Crust movement in California

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Precisely how much additional relative plate motion is accommodated across other faults in California is uncertain, although the amount is probably very little. According to Minster and Jordan (1987), very long baseline interferometric (VLBI) surveying results indicate that oblique extension of the Basin and Range province, directly east of California, is occurring at a rate of 10�2 mm/yr with an orientation of N. 56� �10� W. Depending on the exact rate and orientation of this extension, as well as on the precise direction of relative Pacific-North American plate motion, the residual "missing" plate motion being accommodated in California on faults other than those of the San Andreas system ranges from negligibly small to possibly as much as 10 mm/yr. Thus, although the geodetic coverage in California is far from complete (see figs. 7.2, 7.5), all or most of the zone of significant plate-boundary deformation apparently has been encompassed.

DETAILED DISPLACEMENT-RATE PATTERNS
Considerable detail on the distribution of deformation in the San Andreas boundary zone is provided by the rather complete geodetic coverage available in the San Francisco Bay region and southern California. In the method used to reduce these data, geodetic-line-length changes are used to determine station-displacement rates relative to a point at the center of gravity of the network. Fault-normal displacements are permitted by this method, but their values are minimized in the inversion process (see Prescott, 1981). Gross departures from this constraint would be revealed by notable disagreements between observed and predicted line-length changes, but no such discrepancies were found for the results presented here. Because the fault-normal displacement rates are small and show no consistent trends, they are not plotted on the profiles presented here.

The distribution of deformation varies considerably across the San Andreas boundary zone from north to south of the San Francisco Bay. In the north bay, the integrated right-lateral-displacement rate across the network of 27+/-3 mm/yr. indicates that not all of the boundary zone has been captured within its 110-km aperture. Within about 5 km of the San Andreas fault, rapid change in the gradient of deformation rate indicates that interearthquake strain is concentrated close to the fault. Outside this near-fault region, deformation southwest of the fault appears to be negligible. Northeast of the fault, however, the persistence of significant movements right to the edge of the profile suggests that the 5- to 10-mm/yr deficit in boundary-zone deformation across this profile is being accommodated to the east of the Green Valley fault. Across the central and south bay, movements are more evenly distributed through the network, and the integrated displacement rate of 37�3 mm/yr across the south bay suggests that the entire boundary zone has been spanned. Closer examination of the profile, however, reveals several zones of locally high deformation gradient, one across the San Andreas fault, where it resembles that observed near the fault in the north bay. In addition, rapid changes in the profile across the Hayward and Calaveras faults reflect aseismic slip at rates of 3 to 6 mm/yr on these faults.

In south California deformation across the San Andreas boundary zone notably broadens from the Salton Sea, in the south, northwestward to the Big Bend region of the San Andreas fault north of Los Angeles. At the south end of the Salton Sea, all of the boundary-zone deformation, 35�1 mm/yr, occurs within an area about 50 km wide (profile S)that rapidly broadens to more than 100 km wide north and west of the Salton Sea (profile N) , possibly, broader still by about 50 km farther northwest. North and west of Los Angeles, networks of 100-km aperture capture only 18+/-2 mm/yr of the total right-lateral-displacement rate. In contrast with the northern section of the San Andreas fault, deformation gradients across the fault are smoother, and deformation is not so closely concentrated near the fault.

MECHANICS OF DEFORMATION

The observations described in the previous section point to a range of mechanical behavior for the faults comprised by the San Andreas system, from freely sliding with only minor accompanying seismicity, to completely locked from the surface to seismogenic depths except for abrupt slip during infrequent great earthquakes.
On the 160-km-Iong central section of the San Andreas fault, virtually all fault slip occurs aseismic ally. Slip rates measured at or near the fault are close to the average rate for the entire San Andreas boundary zone, no strain is detectable in the crustal blocks adjacent to the fault, and historical earthquakes of M>5Ѕ have not occurred. Abundant minor seismicity contributes only negligibly to the slip budget, and except for a few small patches of fault surface that are in frictional contact between these small earthquakes, the first-order steady-state model for this segment involves rigid translation of adjacent fault blocks across the freely sliding plane of the San Andreas.
A transitional behavior applies to those fault segments where steady-state fault creep is observed at the surface but historical or prehistoric earthquakes of M>6 have been documented. Examples include the Parkfield and Coachella Valley segments of the San Andreas fault, the Hayward fault, and the Imperial fault. On these segments, during the interseismic phase of the earthquake cycle, the fault is inferred to be freely slipping in its upper few kilometers, in locked frictional contact at seismogenic depths (approx 3-10 km), and once again freely slipping at greater depths. The result of this slip distribution is interseismic creep at the surface fault trace and elastic-strain accumulation in the adjacent blocks.

