Furlong [1993] presented an elegant synthesis of how the northward migration of the Mendocino triple junction removed the Gorda plate from under North America, creating a ``mantle San Andreas transform'' which was to the northeast of the surficial margin; with time, surficial fault activity jumped inboard, as from the San Andreas to the Hayward-Calaveras fault system in the San Francisco Bay area. The complicating effects of a slightly convergent Pacific plate velocity since 7 Ma are being recognized; in particular, seismic studies show probable oceanic crust of the Pacific(?) plate underthrust along the California margin from Morro Bay north to San Francisco [ Page and Brocher, 1993].
The maturity of California neotectonic studies is shown by the fact that Bird and Kong [1994] were able to predict fault slip rates and geodesy to within 3 mm/year in a finite-element model; such models may serve as supplements to hard data in seismic hazard estimation.
In the area of hazard studies, it is being realized that few new (Pliocene-Quaternary) thrust faults in the Transverse Ranges actually break the surface; instead, anticlines involving Holocene sediments should be assumed to overlie seismically dangerous blind thrusts. Shaw and Suppe [1994] used reflection sections and balanced-section methods to infer three active thrusts under the Santa Barbara Channel, with slip rates that add up to 3 mm/year (or half the geodetically-determined rate of shortening). A Wilshire fault with a slip rate of 1.5-3.2 mm/year has also been proposed as the explanation for the Quaternary Wilshire arch under Los Angeles and Hollywood [ Hummon et al., 1994].