The onshore Pilarcitos Fault continues on a trend of about N. 50º W. into the offshore basement and may merge with the San Gregorio Fault. This fault is now an inactive fault (Burns, Cooper, Carlson, McCulloch pg 110).
The Westline Drive Study area is located along the Pacific coastline in a seismically active region of California that is astride the San Andreas Fault Zone. The active trace of the San Andreas Fault Zone is trending to the northwest through the interaction of Westline and Longview Drives, and most of the residences are within the Alquist-Priolo fault-rupture hazard zone established along the San Andreas Fault. The San Andreas Fault represents a significant geologic hazard that could produce a violent ground shaking in the area and result in fault ground rupture (Jacobs Associates 1998).