SCEC Award Number 12044 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title New Slip Rate estimates from Wallace Creek and Phelan Creek Paleoseismic Sites. Re-sampling, Re-dating and Re-Synthesizing
Investigator(s)
Name Organization
Lisa Grant Ludwig University of California, Irvine J Ramon Arrowsmith Arizona State University
Other Participants Sinan Akciz, Co-PI
SCEC Priorities 1d, 2a, 4b SCEC Groups Geology, SoSAFE, WGCEP
Report Due Date 03/15/2013 Date Report Submitted N/A
Project Abstract
 Sieh and Jahns (1984) measured the slip rate of the San Andreas fault (SAF) at Wallace Creek in the Carrizo Plain, and thereby provided an anchor point for nearly all data-driven models of the southern San Andreas fault behavior and hazard. In 2012 we excavated a new trench between Sieh and Jahns' original trenches, trench WC12, to collect new organic samples from the abandoned channel deposits and confirm original stratigraphic correlations which underpin the slip rate at Wallace Creek.
The Wallace Creek channel has been offset by about 130 m since about ~3300 BP. Using the calibrated calendar age (1616-1506 BC) of a sample from the uppermost abandoned channel stratigraphy, the revised slip rate is 38±0.6 mm/yr. Although this value may change slightly after further analysis, this new data indicate that the slip rate is slightly higher than the Sieh and Jahns (1984) result of 33.9±2.9 mm/yr and Noriega et al. (2006) result of 32.4±3.1 mm/yr (since about 1000 BP at the Van Matre Ranch site). Our results are closer to the higher end of the ~36±2 mm/yr velocity gradient across the SAF from decadal timescale geodetic measurements (Schmalzle, et al., 2006). Measurements of slip per event for the last 5 or 6 earthquakes (Liu et al., 2004; Liu-Zeng et al., 2006), when combined with new dates of earthquakes from Bidart Fan site in the Carrizo Plain (Akciz et al., 2010), yield slip rates (up to 50 mm/yr) well above widely accepted values of ~ 35 mm/yr (Sieh and Jahns, 1984; Noriega et al., 2006) and this investigation's new result (38±0.6 mm/yr). The mismatch suggests that either some of the reported slip-per-event values are actually cumulative slip for multiple large events, or majority of the earthquake age determinations are of smaller magnitude earthquakes with minimal slip. Either option should accommodate a relatively constant slip rate of ~36±2 mm/yr.
Intellectual Merit The Wallace Creek channel has been offset by about 130 m since about ~3300 BP. Using the calibrated calendar age (1616-1506 BC) of a sample from the uppermost abandoned channel stratigraphy, the revised slip rate is 38±0.6 mm/yr. Although this value may change slightly after further analysis, this new data indicate that the slip rate is slightly higher than the Sieh and Jahns (1984) result of 33.9±2.9 mm/yr and Noriega et al. (2006) result of 32.4±3.1 mm/yr (since about 1000 BP at the Van Matre Ranch site). Our results are closer to the higher end of the ~36±2 mm/yr velocity gradient across the SAF from decadal timescale geodetic measurements (Schmalzle, et al., 2006).

Measurements of slip per event for the last 5 or 6 earthquakes (Liu et al., 2004; Liu-Zeng et al., 2006), when combined with new dates of earthquakes from Bidart Fan site in the Carrizo Plain (Akciz et al., 2010), yield slip rates (up to 50 mm/yr) well above widely accepted values of ~ 35 mm/yr (Sieh and Jahns, 1984; Noriega et al., 2006) and this investigation's new result (38±0.6 mm/yr). The mismatch suggests that either some of the reported slip-per-event values are actually cumulative slip for multiple large events, or majority of the earthquake age determinations are of smaller magnitude earthquakes with minimal slip. Either option should accommodate a relatively constant slip rate of ~36±2 mm/yr.
Broader Impacts SCEC Intern Terry Cheiffetz was an invaluable part of the project research team.
Exemplary Figure Figure 1. Aerial photograph of the main trace of the San Andreas fault in the Carrizo Plain. Abbreviations: WC=Wallace Creek, PC=Phelan Creeks, PF=Phelan Fan, BF=Bidart Fan.