Stress drop, apparent stress, and radiation efficiency of clustered earthquakes in the nucleation volume of the April 6, 2009, Mw 6.1 L'Aquila earthquake

We investigate the variability of Brune stress drop ( increment sigma) and apparent stress (tau(a)) of 23 earthquakes occurred in a small crustal volume adjacent to the hypocenter of the destructive M-w 6.1 L'Aquila earthquake. Their magnitude range is 2.7 <= M-w <= 4.1. Interevent variability of stress drop and apparent stress results in a factor of 10, well beyond the individual-event uncertainty. Radiation efficiency eta(sw) = tau(a)/ increment sigma varies mostly between 0.1 and 0.2, but in the days immediately before and after the main shock, eta(sw) tends to be smaller decreasing to values as low as 0.06. This may be the consequence of ruptures migrating in those days into a focal volume with higher dynamic strength. The temporal change of eta(sw) is tentatively interpreted as a spatial variation due to the earthquake migration into the locked portion of the fault that originated the main shock. Consistently, no variation in stress drop and apparent stress is observed between foreshocks and aftershocks but the smallest and largest increment sigma result in a good correlation with the largest and smallest b-values, respectively, imaged by other authors in the rupture nucleation volume.