Approximately 230 mm of aseismic vertical offset of the land surface across the Pond-Poso Creek fault in the San Joaquin Valley, California, probably is related to groundwater withdrawal for crop irrigation. The scarp is approximately 3.4 km long and occurs in an area where the land subsided more than 1.5 m from 1926 to 1970. Modern faulting postdates the beginning of water level declines and associated subsidence. Movement detected by precise leveling surveys from February 1977 to March 1979 was seasonal, occurring during periods of water level decline. Fault offset was greater in the year with the lower seasonal low water level. The modern movement probably is caused by localized differential compaction induced by differential water level declines across the preexisting fault.