High pore pressure, or its absence, in the San Andreas Fault

High pore pressure in the San Andreas fault (California) was hypothesized to explain the prevailing weakness of the fault and may have major implications on the mechanics of earthquakes. However, no evidence of high pore pressure was found in the latest drilling into the San Andreas fault (SAFOD: San Andreas Fault Observatory at Depth) in central California (Zoback et al., 2010). If widely applicable, this result would impact our understanding of earthquake mechanisms on this and other active faults around the world. Here, however, I show that the available evidence from the latest SAFOD drilling may not be sufficient to reject the high pore-pressure hypothesis, and that definite knowledge of pore pressure in the fault zone may require long-term monitoring at the SAFOD site. The inference may also be useful for interpreting results from drilling projects on other active faults.