Constraints on seismic anisotropy in ductile rock fabric and application to imaging fault roots in southern California

Vera Schulte-Pelkum, Karl Mueller, Sarah J. Brownlee, Thorsten W. Becker, & Kevin H. Mahan

Published August 14, 2017, SCEC Contribution #7652, 2017 SCEC Annual Meeting Poster #225

Understanding the roots of faults in the deep, ductile crust constrains and validates rheological models of deep shear and informs fault modeling of strain transfer across the base of the seismogenic crust. We show passive imaging results for deep crustal anisotropy in Southern California. We interpret the results using a new set of laboratory anisotropy measurements for ~100 rock samples, with a majority from mid- to lower crustal depths and from shear zones. Hexagonal symmetry is a good approximation for P-to-S receiver function conversions arising from contrasts in anisotropy. Gneisses and schists always show slow-axis hexagonal symmetry, while amphibolites can have either a fast or slow symmetry axis. In contrast to a common assumption, elliptical symmetry is a poor approximation for crustal anisotropy, and deviations from ellipticity increase with the anisotropy amplitude. For receiver functions, the elliptical assumption leads to overestimation of the strength of anisotropy. We propose three scaling relationships for gneisses and schists, amphibolites, and quartzites that relate the degree of non-ellipticity to the strength of anisotropy, and suggest to use those in forward and inverse modeling.

We use our findings to interpret observed anisotropic receiver function conversions near faults in southern and central California. The amplitude of receiver function conversions from anisotropic contrasts scales linearly with the strength of anisotropy, and changes with ray incidence angle relative to foliation dip angle, for fixed strength of anisotropy. Dipping foliation results in conversions with higher amplitude and a degree-1 polarity pattern in backazimuth, compared to smaller conversions with degree-2 azimuthal patterns in the case of subhorizontal foliation. In southern California, the dipping foliation signal dominates over that from subhorizontal foliation, and the strike of the dipping foliation most often parallels nearby surface fault traces, even those of transform faults that are subvertical near the surface. Our results suggest that the listric geometry recently observed at the bottom of the seismogenic zone for some transform faults based on seismicity may be expressed in similarly dipping fabric in the ductile domain. We also compare our results with surface geology to discuss the possible influence of older, pre-San Andreas fabric on observed anisotropy.

Key Words
community rheological model, rheology, receiver functions, anisotropy, shear zones, faults

Citation
Schulte-Pelkum, V., Mueller, K., Brownlee, S. J., Becker, T. W., & Mahan, K. H. (2017, 08). Constraints on seismic anisotropy in ductile rock fabric and application to imaging fault roots in southern California . Poster Presentation at 2017 SCEC Annual Meeting.

Related Projects & Working Groups
Stress and Deformation Over Time (SDOT)