Passive monitoring of anisotropy change for the Parkfield 2004 earthquake

Measuring significant and systematic temporal variations of physical parameters is a major goal of seismologists for monitoring seismogenic zones. Seismic anisotropy is induced by the crack distribution within the continental crust, and it is very sensitive to stress-field changes. To date, anisotropy has been investigated through shear-wave splitting (SWS) measurements of local earthquakes. To avoid the erratic occurrence and spatial uncertainties of these events, we have measured the effects of anisotropy on surface waves recovered from the crosscorrelation tensor of the ambient seismic noise. We find that the polarization angles of the retrieved surface waves provide a very sensitive parameter for monitoring stress changes in seismogenic zones. We processed data continuously recorded by the High Resolution Seismic Network (HRSN) located around the San Andreas Fault (SAF), Southern California, USA. We focused on a two-year period, from 2004, when the Parkfield earthquake occurred (28 September, 2004; Mw 6.0), to 2005, with no significant seismic activity. We identified and separated out two main contributions from temporal changes of surface wave polarization: (1) slow and weak variations due to seasonal changes of seismic noise incident direction; and (2) strong and fast rotations of quasi-Rayleigh and quasi-Love wave polarization angles at the moment of the Parkfield event. After removing part (1), the strong polarization shift may be related to changes in crack properties induced by the co-seismic stress. More precisely, it is observed that the polarization directions began to rotate several months before the earthquake and even continue after it. Contrarily to the SWS, polarization of surface waves is more sensitive to crack distribution rotation than travel-times or phase velocity. We performed synthetic experiments for exploring how an anisotropic medium with a horizontal symmetry axis similar to a cracked medium around a strike-slip fault, affects the polarization of surface waves as a function of azimuth.