Northwest of the Transverse Ranges, and reaching into the area near the Mendocino triple junction, are several low but rugged ranges interspersed with long intermontane valleys. The region constitutes the Coast Ranges. They break off sharply both at the shoreline on the west and with the topographically flat Great Valley on the east. Throughout the mountain belt, the ranges are uplifted and deformed as shown by warped young terraces and surfaces (Buchanan-Banks et al., 1978; Page, 1981). The Coast Ranges are also constructed of ancient rocks that have been involved in a complicated history through geologic time. For example, many of the older strata were formed at a convergent tectonic boundary between lithospheric plates coming in against the continent from Pacific regions. Many faults in such areas have moved again and again, and some may now be active, although it is difficult to document the timing of the most recent displacements owing to the paucity of young offset strata.
The stretch of the San Andreas Fault through the central Coast Ranges is straight and constitutes the near-surface boundary between the lithospheric plates. Activity along it is shown in two ways. First, it is the locus of continuous slow creep, and, second, from time to time noticeable earthquakes take place upon it, such as the Parkfield earthquake of 1966 (Brown et al., 1967). The reach of the fault subject to creep and frequent small earthquakes extends on northwestward from near the town of Parkfield into the San Francisco Bay area. This is the part of the fault system that is under close watch instrumentally by the U.S. Geological Survey. Seismographs and instruments of other types are monitoring the behavior of the walls on either side of the fault in a search for premonitory changes in strain, tilt, magnetism, electrical field, the depth and character of small earthquakes, and the gas contents and water levels in wells (Raleigh et al., 1982). When a major earthquake occurs along this segment of the San Andreas, geoscientists hope to have gathered many kinds of information pertinent to understanding when, where, and why the earthquake happened.
The Coalinga earthquake of May 1983 took place in the central California Coast Ranges, with its epicenter near their margin with the Great Valley and about 30 km from the San Andreas Fault (Eaton et al., 1983; Namson et al., 1983; Wentworth et al., 1983). Although the tectonic setting of the earthquake is still under study, it appears that relatively rigid crustal layers are deforming differently from those at depth. The occurrence of the earthquake at distance from the San Andreas Fault attests to the mobility of a broad belt near the lithospheric plate boundary and also that geologists have much to learn concerning its dynamics.
To the south of Parkfield the fault last ruptured during the Fort Tejon earthquake of 1857, one of the strongest earthquakes in California recorded history (Sieh, 1978a,b). This earthquake displaced ground features about 30 feet right laterally along the fault from the southernmost Coast Ranges, through the Big Bend region of the San Andreas Fault and the central Tranverse Ranges, to the vicinity of the city of San Bernardino.
Within the San Francisco Bay region elongate crustal blocks are separated by several major faults of the San Andreas transform system, such as the San Gregorio, San Andreas (proper), Hayward, and Calaveras (Crowell, 1976; Page, 1981, 1982). These faults are neither exactly parallel to each other nor to the direction of movement between the Pacific and North American lithospheric plates. Where the faults diverge or converge in map view, blocks between them may be either squeezed or stretched. Where they are stretched, the terrain sags to form valleys and sedimentary basins, and where squeezed, terrain rises to make mountains. San Francisco Bay itself may well owe its origin to the sagging of the block between the San Andreas and Hayward Faults so that ocean waters from the Pacific are able to flood eastward upon the continent. In this region, terrains between major faults are undergoing deformation as shown by out-of-place old-erosional surfaces—some have been much uplifted and dissected; others, such as marine terraces, are now depressed far below the levels where they were formed (Atwater et al., 1977). Soft mélanges of the ancient Franciscan Complex stand high and are prone to severe landsliding. The region as a whole is undergoing deformation, but deformation is even more concentrated along the major fault zones (Brabb and Hanna, 1981; Prescott et al., 1981).
North of San Francisco Bay several terrains between discontinuous active faults are recognized (Herd, 1978; Fox, 1983). Shear stress between the two lithospheric plates is apparently spread across a region running through broad lowlands near the city of Santa Rosa. The belt of displacements includes the San Andreas Fault on the west and extends well to the east into the high Coast Ranges, roughly on line northwestward from the active Calaveras and Concord Faults. This region is one where the active plate boundary is broad and diffuse and where crustal slices within it also show “porpoise structure.” Geologic mapping with an emphasis on documenting recent deformation is hampered in the northern California Coast Ranges by widespread dense forests and brush, steep terrain subject to landsliding, and the paucity of datable young rocks.