Near-Field Stress and Pore Pressure Observations Along The Carrizo Plain Segment of the San Andreas Fault in California

David A. Castillo and Stephen H. Hickman

Preliminary observations of wellbore breakouts from 9 wells drilled to depths approaching 5 km and located within 3-10 km of the San Andreas fault in the Carrizo Plain area indicate maximum principal stress orientations (SHmax) 30-40° from the fault trend, consistent with high shear stress resolved unto the fault. Analysis of stress orientation data from additiona wells located >1O km from the fault confirm previous observations that SHmax stresses are at high angles to the fault trend, consistent with low shear stress on the San Andreas. We suggest that the overall variation in SHmax direction with distance from the San Andreas fault indicates tha shear stresses resolved onto the fault may be depth dependent, with greater shear stress at shallower depths. Alternatively, these stress rotations observed In the vicinity of the San Andreas might also reflect the influence of local secondary faulting and folding, variations in lithology and/or slip heterogeneties associated with the 1857 M8+ Fort Tejon earthquake.

Estimates of crustal pore pressure inferred from drilling mud-weights and drill-stem tests from wells in the vicinity (<10 km) of the San Andreas fault indicate near-hydrostatic conditions to depths of about 5 km. However, 20-30 km from the San Andreas fault and within the central portions of the southern San Joaquin Valley, crustal pore pressures approach 60% of the lithostatic load starting at about 3.5 km depth. Thus, our data close to the fault suggests that elevated fluid pressures within the fault zone, as proposed to explain the long-term low-strength of the San Andreas, either do not penetrate far into the adjacent crust and/or are confined largely to deeper portions of the fault zone.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California