Vertical deformation (1952–1987) in the Salton Trough, California, from water level recordings

Analysis of 36 years of water level data from three sites on the shore of the Salton Sea reveals a history of vertical deformation in the Salton Trough. Data from two sites allow estimation of tilt along a 36‐km NW–SE baseline between 1952 and 1970; since 1970 the addition of a 13‐km WNW–ESE baseline allows the full tilt vector to be estimated. We find down to the southeast tilting at an average rate of about 0.01±0.01 μrad/yr from the early 1960s to 1987. An anomalous period of more rapid, 0.2±0.04 μrad/yr, tilting occurs between the mid 1950s and early 1960s, but this coincides with a period from which we have been unable to examine all the original data. A previous analysis of the 1952–1978 data suggested a precursory increase in tilt rate prior to the 1968 Borrego Mountain earthquake, and a tilt reversal in 1972. We show that there was no significant tilt change prior to the 1968 earthquake, and that the apparent 1972 reversal resulted from uncorrected offsets in the water level observations. Tilts derived from water level and from reliable first‐order leveling surveys show limited agreement, but the leveling data contain evidence for nonuniform tilting. Recent analyses of leveling data by others, using different techniques, show 20–30 cm uplift and subsidence events in the Salton Trough between 1968 and 1978. Our data constrain the relative elevations of the three water level sites to be stable within 1–2 cm during these proposed events.