A seismotectonic picture of the inner southern Western Alps based on the analysis of anomalously deep earthquakes

The anomalously deep seismicity beneath the Western Po Plain is here analyzed to shed light on the complex and
still poorly understood tectonic configuration of the internal side of the Western Alps area. The original dataset,
including 590 earthquakes deeper than 20 km recorded during the last 25 years, has been accurately relocated
with HypoDD using both catalog and cross-correlation differential times. We found that the distribution of seismic
events faithfully mirrors the presence of two distinct tectonic domains (axial belt domains 1 and 2), originally
belonging to the Paleogene Alpine wedge and now anomalously juxtaposed beneath the sedimentary infill of the
Western Po Plain. Shallow, low-magnitude earthquakes (b20 km depth) are concentrated in domain 1, and are
possibly triggered by the isostatic reequilibration of the accretionary wedge. Earthquakes between 25 and 75 km
depth, instead, define a NNW–SSE linear cluster along the boundary between domains 1 and 2, and mark an
apparent plane steeply dipping to the ENE. We propose that this plane may represent a major tectonic boundary
of Neogene age, here referred to as the Rivoli–Marene deep fault. Focal mechanisms along the Rivoli–Marene
deep fault are invariably transpressional, and suggest ongoing left-lateral motion in agreement with available
plate motion constraints. The normal throw inferred from surface geology data (N8 km), and accommodated
in correspondence of the fault, could be linked to its Neogene activity, coeval with the northward translation
of the retreating Adriatic slab