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 (<. 20. 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 (>. 8. 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.