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Keywords:

  • calderas;
  • crustal structure;
  • gravity anomalies;
  • inverse problem;
  • seismicity;
  • volcanic structure

SUMMARY

The seismic response of the São Sebastião volcanic crater, located at the SE end of Terceira Island (Azores), is characterized by an amplification of ground movements inside the crater with respect to the surrounding area. This variability of ground motion is also observed inside the volcanic depression itself. To gain insight into this phenomenon, a gravimetric survey with dense coverage of the region where the depression is located was carried out. Data corrections, including the calculation of terrain density by the Nettleton method and filtering of non-correlated anomalies (noise) by least-squares prediction, led to a Bouguer anomaly map with a standard deviation of 0.213 mGal. The analysis of this map and its interpretation is carried out by means of a stabilized non-linear inversion methodology resulting in a 3-D model of density contrasts, which identifies the geometry of the causative sources in the area. The gravity inversion outlines a correlation between the presence of scoria cones eruptive centres and the zones of negative density contrast in the calculated model. At São Sebastião village, another important structure with negative density contrast is located in places where a more destructive and strong anomalous behaviour has been observed during several seismic events. Furthermore, the gravity inversion seems to confirm a collapse model for São Sebastião crater and to indicate the presence of a deep conduit system compatible with the fissural nature of the volcanism in this area.