Active Tectonics Projects II

Holocene-Late Pleistocene geologic slip rates for the Banning strand of the southern San Andreas Fault system, southern California

by Peter Gold, Ph.D. Candidate at UT Austin

The goal of research in active tectonics is to use structural and geomorphic evidence of active faulting and measurements of seismicity, rates of slip, and earthquake timing to understand fault behavior over 10-100,000 year time scales. In the western United States, earthquakes on active faults within the San Andreas transform plate boundary system constitute the most significant natural hazard for this region. In southern California, plate boundary deformation is shared, or partitioned, between multiple faults (Figure 1). Determining which of these active faults is most likely to produce the next major southern California earthquake is related to how much displacement these faults have accommodated in the recent past, which we can estimate by measuring the fault slip rate.

Figure 1. (Click to enlarge)

 

Fault slip rates are time-averaged measurements of how fast crustal rocks on one side of a fault are moving past the other. Geodetic methods such as GPS can be used to measure ‘present day’ slip rates that are applicable to the past 10 years. However, since slip rates may vary over time, rates measured over longer geologic time scales are needed as well. Geologic slip rates are measured by identifying a geologic or geomorphic feature like an alluvial fan axis that has been offset from its source by the fault (Figure 2), usually as the result of many surface-rupturing earthquakes. Combining the length of offset with the age of the offset feature (the time since it was formed) provides a time-averaged slip rate, usually presented in mm/yr.

Figure 2. (Click to enlarge)

To identify and measure offset features we rely in part on Light Detection and Ranging (LiDAR), which is a laser measurement method that provides detailed topographic images of the earth’s surface (Figure 2). In southern California, freely available airborne LiDAR datasets cover most of the known active faults. To date offset features, we use quantitative methods such as in-situ cosmogenic radionuclide exposure or burial dating of clasts and sediments (Figure 3), optically stimulated luminescence dating of sediments, and uranium-series dating of carbonate coatings on buried clasts. Combining one or more of these methods with qualitative descriptions of soil development provides reliable constraints on surface age.

Figure 3. (Click to enlarge)

 

The focus of our work in southern California has been to measure geologic slip rates for strands of the southern San Andreas Fault. In the Coachella Valley, this fault splits into the Banning and Mission Creek strands (Figure 1), each a potential rupture path for future earthquakes. Since the southern San Andreas is considered overdue for a large earthquake, it is important for seismic hazard estimates and our understanding of the Late Quaternary evolution of this fault system to determine which of these two strands the next rupture is likely to follow. The Banning Strand would direct an earthquake towards population centers, so we have focused on measuring the slip rate for this fault.

Using the methods described above, we have measured a slip rate of about 9 mm/yr for the Banning Fault over the past 3000 years. This implies that earthquakes on the southern San Andreas Fault follow the Banning Strand about half the time. Since slip rates may vary along the length of a fault (as well as over time), the next stage of this project will be to measure rates from elsewhere along the Banning Strand, as well as from the Mission Creek Strand.