Shear wave properties and seismic imaging of Mylonite Zones

Abstract

Major crustal fault zones have displayed high reflectivity on observed compressional reflection profiles and synthetic models of laboratory measurements. This high compressional wave reflectivity suggests that shear waves may also produce similar reflectivity. The shear wave properties and reflectivity of mylonites from the Brevard fault zone have been determined using measured shear velocities as a function of confining pressure. The measured shear wave velocities are strongly influenced by mineralogy, especially quartz and oriented micas. The strong oriented fabric of Brevard mylonites creates seismic anisotropy, which, for nonnormal incident propagation produces significant shear wave splitting. A wide range of Vp/Vs ratios within the Brevard fault zone originates from a complex combination of composition and anisotropy, making lithology determination difficult. The shear wave reflectivity of the Brevard fault zone was derived from one‐dimensional normal incidence modeling of detailed geologic fault zone models. The shear wave reflectivity of the Brevard fault zone, which originates from fine compositional layering, is moderate and can be directly correlated with individual geologic formations. Within the fault zone variable shear wave reflectivity, both horizontally and vertically, originates from geologic heterogeneity. The modeled shear wave reflectivity also depends on frequency. Frequencies below 20 Hz will resolve boundary structure, while higher frequencies image detailed geologic features within the fault zone.