (H/V) with the synthetic ones derived by the Diffuse Field Approach and 1D theoretical SH wave amplification functions. Thirdly, we compute the shear wave velocity in the first 30 m obtained from our genetic inversion and compared with the ones estimated by the empirical formulas based on geomorphological conditions. Fourthly, we present a preliminary liquefaction hazard map based on the level of H/V microtremor ratios and the fundamental period of vibration. Finally, we conclude with further recommendations for planning purposes in the city of Port of Spain.

Summary The 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, employing 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 and thickness from shallow to about 60 meters. The marls, which cover the majority of the study area, exhibit shear wave velocities in the range of 700-1600 m/s, 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, while their unweathered state reaches 3000 m/s. 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.