GPS MONITORING OF THE SAN ANDREAS FAULT IN THE SAN BERNARDINO MOUNTAINS AND URBAN SAN BERNARDINO AREA OF SOUTHERN CALIFORNIA

The southern San Andreas fault in the San Bernardino, California area has had no large earthquake for almost 200 years. Two segments of the fault have ruptured to the north more recently (San Francisco, 1906 and Fort Tejon, 1857). It is important to try to understand the strain accumulation in this southern fault segment as a probable earthquake will affect the lives and property of many southern California residents. To track strain accumulation along this area, very-precise GPS tracking is performed annually by undergraduate students and high school teachers at approximately 25 geodetic markers throughout the San Bernardino Mountains and many urban areas in the Inland Empire area of Southern California. A tripod with a GPS antenna was positioned over each survey marker for 40-90+ hours while the exact site location was recorded by a GPS receiver. These data are then compared with results from previous years starting in 2002 or earlier, which then provide calculable velocities for the movement of these sites annually which tell about the rate of strain accumulation for this part of the San Andreas Fault. Most sites have velocities ranging from 5-25 millimeters per year in the northwestward direction, relative to stable North America. The velocities generally increase from northeast to southwest across the region studied, as would be expected. Vertical velocities indicate that stations in the mountains are generally going up while those in the valley are going down. Using these data, we have created a 1-dimensional elastic model to better understand the slip rates of the San Andreas fault and other faults that make up this part of the plate boundary. In this model, we have separated the main active faults into three categories based on their locations relative to each other: western faults (Palos Verdes, Newport-Inglewood and Elsinore faults), San Andreas/San Jacinto faults (SAF & SJF), and the East California Shear Zone (ECSZ) faults. The best fitting models in this approach have slip rate values of 8-14 mm/yr for the three western faults combined, 12-24 mm/yr for SAF/SJF combined, and 12-18 mm/yr for the ECSZ.