A Double-Rupture Source Mw 5.0 Earthquake and Its Aftershock Sequence in Calabria, Italy

Abstract On 1 August 2024, a magnitude Mw 5.0 earthquake struck Calabria, Italy. Seismograms reveal a clear double-rupture source consisting of an initial mb 4.4 event followed 1.1 s later by a stronger rupture. The source kinematics computed from the P-wave first motion polarity for the first rupture and from waveform modeling for the entire event provide markedly different results. The first rupture exhibited normal-fault kinematics with a nearly vertical plane, whereas the low-frequency radiation of the whole earthquake is consistent with strike-slip motion. Nevertheless, both focal mechanisms share a common feature: one nodal plane is nearly vertical, oriented northeast–southwest, and dips to the southeast. The location of more than 400 aftershocks delineates a diffuse cloud of hypocenters over a broad seismogenic volume with a pronounced northeast–southwest elongation, coincident with the orientation of one nodal plane of the mainshock moment tensor solution. Spectral analysis of P waves indicates that the stronger rupture of the mainshock occurred on the northeast–southwest fault with right-lateral slip and a clear directivity effect. Focal mechanisms of 17 aftershocks in the magnitude range 2.0≤ML≤3.1 show highly variable solutions, even among events located close to one another, reflecting all kinematic styles. The mainshock was followed by fewer aftershocks than predicted by the Gutenberg and Richter law, distributed across a broad volume that is probably highly fractured and affected by a complex stress field.