Ground Motion Polarization in Fault Zones:Its Relation with Brittle Deformation Fields

Erice

Many recent studies indicate that ambient noise and seismic signals in fault zones tend to be polarized on the horizontal plane with a predominant orientation. Here we present a summary of past experiments as well as new study cases showing evidence of this 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 predominant polarization characteristic for each fault, and iii) polarization is not parallel to the fault strike as it would be expected for fault-trapped wave generation.
In previous papers, a role of fluid-filled microcracks in the damage zone was hypothesized. If this is true, a correlation is expected between seismic anisotropy and polarization. In the studied faults, when anisotropy results are available, the horizontal ground motion polarization is found to be perpendicular to the fast wave splitting component, confirming the role of fluid-filled microcracks in the damage zone. We have then checked this interpretation in terms of the fracture field orientation in the damage zone by applying the package FRAP3 (Salvini, 2002) to model the brittle deformation field expected in the damage zone of the studied faults. We have found a consistent orthogonal relation between the observed polarizations and the orientation of the predicted fracture systems.
The quick and relatively inexpensive character of the method encourages to further tests for an extensive application to many fields of theoretical and applied geophysics.