GLOBAL POSITIONING SYSTEM CONSTRAINTS ON FAULT SLIP RATES IN SOUTHERNCALIFORNIA AND NORTHERN BAJA, MEXICO

We use Global Positioning System (GPS) estimates of horizontal site ve locity to constrain slip rates on faults comprising the Pacific-North America plate boundary in southern California and northern Mexico. We enlist a simple elastic block model to parameterize the distribution a nd sum of deformation within and across the plate boundary. We estimat e a Pacific-North America relative plate motion rate of 49 +/- 3 mm/yr (one standard deviation), consistent with NUVEL-1A estimates, We are able to resolve robust slip rate estimates for the southernmost San An dreas, San Jacinto, and Elsinore faults (26 +/- 2, 9 +/- 2, and 6 +/- 2 mm/yr, respectively) and for the Imperial and Cerro Prieto faults (3 5 +/- 2 and 42 +/- 1 mm/yr, respectively), accounting for about 86% of the total plate motion. The remaining 14% appears to be accommodated to the west of these fault systems, probably via slip along the San Cl emente fault and/or the San Miguel, Vallecitos, Rose Canyon, and Newpo rt-Inglewood fault systems. These results are highly consistent with p aleoseismic estimates for slip rates implying that off-fault strain ac cumulation within the deforming zone of the plate boundary is largely elastic. We estimate that the seismically quiescent, southernmost San Andreas fault has incurred about 8.2 m of slip deficit over the last f ew hundred years, presumably to be recovered during a future large ear thquake.