Seismic evidence of an early afterslip during the 2012 sequence in Emilia (Italy)

We apply a matched-filter-technique to augment the detected events in the time window between the two mainshocks of the 2012 Emilia seismic sequence (Italy), 20 (Mw6.1) and 29 (Mw6.0) May. Using well-located templates, we increase the number of detections from 1,727 to 7,616. This greater detail allows evidencing migrations of seismic events from the nucleation point of the first shock to the second. Repeating earthquakes are also found between the two mainshocks. The seismicity pattern suggests a transient slip acting immediately after the shallow first event, weakening and loading the volume around the deep nucleation point of the 29 May Mw6.0 earthquake. We interpret migrations and repeaters as the fingerprint of an early afterslip triggered by the first mainshock. By taking into account the earliest repeater we estimate a cumulative slip of approximately 27 cm (Mw5.7). This value could represent the lower bound of the total released afterslip.