Abstract Southern California straddles the boundary between the North American and the Pacific plates. The relative motion between these two plates has been determined from paleomagnetic lineations in the Gulf of California, from global solutions to known slip rates along plate boundaries, from geology, and from geodesy (Minster and Jordan, 1978; Minster and Jordan, 1978; DeMets and others, 1987) to be primarily horizontal at a rate of about 48 mm/yr (DeMets and others, 1987). This results in one of the highest levels of seismicity in the conterminous United States (e.g., Evernden, 1970). In southern California, the deformation is spread over a large area, encompassing numerous normal, strike-slip, and reverse faults. A majority of the plate motion appears to be accommodated by the San Andreas fault, with the rest distributed among the dozen or so other major faults (Weldon and Humphreys, 1986). This is in contrast to the plate boundary in northern California, where the plate motion is more concentrated near the San Andreas fault than it is in southern California (e.g., Hill and others, this volume). The diffuse deformational pattern leads to the high level of seismic activity and to a complicated tectonic structure. On a broad scale, the North American-Pacific plate boundary in California is a transform fault that extends from the Gulf of California to Cape Mendocino (Fig. 1). The San Andreas fault and the transform plate boundary end at the Mendocino Triple Junction in northernmost California. North of Cape Mendocino, the spreading center and subduction zone of the