Strike-Slip Earthquakes At The Northern Edge Of The Calabrian Arc Subduction Zone

We analyzed earthquakes of a swarm started in October 2019 in the Tyrrhenian Sea, at the northern border of the Calabrian arc subduction zone. The swarm is located in the same area where a subduction-transform edge propagator (STEP) shear-zone-oriented east-west is recognized from ocean floor morphology and submarine volcanoes. We computed focal mechanism, relative location, stress drop, corner frequency, and source directivity of the mainshock M-w 4.4 and of some aftershocks in the local magnitude range 2.3-3.7. Results indicate clearly that the mainshock occurred on a northwest-southeast-oriented fault, with right-lateral strike-slip motion, and it was characterized by a strong directivity of the rupture propagation from northwest to southeast. On the contrary, most of aftershocks were located on another strike-slip fault oriented northeast-southwest and had left-lateral kinematics. The kinematic features of these earthquakes indicate a strain field with the P-axis oriented north-south and the T-axis oriented east-west. Fault directions and stress field are in good agreement with the theoretical fracture model of shear zones associated with a STEP.