Experimental study of site effects in the Fort-de-France area (Martinique island)

Abstract

Abstract We used weak-motion recordings of regional events to quantify the site effects in the Fort-de-France (Martinique) area in the frequency band 1 to 25 Hz. The studied area extends over three distinct geological units: the Morne-Pitault volcanic massif (15 to 11 M.A.), the Carbet volcanic massif (4 to 2 M.A.), and the Lamentin alluvial plain (quaternary age). The topography being complex, 31 sites were selected in order to get a representative sampling of the various topographical and geological configurations. The large number of studied sites also allowed us to define a reference site whose selection was not obvious. Large broadband amplifications are observed in the Lamentin plain instead of the expected peak resonances. More surprising, such resonance is observed around 2 to 3 Hz on several sites of the volcanic Vauclain-Pitault chain, though no sedimentary layer is present. On the other hand, in the inner Fort-de-France where significant effects were expected because of the very complex topography, no large amplication is observed except for at one site. Clear three-dimensional (3D) effects are also observed at some stations. Though not very well documented, the low-frequency behavior deduced from teleseismic observations seems rather different, illustrating the difficulty in finding a convenient reference site in such a volcanic area.