We appreciate the support of these organizations. Nils A. Johnson drafted the maps and figures. Edward Cranswick, U.S. Geological Survey, reviewed the manuscript. Jim Gardner, technical editor in Earth and Atmospheric Sciences, edited the manuscript. We thank these people for improving the manuscript, but we are responsible for remaining errors.
Rumpf and Meurisch (1981) conclude that any magnetic field including particularly that of the earth, can deflect the line of sight of compensator or automatic (self leveling) levels. Since the City of Los Angeles has observed many lines of geodetic leveling using a Zeiss Ni‐1 level, a field investigation was designed to assess the potential impact of this error source. The observations were made along a line that had been releveled several times over a period of years. This, generally, north‐south line runs from Tidal 8, San Pedro, to the vicinity of the Jet Propulsion Laboratory in LaCanada (Figure 1). An additional line of leveling was also observed in an east‐west direction. Our investigation seems to confirm the existence of an azimuth dependent error as suggested by Rumpf & Meurisch.
Analysis of the results of repeated levelings through the epicentral region of the M w 6.7, 1994 Northridge earthquake has disclosed the occurrence of differential uplift in this area that preceded the earthquake. Although the distribution of the relevant vertical-control data is somewhat sparse, in both space and time, those data that we have recovered indicate that this uplift exceeded 0.10 m and peaked 20-25 km west of the 1994 epicenter. While our data also indicate that this deformational event must have occurred during the period 1978-1989, evidence based on the character and magnitude of misclosures developed from 1987 and 1989 surveys argue that the deformation occurred largely during the period 1987-1989. The preseismic vertical-displacement field that preceded the Northridge earthquake is similar in form and, less certainly, magnitude to that which preceded the M w 6.7, 1971 San Fernando earthquake; other possible, but less significant southern California analogues include the deformational events that preceded the M w 5.3, 1973 Point Mugu and M w 5.9, 1987 Whittier Narrows earthquakes. The small but growing number of recognized aseismic deformational episodes that preceded small to moderate magnitude earthquakes in southern California suggests that the deployment of the dense array of continuously recording GPS receivers planned for southern California can be expected to detect and more accurately describe such events than was heretofore possible. Moreover, if the relation between the duration of these deformational anomalies and the magnitudes of any ensuing earthquakes can be much more clearly established, the near perfect temporal control on position afforded by GPS suggests that we may be on the threshold of a realistic earthquake warning system.
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