Changes in long‐term extension rates associated with the Morgan Hill and Loma Prieta earthquakes in California

Frequent measurements since mid‐1981 of the distances from a geodetic monument located about 100 km south‐southeast of San Francisco to three monuments 30 to 40 km distant provide an unusually complete record of the deformation before and after two nearby earthquakes, the 1984 Morgan Hill (M_L = 6.2) and 1989 Loma Prieta (M_s = 7.1) earthquakes. Except possibly for the first few months postseismic, the extension rates indicated by these measurements appear to be steady over the four or five years both preceding and following those earthquakes. However, the preseismic and postseismic rates differ significantly for at least one of the baselines measured for each earthquake. The data over the four to five year postseismic records available are not adequate to demonstrate whether the postseismic rates are relaxing back to the preseismic rates.