LAUSR

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 Mw 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.