GEOMETRY AND SEISMOGENIC BEHAVIOR OF THE MONTELLO BLIND THRUST (SOUTHEASTERN ALPS, ITALY) UNVEILED BY A HIGH-RESOLUTION SEISMIC NETWORK

Microseismicity pattern analysis is one of the most effective tools to unveil the existence of fault planes, mainly when active faults don’t outcrop or don’t have a clear surface expression.

The Montello thrust is a blind structure that belongs to the outer front of Southeastern Alps (Italy). Geodetic, geomorphological and geophysical data reveal its tectonic activity, but the seismic behavior (i.e., seismogenic or creeping fault) is still doubtful, as no strong earthquakes can be associated to it. The Montello thrust hanging-wall anticline hosts a gas storage reservoir monitored by a high-resolution seismic network able to detect possible induced seismicity besides to tectonic events.

In this work we analyze more than 1600 microearthquakes (98 % with M_L ≤ 2) detected by the network in the first 6 years of monitoring (2012-2017), and located with a 1D velocity model estimated on purpose for the Montello area. We plot this background seismicity on cross-sections cutting the Montello thrust along its dip and strike. Clear alignments of earthquake hypocenters depict a distinct N-NW dipping surface that cuts two sub-vertical transversal planes. This foci arrangement is geometrically compatible with the Montello thrust surface and two pre-existing high-angles faults rooted in the basement. Furthermore, this peculiar seismicity pattern strongly suggests a creeping behavior at seismogenic depth for the Montello thrust, with local re-orientation of stress-axes supported by focal mechanisms. At the same time, we cannot exclude that some parts of the plane not microseismically active are locked and under stress loading condition.

Last but not least point is about the tectonic origin of the recorded seismicity, mainly supported by the lack of space-time correlation with the gas storage activities.