Crustal movements and earthquake‐related deformation

The past four years have seen significant advances in delineating the current movement pattern at and near major plate boundaries, modeling the earthquake‐related deformation occurring there, and developing new high‐precision measurement techniques. At the same time, interpretation of both historic geodetic survey data and new precise measurements in the western U.S. has uncovered some unexpected results, emphasizing that despite notable progress in modeling observed movements, a number of significant features of plate boundary deformation are not yet well understood. The Southern California Uplift Undoubtedly the most notable development since 1975 has been the discovery by R. O. Castle and his colleagues (Castle et al., 1976) of a remarkable episode of ground uplift in southern California. Analysis of a very large collection of repeated leveling measurements has shown that between 1959 and 1974 approximately 90,000 km 2 of southern California was uplifted by 200 mm or more. In contrast to these rapid movements, earlier measurements indicate negligible aseismic vertical deformation between at least 1926 and 1959. A very similar though not precisely identical episode of uplift occurring sometime between 1897 and 1914 is also documented by the leveling observations (Castle et al., 1976). This earlier uplift episode included much of the region that sustained rapid upwarping after 1959, but also encompassed most of the Peninsular Ranges, an area not significantly elevated in the most recent phase of uplift (Wood and Elliott, 1979). In the case of both deformation events, uplift of as much as 400 mm was followed by a regional partial collapse that averaged near 50% of the initial upwarping. Some observations of contemporaneous horizontal deformation are available for the 1959–74 uplift and its subsequent partial collapse; triangulation data indicate some anomalous changes in the rate and orientation of shear strains during the uplift (Thatcher, 1976), and recent geodimeter measurements (1974–78) show a very uniform pattern of nearly uniaxial north‐south compression of ∼0.2 microstrain/yr (Savage et al., 1978) that coincided with the subsequent downwarping. Because of uncertainties concerning the precise pattern of interseismic horizontal deformation related to the San Andreas fault system (and possible temporal fluctuations in this strain field), it is not yet possible to unambiguously distinguish these interseismic strains from those which may be related to the uplift or its collapse, and as a result the proper interpretation of both the triangulation and geodimeter data is correspondingly uncertain. Similarly, although several interpretations have been proposed concerning the origin and geophysical significance of the southern California uplifts (Thatcher, 1976; Wyss, 1977; Rundle, 1978a), no one explanation has yet been universally accepted.