Crustal attenuation for Jamaica, West Indies

Abstract

The S and coda wave spectra of small earthquakes on the island of Jamaica were used to determine the near surface and coda Q attenuation, κ and Qc,respectively. Qc determined by the single-station method was found in the range of 1 to 10 Hertz to be given by the relationship, Qc= 60 ± 5f0.87±0.05. This suggests that the Jamaican crust is highly attenuating which is further supported by the observation of rapid intensity fall-off with distance for earthquakes that have affected the island in the past. κ, determined from S-wave spectra with short travel times was found to be 0.058 ± 0.012 on the central crustal block, which makes up nearly two-thirds of the island, and 0.080 ± 0.014 in surrounding belt sub-regions. The pattern of κ values seems to fit with the surface geology in that the central block has areas of exposed outcrops of older and harder rock than the belts, which are characterized by thicker sedimentary sequences as well as intense fracturing and faulting.Atkinson and Boore (1998) and Atkinson(2001) presented an alternative method to stochastic modelling for ground motion in Eastern North America, whereby California attenuation relationships were modified to account for crustal differences invelocity-depth profile, Q and κ between both regions. Following their example, the California spectral attenuation relation of Boore, Joyner and Fumal (1997) was modified to account for differences between the California and Jamaica crust, resulting in an attenuation relation that is deemed to be more appropriate for Jamaica. Spectral accelerations for Jamaica when compared to California, are especially reduced beyond 20 km from the source and at high frequencies, f≥ 1 hertz.The study concludes that the Jamaican crust, although having an oceanic composition is highly attenuating, which may be a result of intensive tectonic processes, whereas κ is consistent with near-rock conditions on the central block and soft rock conditions elsewhere on the island.