Presenter: Ruei-Jiun Hung
Date: 2016/12/08
Abstract
The study of seismic anisotropy has been widely-discussed in recent decades. The mechanism of anisotropy can be categorized into two groups: One is the elastic properties of a material due to the natural crystal lattices and/or mineral alignments, and the other is the micro-cracks, fractures, or very local heterogeneity compare to a wavelength. Analyzing the anisotropy will enable us to understand, most important of all, the possible local stress states. For example, vertical aligned micro-cracks are usually generated in shallow crust because the maximum stress is usually in horizontal direction, and thus, it will cause anisotropy. To diagnose the anisotropy inside the earth, severa seismological methods are considered, one of these methods is by observing the shear wave splitting. Studies of shear wave splitting are usually focus on two properties, one is the delay time between two distinct shear wave arrivals, the other is the polarization of the fast shear wave. The former describes the degree of anisotropy, and the later focus on the possible direction of fractures, or mineral alignments (usually the polarization of the fast shear wave will parallel to the direction of weak plane) Here, I would like to focus my topic in the crustal cases. I show a paper presenting the fault-zone anisotropy investigation after a major earthquake in California (Mw7.0 Loma Prieta earthquake, 1989). The polarization of the fast shear waves can be divided into three groups, which are parallel to the fault trace (perpendicular to maximum stress), parallel to maximum stress, and complex patterns. To our TCDP anisotropy investigation, shear waves from local earthquakes reveal that most of the fast shear waves with polarization of NW-SE directions are consistent with the convergent direction of Philippine Sea Plate. The other, especially near BHS4 and BHS5 has ENE directions. This may be the possible anisotropy associated with cracks or fractures near the fault zone. Further investigations are necessary to complete our observations in the near future.
Reference
Crampin S., 1985, Evaluation of anisotropy by shear-wave splitting. Geophys., Vol. 50, No.1, 142-152.
Zhang, Z., S. Y. Schwartz, 1994, Seismic anisotropy in the shallow crust of the Loma Prieta segment of the San Andreas Fault system. J. Geophys. Res., Vol. 99, No. B5, 9651-9661.