Space-Time Variations in the Asperity of the Mw 6.1 L'Aquila Mainshock Inferred From Fault-Zone Trapped Waves and Stress Drops of Foreshocks.

A long sequence of foreshocks preceded the destructive Mw 6.1, 6 April 2009 L'Aquila earthquake. On 30 March, the strongest (Mw 4.1) of these foreshocks marked a spatial and temporal change in the seismicity rate and elastic properties of rock in the nucleation volume as testified by variations in the P-to-S wave velocity ratio and shear-wave velocity in the fault zone. In a previous study using fault-zone trapped waves (FZTWs) of repeating earthquakes, we showed a sudden jump of spectral amplitudes a ten hours after the occurrence of the Mw 4.1 foreshock. The amplitude increase was accompanied by a loss of waveform coherence in FZTWs. The concomitance of a consistent change in amplitude with the loss of coherence led us to interpret our observation as due to a sudden temporal variation of the velocity contrast between the fault damage zone and the hosting rocks in the focal volume, depending on physical processes occurring in the preparatory phase. The waveforms of usable repeating earthquakes of the previous study are now used for a stress release analysis by adding 13 events located in a tight area inside the mainshock seismic source. Although estimates of Brune stress drop can suffer huge uncertainties, the use of the same analysis procedure on a closely spaced earthquake cluster reduces significantly the estimate fluctuations. High and low stress drop values in the diffusivity plot show that only low stress drop values fall on the migration front of the diffusivity equation. These results are consistent with an interpretation of the stress drop decrease due to fluid diffusion.