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

Abstract

[1] With the aim of exploring the deep structure of the Larderello-Travale (LT) geothermal field, a high resolution 3-D tomographic inversion of microearthquake traveltimes has been performed. Results show that the deep part of the Larderello-Travale field is characterized by the presence of a structure having a velocity range of 6.0 -6.5 km/s and a convex shape deepening towards the northeastern and the southeastern sides of the field. Earthquakes are mostly concentrated on the top of the high velocity structure and below the 'K horizon' implying a transition of rheological properties at depth. The reported dependence on time of t s -t p observed at one station located above an earthquake cluster suggests that the variation in pore fluid pressure might be responsible for the transition of rheological properties along the contact. In such an area, changes in pore fluid pressure might be related to time-dependent hydraulic mechanisms that are very effective in crustal rocks at elevated temperatures.

Influence of depth, temperature, and structure of a crustal heat source on the geothermal reservoirs of Tuscany: numerical modelling and sensitivity study

“…They are non-uniformly distributed and emplaced at various depths, leading to rather large differences in surface heat flow (100-1000 mW/ m 2 ; Bellani et al 2004) in this inner Northern Apennine area. A correlation between the depth of this heat source and a high-amplitude seismic reflector (called K horizon) has been described, among others, by Bellani et al (2004), Cameli et al (1993Cameli et al ( , 1998, Gianelli et al (1997), Liotta and Ranalli (1999), and Vanorio et al (2004). As stated by Liotta and Ranalli (1999), "this horizon ranges in depth between 3 and 12 km, and can be followed almost continuously from the Tyrrhenian coast to the central part of the Apennines'".…”

Thermal modelling of the Larderello geothermal field (Tuscany, Italy)

“…The K 2 horizon, instead, is a deeper and a more regional reflecting interval, compatible with a lithologic change. However, the shape and depth of this structure, as well as the associated micro-earthquakes hypocenters down to 8 km depth (Vanorio et al 2004), suggest that the K 2 horizon might be a reasonable image of the brittle-ductile transition.…”

Three-dimensional tomography and rock properties of the Larderello-Travale geothermal area, Italy

“…From the initial data set, and as in Vanorio et al (2004), we selected earthquakes with at least 10 5 phases read and characterized by weight ≤2 and ≤3 for the P-wave and S-wave arrival times, 6…”

“…The best initial S-wave velocity model was derived from the 3-D tomographic P-wave velocity 10 model (Vanorio et al, 2004) by choosing an appropriate Vp/Vs ratio, as described below. show that the velocity anomalies with wavelengths of the order of 2 km are reasonably well 5 recovered in the central part of the volume investigated, up to 7-8 km.…”

“…19 Epicentre locations are spread over the whole of this exploited region, even though clusters are 20 seen in particular areas; the hypocentre depths are seen mainly down to 10 km in depth ( Figure 21 3). We have analyzed the same dataset that was used by Vanorio et al (2004) to retrieve the 3-D 22 P-wave velocity structure of the Larderello-Travale field. The data consist of 500 23 microearthquakes that were recorded between January 1994 and September 2000, providing 24…”