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Attenuation and Site Effects in the Region of Guadeloupe, Lesser Antilles

Centro de Investigación Científica y de Educación Superior de Ensenada
(CICESE)
Department of Seismology, Earth Science Division
Km 107 Carretera Tijuana-Ensenada
22860 Ensenada, Baja California, México
raul{at}cicese.mx
(R.R.C.)

BRGM
117 avenue de Luminy, BP 167
13276 Marseille Cedex 09, France
(H.F., M.B., B.I.)

The accelerograph network of Pointe-à-Pitre operated by the Bureau de Recherches Géologìques et Minières 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

where f, r, ß, 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.

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