abstract

Deformation possibly related to an earthquake in Texas has been revealed from analysis of precise leveling data collected by the National Geodetic Survey. Relative elevation changes deduced from first-order leveling surveys conducted between 1910 and 1957 show that at least two areas in western Texas near the epicenter of the 1931 Valentine earthquake (M = 6.4) subsided relative to their surroundings. Apparent subsidence east of Van Horn, Texas, is attributed primarily to the combined effects of groundwater withdrawal and topography-related survey errors. In contrast, relative subsidence near Valentine, Texas, of 11.2 ± 1.0 cm extending over a distance of about 20 km does not appear to be due to either near-surface effects or leveling errors and thus may represent coseismic deformation of the Valentine earthquake. The large magnitude of the vertical movements indicated by the leveling data suggests significant dip-slip movement on the inferred earthquake fault. This is consistent with the Cenozoic structure of the region which is characterized by Basin and Range morphology. If the observed subsidence was caused by the 1931 earthquake, the leveling data suggest that the epicenter for this event lies considerably closer to Valentine than originally reported. This new location is supported by a recent relocation for the Valentine earthquake, and by local intensity reports.

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