Content Previous Next

INTRODUCTION
[c7, p189-190]

Contemporary crustal movements in California are concentrated within a plate-boundary deformation zone that is typically 50 to 200 km wide, centered approximately on the San Andreas fault. Observations of coseismic, postseismic, and interseismic movements define the earthquake deformation cycle and constrain models of strain accumulation and release for strike-slip plate boundaries.

Crustal movements measured in California today sample deformation processes that have continued through at least the past 5 Ma of Pliocene, Pleistocene, and Holocene time. During this interval, several hundred kilometers of right-lateral offset has accumulated across the San Andreas fault system, and many thousands of great earthquakes similar to the historical events of 1857 and 1906 have undoubtedly occurred. The observed deformatIon results from relative right-lateral translation of the Pacific and North American plates far from the main plate-boundary faults, which are either freely slipping and without major seismic activity, or are in locked frictional contact and slip episodically in repeated great earthquakes. Aseismic fault slip (creep), as occurs across the San Andreas fault in central California ( Figure 7.1), causes no crustal deformation beyond a growing offset across the fault, although this offset may be distributed across a zone as broad as a few tens or hundreds of meters. Where the plate-boundary fault is alternately locked aseismically in its upper 10 km or so and abruptly slipping in great earthquakes, deformation extends several tens of kilometers into the plate interiors. Between large events, elastic strains build up in this zone and are episodically released every few hundred years. Subsequent postearthquake recovery processes redistribute the strains aseismically for years to decades after a major shock, and this deformation gradually merges into the steady accumulation of elastic-strain energy that persists until the frictional strength of the fault is again exceeded. This sequence of interearthquake strain accumulation, coseismic strain release, and postseismic readjustment is thus a recurring process, here referred to inclusively as the earthquake deformation cycle.

Several fault-zone features result in measurable deformation spread over an extremely broad plate-boundary zone. This deformation occurs where the San Andreas fault system comprises several subparallel splays, as in both the San Francisco Bay region and southern California. There, both aseismic slip and major strike-slip earthquakes have been documented, some on the same fault strand, and the entire zone of plate-boundary deformation is exceptionally broad. Major changes in fault strike also play a role in redistributing plate-boundary deformation and diffusing it over a wider zone. Compressional bends introduce uplift, crustal thickening, and subsidiary reverse faulting, such as in the Transverse Ranges of southern California. Extensional bends are characterized by subsidence, basin filling, and, possibly, volcanism, as occurs in the Imperial Valley. Extensional and compressional features are more localized at the smaller-scale discontinuities and changes in fault strike that occur throughout the San Andreas system.

Crustal movements observed at the surface reflect deformation processes occurring at depth in the lithosphere. Both laboratory rock-mechanics experiments and studies of exhumed fault zones define the nature of these processes, which, in turn, constrain the classes of large-scale faulting models consistent with surface measurements. In the cool and brittle seismically active parts of the crust, elastic processes are dominant, the frictional strength of active faults increases linearly with depth, and faulting is controlled by Coulomb failure. The transition from brittle seismic behavior to ductile aseismic deformation occurs in the midcrust. Although it is generally agreed that this transition occurs as a result of increasing temperature, its precise mechanism is uncertain. If deformation in the midcrust is concentrated within a narrow vertical shear zone lying beneath the seismically active fault plane, then the brittle/ductile transition may reflect either the increasing importance of ductile or cataclastic flow at depth (Sibson, 1982) or a thermally controlled transition from unstable to stable frictional sliding (Brace and Byerlee, 1966; Tse and Rice, 1986). However, if ductile deformation is broadly distributed in the midcrust, then the cyclic buildup and relief of stresses in the brittle seismogenic crust is controlled by the stress transfer between the elastic lithosphere and ductile "asthenosphere" and the flow properties of the latter.

Both the steady, aseismic movements within the San Andreas plate-boundary zone and the coseismic strain release in large earthquakes are well within the range of detectability of repeated geodetic-survey measurements. The purpose of this chapter is to summarize the salient features of these observations, demonstrating the constraints they place on the amount of present-day plate motion occurring across the San Andreas plate-boundary zone and showing how measurements shed light on the mechanics of the cycle of strain accumulation and release. The emphasis is necessarily on movements close to the main strands of the San Andreas fault system, where observations are most numerous, although some networks extend as far as 100 km from the major faults. The measurements include triangulation, repeated observations of the angular separation of permanent survey markers, for which useful data date back to about 1850, when gold was first discovered in California; trilateration, repeated line-length measurements made by laser ranging since about 1970; and local measurements of aseismic fault slip made periodically or recorded continuously over apertures of about 10 to 100 m since about 1960.