Non-double-couple microearthquakes in the geothermal field of Larderello, central Italy

Abstract Earthquakes with focal mechanisms deviating from a simple double-couple have been observed in volcanic and geothermal areas. In some cases these events exhibit radiation patterns characterised by prevalence of compressive fields over the focal sphere, which could result from sources containing an enhanced volumetric component. This study makes an attempt to comparing different source mechanism models fitting first onset amplitudes for a set of microearthquakes recorded by a dense local seismograph network deployed by ENEL, the Italian National Power Company, around the geothermal field of Larderello, central Italy. The purpose of this network was mainly that of monitoring eventual induced seismicity in connection with fluid injection routinely carried out in deep wells in order to waste hot water and refill the steam reservoir. Three source models have been tested: (a) simple double-couple (three parameters, after amplitude normalisation), (b) double-couple associated with tensile crack (four parameters), and (c) double-couple associated with compensated linear vector dipole (four parameters). The results, obtained making use of statistical criteria, show a significant component of non-double-couple and volumetric mechanism for a small number of events. The instability of the solutions for shallow earthquakes and the large standard deviation of amplitude residuals suggest the opportunity of refining the model used to describe the velocity and the Q-structure.