The faults of San Mateo County are characterized by both strike-slip and dip-slip components of displacements. There are three major faults systems that display large right-lateral offsets: the San Andreas, the Pilarcitos, and the San Gregorio fault zones. These faults trend roughly N30W. Most of these fault systems include many strands in a broad zone. The San Andreas Fault was displaced up to several meters in San Mateo County during the 1906 earthquake. Currents estimates of total offset is about 35 km for the San Andreas fault zone in San Mateo County, 120 km for the Pilarcitos fault zone, and 155 km for the San Gregorio fault zone (USGS).
The 1906 trace of the fault trends about 48 degrees northwest through the Mussel Rock area. The San Andreas Fault is generally considered to be about a mile wide and the entire Mussel Rock landslide area probably lies within the fault zone (Burns, Cooper, Carlson, McCulloch pg 85).