Attenuation and site effects in the region of Guadeloupe, Lesser Antilles

The accelerograph network of Pointe-a-Pitre operated by the Bureau de Recherches Geologiques et Minieres recorded strong motions from earthquakes located in the northern Lesser Antilles. Twenty-three small to moderate earthquakes (M 2.9-5.5) were used to estimate S-wave attenuation and site effects. We characterized the spectral amplitude decay with distance at discrete frequencies between 0.6 and 25.0 Hz using a nonparametric approach. The resulting attenuation functions were used to correct the strong-motion records, and then we separated source and site effects using a spectral inversion technique. We compared the site functions obtained with previous estimates reported by Le Brun et al. (2001) using a standard spectral ratio technique and the Nakamura method (1989). We found that the three approaches give similar site-response functions. The site amplification, inferred from the inversion results, is significant at several sites. For instance, the soft soil sites at station GS show a maximum amplification factor of 18 at 1.3 Hz, station SF a factor of 9.5 at 5 Hz, and station T2 a factor of 8.5 at 1.0 Hz. To estimate the quality factor Q of the S waves, we model the empirical attenuation functions using a parametric attenuation function of the form A(fr) = G(r) (.) e(-pifr/Qbeta), where f, r, beta, and G(r) are frequency, hypocentral distance, mean shear-wave velocity, and the geometric spreading function, respectively. We constrain G(r) = 10/r(b), finding that the coefficients b and Q are both frequency dependent. In the frequency band analyzed (0.3 < f < 25 Hz) Q takes values between 20 and 500, and b between 0.6 and 2.3.