Analysis of Microseismicity Recorded in the Geothermal Area of Mt. Amiata (Italy)

Abstract

Mt. Amiata is an extinct volcano whose last eruptive activity was dated about 200ky ago. Today, its underlying crustal volume is still characterized by a high geothermal gradient, which makes the area particularly suitable for geothermal exploitation. The structure overlaying the former magmatic reservoir is characterized by permeable layers of highly fractured volcanic rocks, saturated with hot water and steam. Geothermal exploitation from these layers started in the 1960's. Since then, earthquakes close to the depth of the geothermal production level at about 3.500 m depth occurred, shallower than the typical seismicity in the upper Tuscany crust. However, because of the sparse permanent monitoring network at Mt. Amiata, depth resolution and magnitude of completeness (Mc) have been poor. To improve the seismic monitoring inside the geothermal field of Mt. Amiata, we installed in 2015 for a 3-years recording period a dedicated 8-station seismic network in the vicinity of the productive geothermal power plants. The aim of the experiment was achieve smaller completeness magnitude Mc and to reduce depth uncertainty, in order to better understand the seismicity and to be able to discriminate between natural and anthropogenic events. We scan the large dataset using an efficient waveform beam-forming grid search approach (LASSIE) for robust detection and rough locations. In a second processing step associated earthquakes are re-located by a refined waveform-based locator (LOKI). Obtaining an improved seismic catalog with reduced Mc, we suggest to approach the discrimination problem by spatial-temporal correlation of seismicity with geotechnical time-series of geothermal production.