SCEC Award Number 15139 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title The Role of Climate in the Formation of Geomorphic Features Used for Fault Offset Measurement
Investigator(s)
Name Organization
J Ramon Arrowsmith Arizona State University Sinan Akciz University of California, Los Angeles Lisa Grant Ludwig University of California, Irvine Thomas Rockwell San Diego State University
Other Participants J. Barrett Salisbury (Ph.D. student)
SCEC Priorities 2a, 1a, 1d SCEC Groups SoSAFE, Geology, WGCEP
Report Due Date 03/15/2016 Date Report Submitted 03/14/2016
Project Abstract
 We targeted the Van Matre Ranch (VMR) reach of the San Andreas fault (SAF) in the southern Carrizo Plain to investigate the age of short-lived offset geomorphic features. Here, several closely-spaced channels from similar-sized drainages are offset along the relatively simple, well-expressed SAF. We excavated four subtle topographic depressions previously interpreted as beheaded channels (as confirmed in the B4 lidar data) less than 10 m from feeder channels. Three of the four beheaded gullies have associated sub-surface channel deposits from which we refined existing (and debated) surface slip measurements and sampled for radiocarbon and optically stimulated luminescence (OSL) estimates of initial channel incision/fill ages. At one site, we found no associated sub-surface channel deposits from the “beheaded gully” and conclude that the swale is actually a fosse (depression) between two small (~10 m radius) offset alluvial fans. In the same trench, we found three buried channel deposits with >10 m of displacement. In total, we have four distinct fault-offset geomorphic features (the ~10 m radius alluvial fan and three sub-surface channel fills) potentially sourced from the same channel feeder offset 3.8 m, 13.8 m, 16.4 m, and 26.3 m, respectively (with slip increments of 3.8 m, 10 m, 2.6 m, and 9.9 m). Based on reconstruction of the alluvial fan apex, these data suggest that slip in the Mw 7.9 1857 event was ~4 m at the VMR site and the additional 10 m of offset for the buried channel fill occurred in at least one and possibly more earthquakes.
Intellectual Merit Fault-offset fluvial features are commonly cited in paleoseismic and slip-per-event studies as indicators of slip magnitude for particular earthquake events (both historic and prehistoric), but the individual small scale offsets (up to 10’s of m) are rarely dated, challenging attribution of slip to dated earthquakes. This project attempts to link fault-offset geomorphic features to causative storm events or multi-year climate episodes for a better understanding of under what conditions fluvial cut/fill sequences are creat-ed and preserved in the southern Carrizo Plain. Coupling this understanding with knowledge of recent earthquake timing along the southern San Andreas will help us improve earthquake chronologies and may be a key to refining slip-per-event along the SAF system.
Broader Impacts This collaborative, largely field-based investigation required help from a range of participants. We em-ployed 5 undergraduate student researchers (2 from ASU, Sutton (F) and Fischman (F); 1 from SDSU, Marquez (F); and 2 from UCI, Matt Martin (M) and Juliet Olsen (F)), a USGS undergraduate intern (Midttun (M)), 2 graduate students from ASU (Salisbury (M) and Williams (F)), and four professional investigators. The undergraduates learned basics of 3-D trench excavation and logging, balloon aerial photography, and structure from motion (SfM) generation of high-resolution models of trench exposures and surface topography. Students helped with post-field data assimilation and attended the 2015 SCEC annual meeting.
Exemplary Figure Figure 3, Annotated air photo showing T1 with additional hand trenches. Approximate fan outlines are shown with white dashed lines and buried channel thalwegs are shown as dashed blue lines. All offset measurements are made using T1-T10 projections into the fault trace (pinpointed in T9 and T5-6-7). Salisbury, Arrowsmith, Rock-well, Akciz, Grant Ludwig.