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

We investigate the variability of Brune stress drop (∆σ) and apparent stress (τ 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. Inter-
event variability of stress drop and apparent stress results in a factor of ten, well
beyond the individual-event uncertainty. Radiation efficiency η sw = τ a /∆σ varies mostly
between 0.1 and 0.2 but, in the days immediately before and after the main shock, η 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 η 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 ∆σ result in a good correlation
with the largest and smallest b-values, respectively, imaged by other authors in the
rupture nucleation volume.

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