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Mud volcanoes are geological systems often characterized by elevated fluid pressures at depth deviating from hydrostatic conditions. This near-critical state makes mud volcanoes particularly sensitive to external forcing induced by natural or man-made perturbations. We used the Nirano mud volcanic field as a natural laboratory to test pre- and post-seismic effects generated by distant earthquakes. We first characterized the subsurface structure of the Nirano mud volcanic field with a geoelectrical study. Next, we deployed a broad-band seismic station in the area to understand the typical seismic signal generated by the mud volcano. Seismic records show a background noise below 2 s, sometimes interrupted by pulses of drumbeatlike high-frequency signals lasting from several minutes to hours. To date this is the first observation of drumbeat signal observed in mud volcanoes. In 2013 June we recorded a M4.7 earthquake, that occurred approximately 60 km far from our seismic station. According to empirical estimations the Nirano mud volcanic field should not have been affected by the M4.7 earthquake. Yet, before the seismic event we recorded an increasing amplitude of the signal in the 10-20 Hz frequency band. The signal emerged approximately two hours before the earthquake and lasted for about three hours. Our statistical analysis suggests the presence of a possible precursory signal about 10 min before the earthquake. [ABSTRACT FROM AUTHOR]

In northern Italy in 1997, two earthquakes of magnitudes 5.7 and 6 (separated by nine hours) marked the beginning of a sequence that lasted more than 30 days, with thousands of aftershocks including four additional events with magnitudes between 5 and 6. This normal-faulting sequence is not well explained with models of elastic stress transfer, particularly the persistence of hanging-wall seismicity that included two events with magnitudes greater than 5. Here we show that this sequence may have been driven by a fluid pressure pulse generated from the coseismic release of a known deep source of trapped high-pressure carbon dioxide (CO2). We find a strong correlation between the high-pressure front and the aftershock hypocentres over a two-week period, using precise hypocentre locations and a simple model of nonlinear diffusion. The triggering amplitude (10-20?MPa) of the pressure pulse overwhelms the typical (0.1-0.2?MPa) range from stress changes in the usual stress triggering models. We propose that aftershocks of large earthquakes in such geologic environments may be driven by the coseismic release of trapped, high-pressure fluids propagating through damaged zones created by the mainshock. This may provide a link between earthquakes, aftershocks, crust/mantle degassing and earthquake-triggered large-scale fluid flow. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]