SCEC Award Number 13146 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title Refining the south-central San Andreas Fault slip rate at the 6 ka timescale: Phelan Creeks
Investigator(s)
Name Organization
Ramon Arrowsmith Arizona State University Lisa Grant-Ludwig University of California, Irvine Sinan Akciz University of California, Los Angeles
Other Participants Arrowsmith graduate students including Tsurue Sato
SCEC Priorities 1, 2, 4 SCEC Groups Geology, SoSAFE, WGCEP
Report Due Date 03/15/2014 Date Report Submitted N/A
Project Abstract
 Phelan Creeks are located along the San Andreas Fault (SAF) 1.6 km southeast of Wallace Creek (WC) in the Carrizo Plain. The geologic slip rate is defined from few published sites with limited age constraints: Van Matre Ranch (VMR) channels offset ~26 m in ~850 yrs (~31 mm/yr), and WC channels offset 128 m in ~3,780 yrs (~34 mm/yr) and ~475 m in ~13,250 yrs (~36 mm/yr). Phelan Creeks consists of three downstream channel complexes on the SW side of the SAF and two feeder channels NE of the SAF. These channels have recorded progressive incision, offset, and abandonment along the SAF (~16.5 m, ~130 m, and ~230 m). Dating the ~230 m offset channels at Phelan Creeks fills a spatio-temporal gap in age-offset constraints (between VMR at 103 yrs & WC at 104 yrs) and test the hypothesis of minimal slip-rate variability along the south-central SAF in the Carrizo Plain during the last 13 ka. Trenches exposed cut-and-fill stratigraphy representing initial incision and subsequent channel processes as they were fault-offset to the NW and eventually beheaded. Alluvial fan layers, channel deposits, and interfingering colluvial wedges were sampled for 14C and Optically Stimulated Luminescence (OSL) dating. Preliminary results from OSL and 14C from trench 3 channel fill deposits are 5.6 + 0.3 ka from OSL and 6.0 + 0.4 ka, and 4.23 + 0.2 ka, and 4.22 + 0.15 ka for 14C.
Intellectual Merit Data on the rupture history of a fault is a fundamental element to construct fault-behavior model useful for seismic hazard analysis. Models of fault behavior generally divided into several types with two end members: the time-and slip-predictable models and the more complex systems where fault can be described as a group but the behavior of the constituent faults cannot be predicted individually. The overall behavior of the SAF can be addressed through analysis of channel evolution in response to faulting at Phelan Creek. Dating of multiple earthquake cycle represented by multiple period of channel abandonment at Phelan Creek is essential to fill the spatio-temporal gap in age-offset constraints from other sites. Our findings will help elucidate whether the simple geometry (i.e., few significant structures nearby) of the SAF between the Parkfield and Big Bend promotes a constant long-term slip rate with minimal along-fault slip-rate fluctuations. Dates obtained from our 14C and OSL samples bracketing the basal incision surface and it will provide age control for the ~230 m offset. Continued results will test whether the new dates support the hypothesis that the slip is constant at the millennial scale for the south-central SAF in the Carrizo Plain.
Broader Impacts The proximity of the SAF to the dense populated region makes the risk associated with this fault a major threat to the community. The findings of this research will provide additional insights on the temporal behavior of past earthquakes in the Carrizo Plain that are useful in predicting the timing of future damaging earthquakes. Eventually, it will greatly reduced the earthquakes threat to life.
Exemplary Figure Figure 3: The Phelan Creeks site is located along the San Andreas Fault (SAF) 1.6 km southeast of Wallace Creek (WC) in the Carrizo Plain. The goal of this study is to fill in the time gap between other known offsets in the Carrizo Plain at Wallace Creek (128 m in ~3,780 yrs, ~34 mm/yr and ~475m in ~13,250 yrs) and Van Matre Ranch (VMR, ~26m in ~850 years, ~31 mm/yr). Digital Elevation Model (DEM) of Phelan Creeks Paleoseismic site, Carrizo Plain, Central California. This model was built using a new method to the active tectonics community called Structure from Motion (SfM) which uses overlapping photos from a handheld digital camera to create ground models. a) Oblique aerial view of the Phelan Creeks channel complex showing the location of Little and Big Plelan channels as feeders for the offset channels as well as the locations of trenches excavated in summer 2013. b) Side view of flight path of balloon and location of images used to build the model relative to the ground (71 camera locations at ~30 m, ~100 m, and ~ 290 m). c) Orthophotograph in map view with surface measurements of offset channels. Samples taken at the trenches for geochronologic dating are expected to fill the time gap between geologic slip rates from WC and VMR. The study will investigate whether the new dates support the hypothesis that the slip is constant at the millennial scale for the south-central SAF in the Carrizo Plain.
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