UPPER CRUSTAL TOMOGRAPHIC-IMAGES OF THE AMIATA-VULSINI GEOTHERMAL REGION, CENTRAL ITALY

We computed the three-dimensional P wave velocity structure of the upper crust beneath the adjacent Amiata and Vulsini Quaternary volcanoes (central Italy), inverting local earthquake arrival times. We used 676 selected earthquakes out of more than 3000 (in the M range 1.5-4.0) located between 1977 and 1992 by two contiguous microseismic networks operated by Ente Nazionale Energia Elettrica to monitor three geothermal areas (namely, Latera, Torre Alfina, and Mount Amiata). The resolving capability of the data set has been verified both with a priori synthetic tests and an a posteriori analysis of the resolution matrix. We find a 40x25 x7 (h) km(3) crustal volume where we can resolve: details as small as 2 to 4 km. Our analysis is favored by numerous geological and geophysical data collected for geothermal exploration. These independent data give us specific constraints for both choosing a starting model and interpreting tomographic results. We find high-velocity anomalies in the upper 3 km of the crust beneath Latera, Torre Alfina, and Mount Amiata geothermal fields, related to uplifted limestone units. The lateral extension of Limestone units is defined by the three-dimensional velocity pattern. A negative anomaly clearly identifies the Radicofani Graben, a Plio-Quaternary depression filled with low velocity clayey sediments, between 1 and 3 km depth. A sharp gradient between high velocities (similar to 6.9 km/s) in the northern region and lower velocities in the southern one is recovered at 7 km depth. This feature may indicate the transition between the Tuscan province uplifted metamorphic crust and thickened carbonate series of the Latium province.