Stress field assessment from ill-defined fault plane solutions: an example from the Larderello geothermal field (western Tuscany, Italy)

A new approach is presented for the definition of the present-day stress field from the analysis of seismic fault plane solutions. In particular, possible principal stress axes are detected by taking into account, in the framework of a fully distribution-free and non-parametric approach, the effect of uncertainty affecting experimental fault plane solutions. The approach has been applied to a data-set comprising 190 small earthquakes that occurred from 1994 to 2000 in the Travale zone, located in the eastern part of the Larderello geothermal field. Results indicate that, despite the apparent heterogeneity of available fault plane solutions, 85% of them are compatible with a single stress field affecting the whole study area. When a partition of the study area into three subdomains is considered, 93% of events in each subdomain are compatible with the relevant stress field. The stress regime affecting the most active subdomain, close to an Andersonian one, has subhorizontal maximum and minimum stress axes trending E-W and N-S, respectively. This configuration is compatible with the kinematics of a strike-slip structure recognized in the study area by recent tectonic studies.