Loading Rate Variations Along a Midcrustal Shear Zone Preceding the Mw6.0 Earthquake of 24 August 2016 in Central Italy

To identify greater detail in the seismicity pattern preceding the 24 August 2016 Mw 6.0 earthquake in Central Italy, we apply waveform matching using 1,028 events as templates. In the 8 monthsbefore the mainshock, we find ~2,000 additional earthquakes mostly located along a subhorizontal shearzone (SZ) bounding at depth the extensional fault system. Asynchrony is observed in the occurrence ofevents nucleating along the SZ compared to the ones on fault portions embedded in the shallower uppercrust, with the former anticipating the latter. Within the SZ, we also observe along-strike seismic migrationepisodes with earthquakes pointing toward the Mw 6.0 mainshock nucleation zone. These episodes arefollowed by an apparent quiescence within the main fault area. We suggest that the variations in the seismicactivity along the SZ represent the brittle signature of the tectonic loading process enabling portions of theoverlaying normal faults to become unlocked.