Remotely triggered micro-earthquakes in the Larderello-Travale Geothermal Field (Italy) following the 2012 May 20, Mw 5.9 Po-plain earthquake

We report observations of remotely triggered earthquakes at the Larderello-Travale Geothermal Field (Italy), following the Mw=5.9 Po-Plain earthquake on 20 May 2012. Four distinct triggered events are recognized within a short (similar to 25s) time interval accompanying the sweeping of similar to 10s Rayleigh waves. Triggered sources are clustered at depths in between 4km and 6km. The magnitude and distance of the mainshock agree well with the triggering threshold previously proposed for The Geysers, California. For three out of four earthquakes, the Rayleigh wave dynamic stresses are mostly associated with extensional vertical (szz) and shear (sxz) components, which range up to 5KPa. Once considering the structural setting of the area, the most likely triggering mechanism involves the rupture of normal faults which are kept close to failure by high-pressure crustal fluids.