Source Parameters and Radiation Efficiency in the nucleation volume of the Mw 6.1 L’Aquila Earthquake

Washington

We analyze Brune stress drop and apparent stress of 25 earthquakes, mainshock, foreshocks and
aftershocks, occurring in a 2.5 x 2.5 x 2.5 km 3 volume adjacent to the hypocenter of the destructive
Mw 6.1 L’Aquila earthquake. In a previous paper, we used a part of these earthquakes to detect a
transient anomaly of shear wave velocity in the fault zone one week before the main shock. The
purpose of this study is to enlarge the previous dataset with further close earthquakes (2.2 ≤ Mw ≤
4.1) to investigate if the same area concomitantly shows temporal and/or spatial variations in the
stress parameters. Brune stress drops were computed in a conventional EGF-deconvolution
approach using local seismological stations, the resulting values vary in the range 0.2 < Δσ < 8.4
MPa. Apparent stress was derived from the waveform energy corrected for propagation and site
effects, it varies in the range 0.03 <  a < 1.10 MPa. The ratio η sw =  a / Δσ, which provides the
radiation efficiency, for the largest part of the investigated earthquakes yields values close to 0.12,
which is a value lower than expected for self-similar ruptures. An exception was the immediately
following foreshocks and aftershocks (since a four hours before up to a four hours after the main
shock, respectively) that result in values of η sw as smaller as 0.06, consistent with ruptures in
seismogenic zones with high dynamic strength. The largest Δσ of these events indicates that the
temporal variation is associated to a spatial variation due to the main shock asperity rupture, and a
satisfactory consistency is found between the smallest and largest Δσ with the largest and smallest
b-values, respectively, imaged by other authors in the nucleation volume.