GROUND MOTION POLARIZATION IN FAULT ZONES : RELATION WITH BRITTLE DEFORMATION FIELDS

San Francisco (CA)

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.

Boness, N. L. & M. D. Zoback (2004). Stress-induced seismic velocity anisotropy and physical properties in the SAFOD Pilot Hole in Parkfield, CA. Geophys. Res. Lett., 3
1, L15S17, doi:10.1029/2003GL019020.
Caine, J.S., Evans, J. P., Forster, C.B. (1996). Fault zone architecture and permeability structure. Geology, 24,1025-1028
Cucci, L., Pondrelli, S., Freopoli, A., Mariucci, M.T., Moro, M. (2004). Local pattern of stress field and seismogenic sources in Meandro Pergola basin and in Agri valley (Southern Italy). Geophys. J. Int., 156, 575583
Di Giulio, G., Cara, F., Rovelli, A., Lombardo, G., Rigano, R. (2009). Evidences for strong directional resonances in intensely deformed zones of the Pernicana fault, Mount Etna, Italy. J.Geophys. Res., 114, doi:10.1029/2009JB006393.
Hobbs, B.E., Means, W.D., Williams, P.P. (1976). An Outline of Structural Geology, Wiley, New York, N.Y, p. 571.
Jurkevics A. (1988). Polarization analysis of three component array data, Bull. Seismol. Soc.Am., 78, 1725-1743.
Lewis, M. & Ben Zion, Y. (2010). Diversity of fault zone damage and trapping structures in the Parkfield section of the San Andreas Fault from comprehensive analysis of
near fault seismograms. Geophys. J. Int., doi: 10.1111/j.1365-246X.2010.04816.x.
Mandl, G. (2000). Faulting in Brittle Rocks; an Introduction to the Mechanics of Tectonic Faults, Springer, Berlin, Germany 434pp.
Pastori, M., Piccinini, D., Margheriti, L., Improta, L., Valoroso, L., Chiaraluce, L., Chiarabba, C. (2009). Stress aligned cracks in the upper crust of the Val d'Agri region as revealed by shear wave splitting. J. Geophys. Res., 179, 601614
Pastori, M. (2011). Crustal fracturing field and presence of fluid as revealed by seismic anisotropy: case histories from seismogenic areas in the Appennines. PhD Thesis,
University of Perugia (Italy).
Riedel, W. (1929). Zur mechanik geologischer Brucherscheinungen. Zentralblatt Mineral Geol Palaont B:354368.
Spudich, P & Xu, L. (2003). Documentation of software package ISOSYN: Isochrone integration programs for earthquake ground motion calculations, CD accompanying
IASPEI Handbook of Earthquake and Engineering Seismology, 72 pp.
Valoroso, L., Improta, L.,Chiaraluce, L.,Di Stefano, R., Ferranti, L.,Govoni, A., Chiarabba, C. (2009). Active faults and induced seismicity in the Val d'Agri area (Southern Apennines, Italy). Geophys. J. Int., 178(1), 488502.