Control of the 2009 L'Aquila earthquake, central Italy, by a high-velocity structure: A receiver function study

Receiver functions (RFs) analyzed at two permanent broadband seismic stations operating in the epicentral area of the M-w 6.3, 2009 L'Aquila earthquake (central Italy) yield insight on crustal structure along the fault rupture. The harmonic decomposition of RFs highlights a subsurface structure in which both isotropic and anisotropic features are present. We model the waveforms using recently developed Monte Carlo methods. The retrieved models display a common depth structure, between 10 and 40 km depth, consistent with the under-thrusting of the Adria lithosphere underneath the Apennines belt. Along the fault, in the uppermost crust, the S wave velocity structure is laterally heterogeneous. Right above the hypocenter, we find a 4-6 km thick, very high S wave velocity body (V-s as high as 4.2 km/s) that is absent in the SE portion of the fault, where the earthquake propagated. The high-V-s body is coincident with the area of fewer aftershocks and is anticorrelated with the maximum slip patches of the earthquake, as modeled by differential interferometric synthetic aperture radar (DInSAR) and strong motion data. We interpret this high-V-s body as a high-strength barrier responsible for the high peak ground motion in the near field, observed in the L'Aquila city and surroundings, and for the complexity in the rupture evolution. The retrieved seismic S wave velocity of this body far exceeds common V-s values in the upper crust and it is more compatible with values observed in mafic basement rocks.