Crustal velocity structure of the Larderello geothermal field determined from local earthquake arrival time data

Abstract

A joint hypocenter-velocity inversion of local earthquake arrival time data has been performed in the Larderello Geothermal Field in Tuscany, Italy. More than two thousand local earthquakes have been recorded in the Larderello area since 1977. Most of the earthquakes occur shallower than 8 km depth, although a few occur as deep as 30 km. The corner frequency of the data is typically 25 Hz. P wave and S wave arrival times from 269 of these earthquakes were used to simultaneously determine the hypocenter parameters and the three-dimensional P wave velocity and Vp/Vs structures. Due to a variety of rock types present in the uppermost 2 km, as well as a complex tectonic history, the shallow velocity structure is very complicated. This shallow structure was determined from seismic reflection and vertical seismic profiling (VSP) data and kept fixed during the joint hypocenter-velocity inversion.