The Belleplaine test site, located in the island of Guadeloupe (French Lesser
Antilles) includes a three-accelerometer vertical array, designed for liquefac-
tion studies. The seismic response of the soil column at the test site is
computed using three methods: the spectral ratio method using the vertical
array data, a numerical method using the geotechnical properties of the soil
column, and an operative fre- quency domain decomposition (FDD) modal analysis
method. The Belleplaine test site is characterized by a mangrove layer overlaid
by a stiff sandy deposit. This con- figuration is widely found at the border
coast of the Caribbean region, which is exposed to high seismic hazard. We show
that the buried mangrove layer plays the role of an isolation system equivalent
to those usually employed in earthquake engineering aimed at reducing the
seismic shear forces by reducing the internal stress within the structure. In
our case, the flexibility of the mangrove layer reduces the distortion and the
stress in the sandy upper layer, and consequently reduces the potential of
liquefaction of the site
