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[c7, p203]

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 ( Fig. 7.9). 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 (see chap. 9). 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.