Is the San Andreas Fracture a bayonet-shaped fracture as inferred from the acoustic body waves in the SAFOD Pilot hole ?

Abstract

The method using the propagation of acoustic body waves within the stress modified areas around a vertical borehole has been applied to the granitic formation penetrated by the SAFOD Pilot hole near the San Andreas Fault trace. This method allows us investigating the horizontal in situ stresses. Only P waves supplied useful and surprising information. A depth of 1270 m separates an upper region of uniform thickness of stress modified areas, possibly corresponding to a shear domain, and a lower region where there are simultaneously two values of the thicknesses of the stress modified areas (particularly between 1500 and 1600 m of depth) possibly corresponding to a compressive and a shear domain. In order to integrate the contradictory effects of the simultaneity of shear and compressive domains at some depths, as well as the presence of three shear zones at particular depths, we propose that the San Andreas Fault could be bayonet-shaped instead of planar. Other recent available information in the literature about this fault, such as the presence of a fault zone of low shear wave velocity, stress rotation measured with depth, and the large angles of the frictional coefficients, can be logically explained by this kind of fault geometry