Scaling of Source Spectra of the April 2009 L’Aquila, Italy Earthquakes

We use EGF technique to study scaling of source spectra of the destructive L’Aquila earthquake of April 6, 2009 (Mw6.3) and two of larger aftershocks (April 7, Mw5.6; April 9, Mw5.4) within the framework of omega-square model. The first large aftershock, which occurred SE of the mainshock, is deeper (15 km) than other events (around 10 km) in the sequence and gave rise to very aftershocks. For each of these three earthquakes we searched for co-located small aftershocks with similar focal mechanism. We find that the mainshock, the Mw5.4 aftershock, and smaller aftershocks of these two events scale as omega-square model with Brune stress drop increasing from ~0.1 to 4 MPa for Mw from 3.0 to 6.3. However, the source spectrum of the deeper, large aftershock (Mw5.6) seems to deviate from the omega-square model, showing a flattening in 0.1-1.0 Hz range. The median radiated energy, Es, of the mainshock, using the EGF technique, is 1.24x10^14 J, which yields Es/Mo=3.6x10^-5 and an apparent stress drop, τ, of 1 MPa. The corresponding parameters for the deeper, Mw5.6 aftershock are: Es=6.9x10^12 J, Es/Mo=2.7x10^-5, τ=0.9 MPa. We note that the spectral shape and the source parameters of the deeper, large aftershock may have been affected by inappropriate selection of EGF. This possibility is presently being carefully explored.