Active faults, probable active faults, and associated fracture zones, San Mateo County, California

As used on this map, the term active fault is defined as a fracture or fracture zone several miles long along which the two sides either are moving relative to one another and parallel to the fracture or are judged likely to move in such fashion within the useful life of existing man-made structures.  The judgment that a fault is active may be based on: 1) historic records of surface faulting and earthquake activity, 2) topographic evidence that demonstrates repeated and systematic displacements along the fault trace, 3) systematic displacement of Holocene or Pleistocene geologic units along the fault trace, 4) a linear pattern of small-earthquake activity either along the surface trace of the fault or parallel to it and within 2 miles of the surface trace, or 5) current and measurable systematic displacement across the surface trace of the fault.  Most active faults shown on the map are recognized as active on the basis of two or more of these criteria.