Horizontal deformation in the Imperial Valley, California, between 1934 and 1980

A triangulation‐trilateration survey was performed by the National Ocean Survey/National Geodetic Survey four months prior to the October 15, 1979, Imperial Valley M 6.6 earthquake. The survey was repeated in February–March 1980 to gauge the coseismic displacement. These data combined with triangulation surveys performed in 1934–1935, 1939, 1941, 1954–1955, and 1967 provide a historical record of crustal deformation over a time period that also includes the 1940 El Centro M 7.1 earthquake. The total data set was analyzed in terms of a dislocation model that relates the observed surface deformation to slip on prescribed geologic faults. According to the model, the 1941–1980 fault slip at depths 10 km and greater is 1.9±0.3 m. This value is approximately twice the 1.0±0.1 m estimated for the 1941–1980 slip on the Imperial fault in the 0‐ to 10‐km depth range. This discrepancy can be explained by a range of possibilities including the following: (1) some of the deep fault slip is due to aftereffects of the 1940 earthquake, in which case the seismic cycle over which deep slip and shallow slip are equal is not the interval between large earthquakes but is delayed a few years; (2) the deep slip should be equated to the shallow slip over several parallel faults; (3) the crust experienced permanent inelastic deformation; and (4) deep deformation is accommodated by a mechanism other than slip.