- Top of page
- 1. Introduction
- 2. Near-Surface Ultrasonic P-Wave Velocity and Physical Property Estimates
- 3. Upscaling in Frequency and Space
- 4. Conclusions
- Supporting Information
 Understanding fault architecture at multiple scales is crucial to delineate in situ fault zone physical properties and rupture dynamics through modeling and geophysical imaging/monitoring. An exposure of the active large-offset, strike-slip San Gregorio Fault at Moss Beach, CA provides a unique field site to relate the well-mapped fault zone architecture with compressional wave velocity (Vp) structure measured at centimeter to meter scales. Laboratory ultrasonic velocities of fault zone samples, adjusted for fluid-related frequency and structural dispersion, indicate that (i) a seismic velocity reduction of ∼30% characterizes the central smectite-rich clay gouge relative to the rocks 100 m away in the relatively undeformed host rocks, and (ii) the across-fault velocity profile trends for the seismic to ultrasonic bandwidth correlate almost exactly to the previously mapped macroscale fault zone structure. These results highlight the value of conducting multiscaled investigations when measuring fault zone properties defined by physical elements at multiple scale lengths.