Does the Holocene Slip History for the Mojave Section of the San Andreas Fault Indicate Secular Variation in Slip and a Discrepancy with Geodetic Rates?

University Of California-Davis, Davis CA

Investigators

Abstract

In this project a team of scientists from the University of California, Davis, in collaboration with researchers at the United States Geological Survey, will use geologic observations to measure the history of fault slip along the Mojave section of the San Andreas fault in Southern California (from Frazier Mountain in the northwest to Cajon Creek in the southeast). The primary goal of the project is to test the hypothesis that the average rate of motion along the Mojave section of the San Andreas fault alternated between periods of fast slip (approximately 45 mm/yr) at 1100-4500 and 6500-8000 yr ago and periods of slower motion (approximately 16 mm/yr) at 0-1100, 4500-6500, and 8000-10000 years ago. To do so, the research team will determine the history of fault motion along this section of the San Andreas fault over the last ten thousand years by measuring the ages and offsets of ten different faulted stream channels. Because each channel formed at a different time, each channel records a different period of fault motion. In particular, each channel experienced a different number of ancient earthquakes and thus accumulated a different amount of fault slip, with offsets ranging from 26 (+4/-3) m for the youngest channel to 261 (+46/-26) m for the oldest. To precisely measure the total age and total offset for each of these channels, and thus measure the slip rate since the channel formed, the research team will use careful field mapping, analysis of detailed topographic measurements, and excavations at each site. These age and offset data will then be analyzed to determine both the slip rate as averaged over the last ten thousand years and the extent to which there were statistically significant faster and slower pulses of slip within that period of motion. In general, active faults remain locked until they suddenly break and slip to produce an earthquake. Such motion is roughly analogous to that of a car in stop-and-go traffic. During earthquake ruptures, surface features such as stream channels or roads are often cut and displaced by the rapid fault motion. Thus, repeated earthquakes over time can result in both an accumulation of surface displacements along the fault and a time-averaged slip rate. Because faster-moving faults will produce more frequent earthquakes, determining the average rates at which faults slip is of fundamental importance for evaluating their earthquake hazards. Geological records of ancient earthquakes in the Los Angeles Basin and Eastern California Shear Zone appear to indicate that fault activity has rapidly switched between the Mojave section of the San Andreas fault and the Eastern California Shear zone, as well as other fault systems in southern California. This model has important implications regarding seismic hazards and if correct, would significantly revise understanding of the mechanical behavior of major continental fault systems.