Nearfield vertical displacement in the creeping segment of the San Andreas fault, central California, 1975 to 1994

Abstract

Nearfield precise spirit leveling reveals progressive, aseismic (?) height changes in the creeping segment of the San Andreas fault between Gold Hill in Cholame Valley and San Juan Bautista over time periods from 8 to 18 years. Height changes are greatest at each end of the creeping segment, whereas the rate of horizontal creep is greatest in the center of the creeping segment. Both vertical and horizontal displacements take place at the surficial trace of the fault, whereas surficial downwarping is evident as much as 500 m from the mapped fault trace. The large height changes at San Juan Bautista, Parkfield, and Gold Hill may be related to compaction-induced subsidence where groundwater is pumped from unmonitored irrigation wells on one side of the fault. At San Juan Bautista, however, the water table has remained nearly constant according to local farmers, even though the height change there is the greatest among the six sites. Therefore, it is concluded here that the vertical displacements are tectonic and result from local deviations of the direction of fault slip relative to the far-field slip vector, or alternatively to deceleration of horizontal creep and dissipation of horizontal strain into the transition zones between the end of the creeping fault segment and the adjacent locked segments.