Field measurements of azimuthal anisotropy; first 60 meters, San Francisco Bay area, CA, and estimation of the horizontal stresses' ratio from V s1 /V s2

Field measurements taken at two different sites during 1980-1986 revealed a shear-wave azimuthal anisotropy of at least 5 percent in the surficial weathered zone. Simple field techniques were used to determine the in-situ elastic and anelastic properties (V P , V S , Q) of near-surface strata. The azimuthal anisotropy revealed in these field experiments is the result of a combination of circumstances: unequal horizontal stresses (the data were collected near the center of the San Andreas and associated fault systems), fabric anisotropy introduced by the depositional agent, and stress-aligned fluid-filled microcracks, cracks, or pore spaces.