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

Abstract

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 (Spain) and the gravity data published by the Institute Nacional de Meteorologia e Geofisica (Portugal). The resulting Bouguer anomaly reveals the presence of a regional trend, several local features and a component of noise. In separating the regional trend we use a robust polynomial fitting. Further, a covariance analysis and a least squares prediction are applied to filter the noise and to model the local anomaly. ii first signal map represents the main local anomaly, while the secondary signal map shows superficial anomalies. 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 inversion, we obtain a density contrast distribution of the sources of the filtered 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 and the weighted density contrast allow us to obtain a solution as a discrete three-dimensional density contrast model. The obtained anomalous densities model shows a clear relation with the structure of the caldera complex and tectonic trends, with the principal body of negative contrast density being associated with the subsurface structure of the caldera. (C) 1999 Elsevier Science B.V. All rights reserved.