Directional resonance and wavefield polarization in the damage zone of the Campo Imperatore fault, central Italy.

Abstract

To infer the occurrence of directional amplification effects, we performed ambient noise measurements along a 200m transect crossing the Campo Imperatore fault zone (Central Italy), an exhumed analogue of the faults responsible of the L'Aquila 2009 earthquake sequence, We have recently found in several fault zones that ambient noise is not randomly polarized, but it is amplified on the horizontal plane along a specific site-dependent direction. The analysis repeated using earthquake signals revealed that S-coda waves and surface waves show the same polarization direction, independently of the earthquake backazimuth and focal mechanism. We have explained the observed directional amplifications in terms of fractured rocks in the fault damage zone, polarization being oriented orthogonally to fractures produced by the kinematic stress component. Therefore ground motion directional amplification could be related to the higher compliance of fractured rocks. In the other studies the fracture pattern was derived from numerical-analytical modeling based on the fault geometry and kinematics, or compared with the fast direction of shear wave obtained by seismic anisotropy analysis. The aim of this study is to compare observations with fracture measurements (strike, dip, dip-azimuth, spacing, later continuity, etc.) performed in the selected fault zone. We thus acquired ambient noise using 25 stations installed along a transect where detailed structural geological measurements were carried out. Ambient noise was recorded for around 1 hour, and was processed to compute the horizontal-to-vertical noise spectral ratio as a function of frequency and direction of motion. Wavefield polarization was investigated in the time–frequency domain as well. We found that, in spite of the complexity of the seismic data, the observed polarization pattern is generally oriented orthogonal to the measured dominant fracture system, confirming the existence of a high angle relation between ground motion polarization and fracture systems.