IN SITU MEASUREMENTS OF SEISMIC VELOCITY ACROSS AN EXPOSED BRITTLE FAULT ZONE

The effects of lithology, fracturing, and gouge zone mineralization on the seismic properties of fault zones are not very well understood. Our goal is to relate in situ geophysical measurements to internal fault zone properties. Collaborative work has collected representative rock samples from the fault zone and will use laboratory measurements to derive seismic velocity, mineralogy and pore fluid information. We directly measure seismic velocity in situ. Field seismic measurements allow documentation of fracturing effects and produce a continuous profile. The active strike-slip San Gregorio fault at Moss Beach, California, is exposed in cross section on an uplifting and actively eroding wave-cut platform. The fault cuts shallow marine sediments which have been buried to a depth of ~1km. As the fault is unweathered and contains seawater, same as the original pore fluid, it is representative of subsurface faults. A 10-m clay-rich gouge zone is adjacent to the main trace of the fault. A 7-20 m wide brecciated zone juxtaposes the gouge zone on either side. Fracturing decreases away from the fault, resulting in a 100-m wide fault zone. A high-resolution seismic experiment was conducted on the outcrop. Multiple measurements with a dense 1-m shot and receiver spacing were made to record 50-250 Hz P-wave energy, statistically resolving the seismic velocity to a scale of a few meters. Our survey is analogous to a horizontal well log across the fault, and has produced a continuous velocity profile. Several crosslines, parallel to the strike of the fault, were acquired to measure velocity anisotropy in the major deformation zones of the fault. We present a 1-D P-wave velocity profile across the fault and anisotropy values, and relate the seismic properties to the major lithological and structural units. A 40-80 m wide low velocity zone, which had a 25-100% contrast in velocity with the country rock, was observed. Future work will use the ground roll (Rayleigh wave) to derive an S-wave velocity profile.