Trying to discriminate between natural and anthropogenic earthquakes recorded at Mount Amiata Volcano (Italy)

Abstract

The increasing population and the society's increasing energy demand implicates that production plants move noticeably closer to the inhabited areas. Dense seismic networks and arrays reveal that anthropogenic activities as e.g. extraction and reinjection of fluids - can induce or trigger seismic activity. This becomes significant in geothermal areas where the hypocenters of the natural seismicity are often at a similar shallow depth level as the human operations. In Italy the geothermal area of Mt. Amiata volcano is characterized by a high geothermal gradient, extensive fracturing and high porosity values, and can therefore be classified as a Hydrothermal System (HS). Geothermal exploitation at Mt. Amiata started in 1959 by drilling relatively shallow wells (<1000m) for the extraction of vapor and reinjection of cold water, using the natural fracture system of the rock volume for the circulation of the fluids. Due to a general pressure decrease inside the reservoirs during the 80s' the exploitation depths was deepened into a second geothermal reservoir down to a level of 2500 - 3500 m. Compared to the normal seismicity rate in the Apennines, the number of earthquakes observed in the Tuscan geothermal fields is very low. For the last 30 years less than 300 seismic events have been reported at Mt. Amiata inside an area with a radius of 20 km around the power plant. The high heat flow and the consequential ductile behavior of the upper crust may be the reasons for this low seismic activity. The strongest earthquake ever recorded at a geothermal field in Italy was the M3.9 event of Apr 01, 2000, located at 4 km depth beneath the small village of Piancastagnaio. The shallow hypocenter, close to the geothermal production depth, and the WNW-ESE normal fault mechanism are coherent with the seismotectonics of the area. It becomes therefore a big challenge to discriminate in the Tuscan geothermal areas between natural and anthropically triggered seismicity.