Principal component analysis of interseismic deformation in southern California

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Abstract
Eight trilateration networks along the San Andreas fault in southern California have been surveyed repeatedly (8 to 19 times) within an approximately 15‐year interval between 1971 and 1992. The data for each network were analyzed by principal component analysis to represent the observed changes in line length as a superposition of individual modes: Lij ‐ Li = ∑k Aik Ck (tj), where Lij is the length of the ith line measured at the time tj, Li is the average length of the ith line, and AikCk(tj) is the kth mode. Each mode is described by a common time function Ck(tj) that is evaluated only at the times tj of the surveys, and the participation of each line in that mode is described by a constant amplitude Aik for that line. For all networks the deformation is largely described by the first mode, and the first‐mode time function is essentially linear in time. That is, the deformation in each of the networks appears to be steady. Two anomalies in the accumulation of deformation were observed: In the network near Palmdale a transient strain event occurred in the second mode in early 1982, and in the network within Cajon Pass, fluctuations in the accumulation of deformation somewhat greater than would be attributed to measurement error were found. Although both anomalies are formally significant given the estimated measurement error, I am not confident that either anomaly is real. The principal conclusion is that the accumulation of interseismic deformation in the southern California trilateration networks is steady within the precision of measurement.