We explored the detection capability of low-magnitude earthquakes before the 6 April 2009 M w 6.3 L'Aquila event by using a matched filter technique and 512 foreshocks as templates. We analyzed continuous waveforms from 10 broadband seismic stations in a 60 km radius from the epicenter and for 3 months before the main shock. More than 3000 new events, mostly located on the main shock fault, were detected to define the spatial-temporal evolution of micro-seismicity. The foreshock sequence was active northwest of the M w 6.3 hypocenter in January, then migrated toward it at a speed of~0.5 km/day in middle of February. At that time, in a~4 km 2 patch close to the main shock nucleation point, the cumulative number of earthquakes gradually increased until the M w 6.3 event. This patch, characterized by a low b-value, played a key role in controlling the preparation stage to the 2009 L'Aquila main rupture.

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

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