Three-Dimensional Inversion of Teleseismic Travel Times for Velocity Structure in the Larderello Geothermal Field, Italy

Abstract

The analysis of teleseismic travel time residuals observed on the Larderello Seismic Network of central Italy has revealed a sharp low velocity zone in the center of the geothermal production area. Residuals from 101 teleseismic events, automatically determined with the implementation of a simulated annealing optimization procedure, are inverted to produce and image of an anomalous body interpreted to be an intrusive heat source. The top of this body is constrained to be below 6 km depth from local earthquake seismicity patterns and extends to a depth of greater than 20 km. The intrusive body broadens with increasing depth and may extend into the upper mantle. The observed travel time residual anomaly corresponds with observed gravity lows, high heat flow, the shallowing of a dominate upper crustal reflection, and diminished teleseismic P wave amplitudes. This body has elevated the temperatures in the region to over 350 degrees C at 2 km and is the origin of the geothermal energy.