San Andreas Fault North of San Francisco, California
North of San Francisco the width of the San Andreas fault zone is difficult to determine. Failure to recognize slivers within this zone apparently led some earlier writers to false conclusions about stratigraphic relationships along parts of the fault trace. Previous authors9 statements that cumulative right-lateral displacement along the San Andreas fault measures hundreds of miles since Jurassic time are based chiefly on alleged stratigraphic relationships south of San Francisco. Attempts to relate amounts of displacement along the fault south of San Francisco to amounts north of San Francisco (and vice versa) may be invalidated by possible distribution of San Andreas movement along one or more other faults that may branch from or cross the San Andreas fault north of San Francisco. North of San Francisco the San Andreas fault was active before middle Pliocene time, but as yet the evidence seems insufficient to allow determination of either the type or amount of pre-middle Pliocene displacement. Present positions on opposite sides of the fault trace of areas that appear to have been marine entrances to middle Pliocene basins east of the fault trace suggest that right-lateral displacement along the San Andreas fault north of San Francisco has not exceeded 15 miles, more likely has amounted to 4 to 10 miles, and possibly has not exceeded 1 to l½ miles since middle Pliocene time. During the same time, vertical movements have raised the east side of the fault about 500 feet relative to the west side in some areas, possibly everywhere north of Bolinas.