SCEC Award Number 11194 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title Collaborative Proposal: SoSAFE- San Jacinto: Quaternary slip rate of the northern San Jacinto fault from offset landslide
Investigator(s)
Name Organization
Sally McGill California State University, San Bernardino Lewis Owen University of Cincinnati Nate Onderdonk California State University, Long Beach Tom Rockwell San Diego State University
Other Participants Emiko Kent, graduate student, Univ. of Cincinnati
one or more undergraduate students from CSUSB
SCEC Priorities A1, A2, A9 SCEC Groups SoSAFE, Geology, SHRA
Report Due Date 02/29/2012 Date Report Submitted N/A
Project Abstract
 During 2011 we continued our work at two slip-rate sites on the northern San Jacinto fault between Moreno Valley and Redlands, California. Within the San Timoteo badlands, north of Moreno Valley and south of Redlands, rock debris has been shed northeastward across the fault from the highlands on the southwest side of the fault. Mapping conducted in 2011 shows that the clasts within these deposits can be matched to their specific source areas within the bedrock highlands on the southwest side of the fault, thus providing constraints on the amount of offset of these deposits, which we refer to as the Quincy Ridge debris fan. A number of younger landslides have been shed from the Quincy Ridge debris fan into lower-lying areas to the northeast. One of these younger slides is also offset by the San Jacinto fault, by about 255 ± 65 m. We refer to this younger offset landslide as the Ebenezer Canyon slide. New Be-10 dates and new soil descriptions are available from both landslides, as well as a new OSL date on the Ebenezer Canyon slide. The available data indicate a slip rate of 9 (+4.5, -2) mm/yr for the past ~37 ka and a maximum slip rate of ≤ 13-27 mm/yr for the past ≥ 100 ka.
Intellectual Merit This work presents a new late Pleistocene slip-rate estimate of 6-13 mm/yr for the northern San Jacinto fault. Previous studies have yielded widely varying rates from <10 mm/yr (e.g., Prentice and others, 1986)to >20 mm/yr (Kendrick and others, 2002). Our work at Ebenezer Canyon adds to a growing body of evidence in support of a slip rate of about 10-15 mm/yr for the San Jacinto fault. These rates leave unanswered questions, however, as to the distribution of deformation across the plate boundary, because recent studies (McGill and others, 2010, 2012) also suggest lower slip rates for the San Bernardino section of the San Andreas fault than previously thought.
Broader Impacts S. McGill has used this field area to train five undergraduate students (two of whom were female) and one master's student in geologic field mapping for neotectonic studies. Several other graduate students from UCLA, CSU Long Beach and University of Cincinnati have visited the site to observe the geology and to assist with collecting samples for dating. Graduate student, Emiko Kent, from University of Cincinnati and University of Plymouth, conducted the Be-10 lab work, which will be a part of her thesis. This work may benefit society by contributing toward more robust inputs to seismic hazard analysis.
Exemplary Figure Figure 4: Preliminary geologic map of the Ebenezer Canyon landslide (in blue; Qols2), which is offset 255 +/- 65 m across the Claremont fault (northern San Jacinto fault zone). Yellow squares show locations of dated Be-10 samples from boulder tops, which suggest an age of 20-40 ka for the landslide and a slip rate of 6-13.5 mm/yr. (S. McGill and others, 2012).