STRUCTURAL AND TEXTURAL CONTROLS ON FAULT ZONE VELOCITY SIGNATURES AT CENTIMETER LENGTH SCALES

Seismic refraction profiling, tomography, and guided wave studies show that faults exhibit a tabular zone of low seismic velocity relative to undamaged host rock. Laboratory and borehole studies indicate that open fractures, fault gouge and microcracks reduce seismic velocities. Low velocity zone (LVZ) width therefore may reflect areas of fault-related fracturing and alteration; however LVZ width differs from fault zone width defined by mesoscopic structures. A qualitative correlation between fault-related microcrack density and seismic velocity variation along a refraction survey across the Punchbowl fault, CA led Y. G. Li to hypothesize that microstructures control fault zone velocity distribution. Such variations in the distribution and density of structures define fault zone internal architecture. The Punchbowl fault has a well-characterized architecture consisting of a continuous ~0.3m wide ultracataclasite core surrounded by a ~30m wide damage zone dominated by fractures and small faults. Fault-related microcrack density defines a ~100m damage zone. Multi-scale velocity characterization is needed to provide the resolution necessary to refine interpretation of LVZs in terms of distribution and length scales of velocity-reducing structures within fault-zone architectural elements. This integrated structural and geophysical study begins to establish a detailed, structurally correlated velocity profile across the Punchbowl fault, CA at a range of length scales.

New data from the Punchbowl fault show brittle structures at a scale between previously recognized fractures and microcracks. Preliminary velocity data obtained from laboratory measurements of hand samples and cores suggest that features, such as fault-related calcite cementation and mesoscopic fracture fabrics, in addition to microcrack density control the fault zone velocity structure. These measurements provide structurally correlated velocity ‘snapshots’ for rocks across the Punchbowl fault, thus allowing velocity reduction produced by different length-scale structures to be determined. Distribution of fault-related cementation and mesoscopic fracture fabrics within the fault zone account for irregularities in the seismic velocity profile documented by Y. G. Li.