High-pressure fluid at hypocentral depths in the L'Aquila region inferred from earthquake focal mechanisms

We apply a new analysis technique using earthquake focal mechanisms to infer the 3-D fluid pressure field at depth in the source region of the A. D. 2009 L'Aquila earthquake/aftershock sequence. The technique, termed focal mechanism tomography, inverts for fluid pressure by examining the fault orientation relative to the regional tectonic stress pattern. We identify three large-scale pockets of high fluid pressure (up to 50 MPa above hydrostatic pressure) at depths of 7-10 km that strongly correlates with an independent data set of well-located foreshocks and aftershocks. The shape of overpressured regions and the evolution of seismicity indicate a plausible scenario that this sequence is being driven in part by the poro-elastic response of trapped reservoirs of high-pressure fluid, presumably CO2, and postseismic fluid flow initiated by the main shock.