Study of Ground Motion Polarization in Fault Zones: a Relation with Brittle Deformation Fields ?

Several recent studies indicate that ambient noise and seismic signals in fault zones tend to be polarized on the horizontal plane with a clear preferred orientation direction. Here we present a summary of past experiments as well as new study cases showing evidence of this effect: the Val d’Agri, the Pernicana and the Paganica faults in Italy, and the Hayward fault in California. We also analyze data recorded by the HRSN network at the Parkfield section of the San Andreas fault and find that stations MM and GH that are close to the fault damage zone show a similar persistent and marked polarization effect. The approach combines the H/V technique in the frequency domain with the covariance matrix diagonalization method in the time domain. Common features are: i) a high stability of results at each site, independently of the nature and location of the source of seismic signals, ii) a characteristic polarization for each fault, and iii) the preferred polarization is close to the fault-normal direction, rather than being fault parallel as would be expected for generation of fault zone trapped waves. In previous papers, the role of fluid-filled microcracks in the damage zone was hypothesized. We have then explored an hypothesis based on the fracture field orientation in the fault damage zone by applying the package FRAP3 (Salvini, 2002) to model the brittle deformation field expected for the studied faults. We have found a consistent orthogonal relation between the observed polarizations and the orientation of the predicted synthetic fracture systems. When anisotropy studies are available, the horizontal ground motion polarization is consistently found to be perpendicular to the fast wave splitting component. The results may reflect reduced stiffness in the fault-normal direction produced by the presence of damage fault zone rocks.