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The Alto Tiberina normal fault (ATF) in Central Italy is a 50 km long crustal structure that deepens at a low angle (15-20◦). Events on the fault plane are about ten times less than those located i...

We explored the detection capability of low-magnitude earthquakes before the 6 April 2009 Mw 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 Mw 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 km2 patch close to the main shock nucleation point, the cumulative number of earthquakes gradually increased until the Mw 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.

[1] Shallow earthquakes occurring in the southern part of the Tyrrhenian basin excite a long-duration low-frequency coda that propagates efficiently along paths crossing the Tyrrhenian Sea. Seismograms recorded at regional distances in the Italian peninsula are analysed in the frequency-time domain and compared with synthetics computed in simple vertically varying media. Based on results of our modeling, we interpret the coda as short-period (5–10 s) suboceanic Rayleigh waves. Their large amplitude and long duration, that we emphasize here, are explained as due to the influence of the Tyrrhenian water layer over a fast seafloor with little S-wave attenuation (QS > 300). A satisfactory fit of amplitude and duration of the observed waveforms is obtained assuming a young oceanic model with an average water column of 1 to 2 km and relatively high S-wave velocity (2.5 km/s) in the shallow crust.