Preliminary examination of the historic geodetic record has disclosed crustal uplift of 0.15 to 0.25 meter that apparently began around 1960 and has since grown to include at least 12,000 square kilometers of southern California. This uplift extends at least 150 kilometers west-northwestward along the San Andreas Fault from Cajon to Maricopa, southward from the San Andreas into the northern Transverse Ranges, and eastward from Lebec into and including much of western Mojave block. It seems to have grown spasmodically eastward from a center near the junction of the San Andreas and Garlock faults and has occurred largely within an area that has remained virtually aseismic since at least 1932. Although much of this area has been characterized by crustal mobility since at least the turn the century, the described uplift seems to be an unusually large and probably unique event superimposed the existing pattern of continuing deformation.
Comparison of preearthquake vertical geodetic surveys with postearthquake surveys in the region affected by the magnitude 5.7 Oroville earthquake of August 1, 1975, indicates appreciable elevation changes. The data are consistent with 0.36 m of normal slip on a 12×10 km2 rectangular fault dipping 60° to the west. Comparison of level surveys run 1 and 6 months after the earthquake suggests an additional 0.08 m of postseismic slip on the fault. Horizontal geodetic data do not show any significant changes, but the horizontal data are noisier than the vertical data and allow as much as 0.7 m of coseismic left lateral slip within a 95% confidence limit. There is no evidence for postearthquake dilatancy recovery, but the data are not adequate to exclude small amounts (10 mm) of relaxation, or even larger amounts if the dilatant region were very broad (80 km in diameter).
Postearthquake changes in elevation across the Tujunga segment of the San Fernando fault in the period March 1971 to 1973-1974 indicate deformation similar in distribution to, but on a much smaller scale than, the coseismic deformation (the maximum postearthquake uplift is about 60 mm compared to the 2 m of coseismic uplift). The postearthquake elevation change just east of the 1971 rupture is a nearly uniform increase in elevation to the north that reaches about 60 mm at the end of the profile. The postearthquake elevation change across the Sylmar segment of the San Fernando fault is more subdued and apparently in the opposite sense from the coseismic deformation; it may be due to local compaction. The horizontal deformation observed in the period August 1971 to March 1973 across a geodimeter network that spans the San Fernando fault is minor (displacement not more than 10 mm). If the explanation of the observed vertical deformation across the Tujunga segment of the fault is indeed afterslip, the deformation must have occurred chiefly in the period March to August 1971.
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