A sudden increase in the scale of seismicity has occurred as a long-term precursor to twelve major earthquakes in California and Northern Mexico. These include all earthquakes along the San Andreas system during 1960-2000 with magnitude M ?6.4. The full list is as follows: Colorado Delta, 1966, M 6.3; Borrego Mt., 1968, M 6.5; San Fernando, 1971, M 6.6; Brawley, 1979, M 6.4; Mexicali, 1980, M 6.1; Coalinga, 1983, M 6.7; Superstition Hills, 1987, M 6.6; Loma Prieta, 1989, M 7.0; Joshua Tree, 1992, M 6.1; Landers, 1992, M 7.3; Northridge, 1994, M 6.6; Hector Mine, 1999, M 7.1. Such a Precursory Scale Increase () was inferred from the modelling of long-term seismogenesis as a three-stage faulting process against a background of self-organised criticality. The location, onset-time and level of ? are predictive of the location, time and magnitude of the future earthquake. Precursory swarms, which occur widely in subduction regions, are a special form of ? ; the more general form is here shownto occur frequently in a region of continental transform. Other seismicity precursors, including quiescence and foreshocks, contribute to or modulate the increased seismicity that characterises ? . The area occupied by ? is small compared with those occupied by the seismicity precursors known as AMR, M8 and LURR. Further work is needed to formulate as a testable hypothesis, and to carry out the appropriate forecasting tests.