The preparatory phase of the 2009 Mw 6.3 L'Aquila earthquake by improving the detection capability of low-magnitude foreshocks

We explored the detection capability of low-magnitude earthquakes before the 6 April 2009 M-w 6.3L'Aquila event by using a matched filter technique and 512 foreshocks as templates. We analyzed continuous waveforms from 10 broadband seismic stations in a 60km radius from the epicenter and for similar to 3months 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 similar to 0.5km/day in middle of February. At that time, in a similar to 4km(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 2009L'Aquila main rupture.