SCEC Award Number 13073 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title Collaborative Research: Assessing slip rate variations on the Garlock fault using newly developed luminescence sediment dating
Name Organization
Edward Rhodes University of California, Los Angeles James Dolan University of Southern California Sally McGill California State University, San Bernardino
Other Participants Lee McAuliffe, PhD, USC; Mike Lawson, PhD, UCLA; Evan Wolf, MS, UCLA; Steve Okubo, MS, UCLA; Wendy Barrera, U/G, UCLA; Tom Capaldi, U/G, UCLA
SCEC Priorities 2a, 2d, 4c SCEC Groups Geology, SDOT, EFP
Report Due Date 03/15/2014 Date Report Submitted N/A
Project Abstract
The objectives of this project were to assess the degree to which the Garlock fault, California, exhibits transient strain accumulation with periods of increased slip and frequent earthquakes, separated by periods of little or no slip and few accompanying seismic events. The methodology involved determining minimum slip rates over the past 2000 years at a number of offset alluvial fan features. Age control was provided by newly developed post-IR IRSL (infra-red stimulated luminescence) using sand-sized single grains of K-feldspar. The project has been successful, with age control now developed for five locations, plus several modern sample locations to assess performance of the technique. Additionally, eight further samples from locations with slightly more ambiguous geomorphic offsets were dated, contributing to the paleoclimate record for this part of the Mojave desert. The results are significant, demonstrating slip rates higher than the Holocene average rate, with uncertainty margins that clearly demonstrate an increase in slip rate over this period.
Intellectual Merit The project contributes to the intellectual merit of SCEC by providing clear evidence for irregular fault slip rates for the Garlock fault. This result is significant in requiring modification to simple earthquake cycle models with uniform strain accumulation, with the prospect of developing improved models for strain release and seismic events for this fault, and possibly in other contexts. The new luminescence chronological developments are also providing a means to date sediments on timescales of 10 to 100,000 years, allowing fault slip rates and paleoseismic records to be measured in contexts where no organic material for radiocarbon exists, or where the events are too old (>40,000 years) or too young (post AD 1750) for that technique.
Broader Impacts This project has provided opportunities for training and teaching; In 2013, two UCLA masters projects have been based around the target sites, and one USC PhD student project. UCLA and Caltech undergraduate and graduate students took advantage of the site to be trained in field geophysical methods. The advantages to society include a cohort of aware students, some with specific skills, taking up positions in academic or industrial settings, and improvements in our understanding of earthquake timing.
Exemplary Figure Figure 2. Available slip rate and paleo-earthquake age data from the central Garlock fault. (A) Incremental fault slip rate plotted vs. time. Yellow horizontal band encompasses range of preferred average latest Pleistocene-Holocene rates measured by Clark & Lajoie (1974); McGill & Sieh (1993); McGill et al. (2009); and Ganev et al. (2012). Solid red line shows known rates. Note correspondence of rapid latest Holocene rate measured during our 2013 SCEC studies with cluster of four earthquakes observed by Dawson et al. (2003) at their El Paso Peaks site 25 km west of our slip rate study site. Dawson et al. (2003) documented an absence of earthquakes between 2ka and ~5ka, yielding a 0 mm/yr rate for this interval. For the two earthquakes they documented at ~5ka and ~7ka we assume a range of possible displacements for each event of 2 m (the smallest geomorphic displacement measured along the central Garlock fault by McGill & Sieh, 1991) to 6 m (ave. slip in four earthquakes totalling 24 m as measured at our Christmas Canyon West study site), yielding 4-12 m of slip within this ~2kyr period. Combined with the 24 m of fault slip measured by us over past 2kyr, this yields a total of 28 to 36 m of slip on the Garlock fault since 7ka. Green, gray, and pink boxes show possible incremental rates prior to 7ka that are consistent with the 70 m of total displacement measured by McGill et al. (2009) and Ganev et al. (2012) that has accrued since latest Pleistocene-early Holocene time. Three possibilities are shown, given the uncertainty in the exact time at which the 70 m of displacement began to accumulate: (1) a 19-24 mm/yr incremental rate from 7.5 ka to 9.3ka (preferred age of incision for 70 m offset at McGill et al., 2009, study site); or (2) a 9-11 mm/yr incremental rate from 7.5ka to 11.5ka (following Ganev et al.’s suggestion that incision began during return to wetter conditions at end of Younger Dryas climatic regime); or (3) 5-7 mm/yr incremental rate from 7.5 ka to 13.3 ka (age of incised surface at Ganev et al., 2012, 70 m offset). Note that these three speculative “rates” assume constant steady slip during these time periods, a behavior we consider unlikely, especially for the 4-6 kyr-long periods represented by the latter two possibilities. This method of presentation thus artificially reduces the average rates shown during these intervals. (B) Same data plotted as cumulative displacement through time. Triangular region between 7.5ka and 13.3ka denotes range of possible cumulative displacements. Note the pink and yellow horizontal bands. These show the key positions of our 37.5 m (pink) and 55 m (yellow) offset targets. Dating of these two offsets with our robust post-IR-IRSL225 luminescence dating technique will allow us to complete our characterization of the incremental slip rate of the Garlock fault for the entire Holocene. The addition of our proposed study of the rate based on the 175 m offset should tell us if the longer-term rate on the fault is also comparable to the average latest Pleistocene-Holocene rate. (C) Available paleoseismologic data for the central and western Garlock fault. The well-constrained site of Dawson et al. (2003) lies closest to our study sites.