GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 1.1 GNGTS 2023 Fault and fluid interaction during the 2012 Emilia seismic sequence R. Fonzetti 1, 2 , L. Valoroso 1 , P. De Gori 1 , C. Chiarabba 1 1 Istituto Nazionale di Geofisica e Vulcanologia (INGV), Rome (RM) 2 Università degli Studi Roma Tre , Rome (RM) Keywords: 2012 Emilia seismic sequence; double-difference tomography; hydraulic properties of fault zones. ABSTRACT The 2012 Emilia seismic sequence originated in a seismically active area beneath the Po alluvial plain. It started with a first mainshock on the 20 th of May with a M L = 5.9, followed by a M L 5.8 event on May 29 th activating two adjacent buried thrust segments of Ferrata and Mirandola faults (Figure 1). Different studies discussed the possible anthropogenic origin of the Emilia 2012 seismic sequence resulting in a general agreement about the absence of a significant alteration of stresses from the exploitation of the nearby Cavone oil field. Based on previous tomographic models, a high pore pressure pulse at the base of the carbonate multilayer has been invoked to explain the triggering of the delayed mainshock of May 29 th 2012 (Pezzo et al., 2018). Anyway, the uncertainty about the possible alteration before the first shock or triggering mechanism by fluids is still debated. In this study, we present high-resolution earthquake location and velocity models computed using absolute time, cross-correlation data and double-difference tomography (Schaff and Waldhauser, 2005; Waldhauser and Schaff, 2008; Zhang and Thurber, 2003). TomoDD code simultaneously computes earthquake relative locations and P- and S-wave velocity models. Aftershock relocations highlight a complex fault system. The Ferrara thrust is illuminated by seismicity between 3-15 km depth and a dip about 35°-40°. Instead, the Mirandola thrust is shallower (with seismicity located between 5-10 km depth) and less steep than the Ferrara thrust (about 25° - 30°). Furthermore, seismicity shows small-scale fault segments coalescing in the two thrusts. We observe a broad high Vp/Vs anomaly at seismogenic depth (about 6.0 km) which could be interpreted as a high-fluid pressure-saturated body that connects the two buried thrusts, justifying