Seismotectonics at the Transition Between Opposite‐Dipping Slabs (Western Alpine Region)

Abstract

We analyze a fully reprocessed data set of ~9,000 seismic events recorded in the western Alpine region during the past 30 yr, in order to understand how convergence between Africa and Eurasia is presently accommodated at the transition between the opposite‐dipping Alpine and Apenninic slabs. We confirm that seismicity in the Internal Zone of the Western Alps is clustered along two different arcs (Briançonnais and Piedmont arcs), clearly outlined by events in the 0–12 km depth range. The Piedmont Arc is best outlined by events in the 12–30 km depth range, forming a narrow belt that matches the shape and location of the Ivrea gravity anomaly. In the Internal Zone, σ3, is oblique to the orogen trend. Although the mountain range is spreading gravitationally at a shallow level, spreading occurs intermittently with other earthquakes that are more directly related to plate interactions. Strike‐slip solutions are predominant for events of magnitude Ml > 4, and reverse solutions are dominant along the Piedmont Arc for events of magnitude Ml < 4. Nodal planes have dominant NNW‐SSE and ENE‐WSW orientations that are common to major faults mapped in the study area. Integration with available tectonic and geodynamic constraints indicates that lithology distribution in the subduction wedge, orientation of major faults within and outside the subduction zone, and the exhumation of mantle rocks at shallow depth concurrently determine a complex seismotectonic scenario that may be expected in other subduction zones worldwide.