Thirty-Five-Year Creep Rates for the Creeping Segment of the San Andreas Fault and the Effects of the 2004 Parkfield Earthquake: Constraints from Alignment Arrays, Continuous Global Positioning System, and Creepmeters

We present results from differential Global Positioning System (GPS) surveys of seven alignment arrays and four continuous GPS sites along the creeping segment of the San Andreas fault. Surveys of four alignment arrays from the central creeping segment yield 33to 36-year average minimum slip rates of 21–26 mm/yr. These rates are consistent with previous alignment array surveys spanning a 10-year period and with rates determined by creepmeters, indicating approximate steadystate creep along the central creeping segment for at least 35 years. Motion between continuous GPS sites that span the central creeping segment is 28.2 0.5 mm/yr for two sites that are 1 km apart and 33.6 1 mm/yr for two sites that are 70 km apart. Slip rates therefore increase with distance from the creeping segment of the San Andreas fault. All rates reported here are significantly slower than the 39 2 mm/yr rate predicted for motion between the Sierra Nevada–Great Valley block and the Pacific plate. Repeat surveys of three alignment arrays following the 2004 Parkfield earthquake demonstrate that its coseismic and short-term postseismic offsets decrease rapidly with distance from the epicenter, from 150 mm to 15 mm to 5 mm at respective distances of 9, 36, and 54 km to the northwest. Continuous GPS data confirm that little coseismic and postseismic slip occurred along the central creeping segment. Geodetic and geologic slip rates are compared and different models for the accommodation of transcurrent deformation across the creeping segment are discussed.