The deep structure of the Larderello-Travale geothermal field from 3D microearthquake traveltime tomography

With the aim of exploring the deep structure of theLarderello-Travale (LT) geothermal field, a high resolution3-D tomographic inversion of microearthquake traveltimeshas been performed. Results show that the deep part of theLarderello-Travale field is characterized by the presence ofa structure having a velocity range of 6.0–6.5 km/s and aconvex shape deepening towards the northeastern and thesoutheastern sides of the field. Earthquakes are mostlyconcentrated on the top of the high velocity structure andbelow the ‘K horizon’ implying a transition of rheologicalproperties at depth. The reported dependence on time of ts-tpobserved at one station located above an earthquake clustersuggests that the variation in pore fluid pressure might beresponsible for the transition of rheological properties alongthe contact. In such an area, changes in pore fluid pressuremight be related to time-dependent hydraulic mechanismsthat are very effective in crustal rocks at elevatedtemperatures.