USGS1984
An extensive resurvey of most of the first-order leveling network in southern California, known as the Southern California Releveling Program (SCRP), was carried out during the first 5 months of 1978. The primary scientific purpose of these measurements was to rapidly update the vertical control record throughout a recently uplifted region of southern California in order to more thoroughly document the vertical component of tectonic movement and to provide a reliable base for comparison with future levelings. Analyses of historic first-order leveling results have clearly demonstrated that a broad crustal upwarping, largely contained within a region consisting of the Transverse Ranges province and an area along the intervening section of the San Andreas fault system, had developed between about 1959 and 1974. Unfortunately, there is strong evidence that parts of the 1978 SCRP data are contaminated by the effects of intrasurvey tectonic deformation, limited surficial failures, and, less certainly, magnetically induced systematic error associated with the use of automatic levels. However, any distortions in leveling results caused by these or other factors are not so serious as to render the SCRP data useless. In fact, the bulk of these data can be accepted at face value, and most of the remaining data can be incorporated with some caution to augment the more reliable parts of the network. The evaluation of the 1978 leveling is based on a combination of circuit-misclosures, local timing of the field observations, analysis of profiles of apparent height changes derived from comparisons with previous levelings, and an analysis of the position and orientation of the various routes in relation to the regional structural grain and the gradients of differential vertical motion established by previous investigations. Comparisons of the 1978 SCRP results with the latest of the previous surveys along each route retained in the analysis show that all but about one-third of the uplift established by leveling data from 1959 to the combined 1974/76 survey period had relaxed by early 1978 through tectonic subsidence. Subsequent limited relevelings along several of the 1978 routes show that rapid tectonic subsidence probably continued through at least early 1979. Despite the pronounced down-to-the-northeast (northeastward) tilt that developed between 1976 and early 1978, the overall shape of the uplift was well preserved. Results of repeated trilateration surveys since 1971 demonstrate that a remarkably uniform and nearly monotonic negative dilatational strain-change trend reversed abruptly between 1977 and 1979. The change from tectonic up to tectonic subsidence is associated with this reversal in horizontal strain accumulation. The reversal in strain trend was expressed as a cessation of the essentially uniaxial north-south contraction, which had been accumulating at about 2? 10 -7/yr (or 0.2 ?strain/yr), accompanied by onset of uniaxial east-west extension at about 5 ? 10 -7/yr (or 0.5 ?strain/yr). Minor earthquakes and occasional swarms of moderate earthquakes were particularly abundant during 1978 and 1979 in conspicuously mobile regions such as the eastern Transverse Ranges and the Salton Trough, areas respectively characterized by nearly complete collapse of the previous maximum uplift and by a dramatic enhancement of the previously identified tectonic subsidence.