Microseismicity analysis in the geothermal area of Torre Alfina, Central Italy

The geothermal field of Torre Alfina is located in central Italy at the northern edge of the Vulsini Volcanic District, the northernmost area of the so-called Quaternary Roman Co-Magmatic Province. In the framework of a medium-enthalpy geothermal exploitation project, INGV installed a local seismic network close to the future geothermal production site for monitoring natural local seismicity. In this paper, we show the results of a study of the microseismicity recorded from June 2014 to May 2016 in a small area of about 10 km2 around Torre Alfina. Analyzing seismic signals recorded by a local temporary network of ten short-period stations and by four permanent stations of the INGV national seismic network, we detected 846 local earthquakes. Then, we accurately relocated 799 events using HypoDD code. Our results show that the region of Torre Alfina is characterized by intense microseismicity, with hypocentral depths between 3 and 7 km and with moderate magnitudes between Md = 0.1 and ML = 2.8. Moreover, more than half of the earthquakes are grouped into six main swarm-like clusters each lasting few days. Furthermore, we computed 36 well-constrained fault plane solutions, which show a clear transtensional deformation regime in the whole study area. Three main tectonic directions have been evidenced from the focal mechanisms analysis: E-W, WSW-ENE, and NW-SE. The understanding of the seismogenic structural setting of the Torre Alfina geothermal field, and the study of its background natural seismicity can be of great importance in recognizing any possible future seismicity induced by the exploitation of the field.