Seismic anisotropy, induced by the crack distribution in fractured medium, is very sensitive to stress field changes. So far, anisotropy has been investigated through Shear-Wave Splitting measurements of local seismicity. In order to get rid of the erratic occurrence of these events, we propose to measure the effect of anisotropy on surface waves recovered from the non-diagonal terms of the cross-correlation tensor calculated from the ambient seismic noise. Noise cross-correlation technique is applied to continuous data recorded by the HRSN located around the San Andreas Fault (SAF), both in 2004, when the Parkfield earthquake occurred (28/09/04, Mw 6.0), and in 2005, with no significant seismic activity. Focusing on the noise-extracted surface wave, after removing seasonal variations of seismic noise sources, rotations of quasi-Rayleigh and quasi-Love wave polarization angles are recovered, which display significant jumps before and after the Parkfield earthquake at some receiver pairs, in particular with pairs involving one of the closest seismic stations to the SAF. We propose an interpretation in terms of changes of anisotropy due to crack distribution rotation. Our new approach might be implemented on a routine basis for the purpose of continuously monitoring stress changes in fractured zones, in oil and gas reservoirs.


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