Seismicity and Velocity Images of the Roman Magmatic Province

Abstract

Seismic tomography provides velocity images of the crust beneath Quaternary volcanoes and geothermal areas with a detail of 1-4 km. We describe the three-dimensional P-wave velocity structure of the upper crust beneath the adjacent Amiata and Vulsini Geothermal Regions, and beneath the Alban Hills Volcano (Central Italy), obtained by inverting local earthquake arrival times. In the three study areas, we find high-velocity anomalies in the upper km of the crust revealing the presence of uplifted limestone units (metamorphosed at depth). The 3-D geometry of the limestone units (the main geothermal reservoir in the region) is clearly defined by the three-dimensional velocity pattern. Negative anomalies at 1 and 3 km depth identify Plio- Quaternary depressed structures, filled with low velocity clayey sediments. Earthquake hypocenters are confined in the high velocity carbonate units, in the upper 6-7 km of the crust. Abscence of seismicity beneath 6-7 km suggests ductile behavior of rocks and the presence of higher temperatures probably generated by deep sub-volcanic bodies. We observe that the seismicity occurs within the uplifted limestone units (or in the metamorphic basement beneath We also hypothesize that overpressured fluids within the seismogenic layer favors the earthquake generation.