Earthquake relocation and three-dimensional V p and V p/V s models along the low angle Alto Tiberina Fault (Central Italy): evidence for fluid overpressure

Abstract

The active extension in the central portion of the northern Apennines is believed to be accommodated by an ENE-dipping low-angle normal fault cutting the upper crust, called Alto Tiberina Fault (ATF). The physical properties and mechanics of such a severely misoriented fault are still unknown and debated. In this study, we constrain the deep geometry of the ATF and investigate the material properties of the fault system by using microseismicity recorded by a dense local seismic network. The inversion of local earthquake P- and S-wave arrival times yields high resolution Vp and Vp/Vs images of the fault system. We find positive Vp anomalies that define the eastward verging thrust-and-fold belt developed in the sedimentary cover, which inner bulk consists of Triassic evaporites, remains of the older compression. Microseismicity reveals the 20°NE-dipping ATF, nearly planar from 3 to 14 km depth. No sharp velocity contrasts are observed across the fault, but only a gentle warping of the velocity anomalies and the dislocation of the Triassic evaporites top at shallow depth. On the fault hangingwall, normal Vp and strong high Vp/Vs indicate the presence of a fractured high fluid pressure volume, within which steep normal fault segments are located. This evidence suggests that fluid pressure plays a dominant role in triggering the shallow background seismicity. Along the ATF, low Vp/Vs anomalies are observed, suggesting a different level of cracking and pore saturation and that the fault acts as impermeable barrier. We speculate that the microseismic activity on the ATF is favoured by the entrapped and locally overpressured CO2 testified by the low Vp/Vs volumes.