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 M_d = 0.1 and M_L = 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.

中文翻译：

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意大利中部Torre Alfina地热区的微震分析

Torre Alfina的地热场位于意大利中部，位于伏尔西尼火山区的北部边缘，即所谓的第四纪罗马共岩浆省的最北端。在中等焓地热开发项目的框架内，INGV在未来的地热生产基地附近安装了一个本地地震网络，以监测当地的自然地震活动。在本文中，我们显示了2014年6月至2016年5月在约10 km ²的小区域内记录的微震研究结果。Torre Alfina附近。通过分析由INGV国家地震网络的10个短周期站点和4个永久站点组成的本地临时网络记录的地震信号，我们检测到846次本地地震。然后，我们使用HypoDD代码准确地重新定位了799个事件。我们的结果表明，Torre Alfina地区的特征是强烈的微震性，震中深度在3至7 km之间，中等幅度在M _d  = 0.1至M _L之间。 = 2.8。此外，超过一半的地震被分为六个主要的类群，每个组持续数天。此外，我们计算了36个约束良好的断层平面解，这些解在整个研究区域内均显示出清晰的张应变变形形式。震源机制分析证实了三个主要的构造方向：EW，WSW-ENE和NW-SE。对Torre Alfina地热田的地震成因构造背景的了解，及其背景自然地震活动的研究，对于认识到由该油田的开采而引起的任何未来可能的地震活动，可能具有重要意义。