Analysis of gravimetric anomalies in Furnas caldera (Sao Miguel, Azores)

Furnas Volcano (Sao Miguel Island, Azores) is a steep-sided caldera complex resulting from several collapses. In this paper, we analyse and interpret the gravity survey carried out by the Institute de Astronomia y Geodesia (S pain) and the gravity data published by the Institute Nacional de Meteorolo gia e Geofisica (Portugal). The resulting Bouguer anomaly reveals the prese nce of a regional trend, several local features and a component of noise. I n separating the regional trend we use a robust polynomial fitting. Further , a covariance analysis and a least squares prediction are applied to filte r the noise and to model the local anomaly. ii first signal map represents the main local anomaly, while the secondary signal map shows superficial an omalies. This distribution reveals a local main minimum close to the 1630 A .D. and Gaspar craters. Several possible alignments are shown that lead to a model, mainly with SW-NE and SE-MY orientations, of the negative anomaly, with the pattern of the low density zones showing a relationship with the volcanic features. Applying a stabilized Linear method of gravimetric inver sion, we obtain a density contrast distribution of the sources of the filte red local Bouguer anomaly, considering as unknowns the anomalous densities of a fixed discrete distribution of rectangular prisms which represents the subsoil volume. A mixed minimization criteria for the weighted residuals a nd the weighted density contrast allow us to obtain a solution as a discret e three-dimensional density contrast model. The obtained anomalous densitie s model shows a clear relation with the structure of the caldera complex an d tectonic trends, with the principal body of negative contrast density bei ng associated with the subsurface structure of the caldera. (C) 1999 Elsevi er Science B.V. All rights reserved.