SUMMARY
Contemporary crustal movements in California are concentrated within a plate-boundary-deformation zone that is typically 50 to 200 km wide, approximately centered on the San Andreas fault. Integrated right-lateral displacement rates across this zone range from 33 to 37 mm/yr, representing about 75 percent of the Pacific-North American relative plate motion. Most or all of the rest may be taken up east of the San Andreas fault system in the Basin and Range province. Although aseismic fault slip (creep) is a locally important component of this relative plate motion, most of the geodetically measured deformation represents elastic strain on the crustal blocks adjacent to faults of the San Andreas system. Rates of secular (interseismic) shear strain are a maximum on the two currently locked segments of the San Andreas fault, sites of the great 1857 and 1906 earthquakes. Values range from 0.4 to 0.6 parts per million per year (ppm/yr) at the fault to 0.1 ppm/yr 30 to 80 km from it. Deformation occurring at the times of large strike-slip earthquakes (coseismic strain) is concentrated within a few tens of kilometers of the surface fault rupture, indicating that earthquake fault slip is largely confined to the upper 10 to 15 km of the crust. After major events, postseismic shear strain occurs at transiently high rates (more than 2 ppm/yr) that decay to background interseismic rates over a time scale of years to tens of years.
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. Observations are fitted equally well by two contrasting models. In the first model, the depth of coseismic faulting is much less than the thickness of the elastically strong lithosphere, and postseismic and interseismic deformation result from transient and steady aseismic slip on the downward extension of the earthquake fault plane. At the other extreme, if earthquake slippage extends through most or all of the elastic lithosphere, interearthquake deformation is due to transient or steady flow in the underlying weak substrate ("asthenosphere").
FUTURE DIRECTIONS FOR RESEARCH
Although the broad outlines of current movement across the San Andreas boundary zone are now known and the main features of the cyclic deformation expected from great strike-slip earthquakes have been delineated, many issues still remain to be explored. Although all of the relative Pacific-North American plate motion occurring across California may have been measured geodetically, this determination is not yet definitive, and as much as 10 mm/yr of motion may be accommodated east or west of the approximately 100-km-wide zone defined by current measurements. Furthermore, the thickness of the elastically strong crust is uncertain by at least a factor of 3, and so major alternative models of the earthquake deformation cycle cannot be distinguished. Because surface-deformation observations cannot themselves resolve this ambiguity, other data, possibly gravity-field observations and lithospheric-deflection models (for example, McNutt, 1980), are needed.
Few details exist on the preseismic and postseismic movements related to large plate-boundary earthquakes. Whether detectably anomalous crustal movements precede large earthquakes is uncertain. Theoretical models and fragmentary observations suggest that precursory slip may occur on or beneath the eventual coseismic rupture plane. However, except for the observation that premonitory deformations must be small relative to coseismic movements (for example, Johnston and others, 1987), precursory slip is otherwise unconstrained. Existing data are sufficient to demonstrate that postseismic movements, at least those from great earthquakes, are large-commonly, 10 to 30 percent of the coseismic deformation (Thatcher, 1984)-but the time scale and spatial distribution of these motions are not well determined at strike-slip plate boundaries. Laboratory experiments on lower-crustal rock types suggest that their ductile behavior is not approximated well by linear viscoelasticity, as assumed in the thin-lithosphere model, but postseismic observations are not yet sufficiently detailed to confirm this expectation.
Furthermore, vertical crustal movements in California are not well understood. Though not dominant in California''s largely strike-slip-faulting environment, vertical movements can nonetheless be locally important in such regions as the Los Angeles and Ventura Basins, the Transverse Ranges of southern California, and the Cape Mendocino area of northern California. Current and future work that integrates geologic and geodetic information in these regions should begin to shed light on long-term, secular vertical-movement patterns and their origins.
Within complex, multistranded fault zones and, possibly, in simpler regions, permanent inelastic deformation of upper-crustal rocks may contribute significantly to the current movement pattern. For example, at subduction boundaries, geologic and geodetic observations indicate a substantial imbalance between cumulative interearthquake strain and coseismic strain release, commonly reflected in long-term uplift or subsidence of coastal and inland regions. However, at such predominantly transcurrent boundaries as the San Andreas, the observable effects of inelastic strain are more subtle. The thermal consequences of such deformation may be the most direct evidence for inelastic strain . However, for California at least, the available data are either contradictory or ambiguous, and the extent to which measured interearthquake movements release elastically stored strain is currently unresolved.

P.S. The terms "lithosphere" and "asthenosphere" are linked to mechanical properties of the Earth''s crust and upper mantle: The lithosphere is the strong elastic layer near the Earth''s surface, and the asthenosphere is the region of ductile flow that lies beneath. Their boundary may thus lie well above the thermal boundary layer that separates the moving plates from the convecting mantle. If so, then at least the upper part of the "asthenosphere" forms part of the tectonic plate and moves with it.
Thus considered, the boundary between "lithosphere" and "asthenosphere" defines the zone of decoupling between surface tectonic processes and those that occur in the ductile region beneath. The location of this boundary is thus of central importance to the broad-scale tectonics of the San Andreas fault, the nature of the earthquake-generation process and its thermomechanical implications (see chap. 9), and the relation between shallow structural features and those inferred at depth (see chap. 8). I explore below the influence of this boundary location on cyclic earthquake-related deformation at the currently locked transform fault zones in the San Andreas, illustrating the contrasting mechanical behavior of the thick- and thin-lithosphere models.
All of the models considered here are two dimensional, and so neither slip nor mechanical properties vary along fault strike. Each model consists of only a single planar, vertical strike-slip fault. However, because the medium properties are linear elastic and (or) viscoelastic, the effects of multistranded fault zones can be obtained by simply superposing the deformation due to slip on individual fault segments. Furthermore, all of the two-dimensional models discussed here have also been considered in three dimensions, and so complexities arising from changes in fault strike, variations in slip along strike, and the finite extent of faulting can be incorporated straightforwardly as necessary. Similarly, except for the transition from elastic lithosphere to viscoelastic asthenosphere, depth variations in material properties are not considered, although, again, solutions have been obtained for faulting in plane-layered elastic and viscoelastic media. Although fault end effects and changes in slip and geometry along fault strike can be locally important, these effects, as well as those due to depth-varying material properties, are generally second order relative to the deformation features described here. More important are the effects of the several subparallel strands that compose much of the San Andreas fault system along its two currently locked sections. In these sections, the interseismic deformation due to each major fault strand contributes significantly to the observed displacement pattern, and as a rule the effects of faults lying off the San Andreas proper cannot be safely ignored in matching models to data across the entire San Andreas boundary zone.

COAST AND TRANSVERSE RANGES
SOUTHERN SECTION OF THE SAN ANDREAS FAULT SYSTEM