SCEC Award Number 13180 View PDF
Proposal Category Individual Proposal (Special Fault Study Area)
Proposal Title Paleoseismology and slip rate of the Garnet Hill Fault at Whitewater Hill
Investigator(s)
Name Organization
Doug Yule California State University, Northridge
Other Participants Several CSUN students will be involved: one MSc graduate student, and several BS geology undergraduate students (all to be determined in spring 2013).
SCEC Priorities 2a, 4a, 4c SCEC Groups Geology, SoSAFE, WGCEP
Report Due Date 03/15/2014 Date Report Submitted 06/29/2017
Project Abstract
 This report describes trenching data that constrain the timing of its most recent surface rupture from a site located near the Hwy 62/I-10 interchange. The precise location of the GHF at this site is not well known due to burial beneath young sediment but is inferred based primarily on structural arguments required to explain the actively growing fold to the north of the inferred fault trace. Our trench revealed no definitive evidence of surface faulting. Nonetheless, we report the following stratigraphy: a charcoal-rich colluvial wedge (CW) at the north end of the trench, with underlying coarse fluvial gravel and overlying finer fluvial material. The youngest charcoal ages from the base and top of the CW are A.D. 1404-1435 and 1474-1635, respectively. These ages indicate that the CW formed between about 611-380 yrs ago. Two possible hypotheses that might explain why we did not detect any evidence of faulting are: 1) the colluvial wedge formed after an earthquake with a thrust tip that is unable to be detected in the coarse fluvial gravels beneath the CW, or 2) the CW formed after the Whitewater River migrated to the north against the hillside and eroded any evidence of faulting. Both explanations provide a minimum age for the timing of the last earthquake at ~600 yrs ago. These results exclude the possibility that the ~A.D. 1690 event observed at Indio and Thousand Palms occurred here, but permit the possibility that the ~A.D. 1400 event observed at Cabazon ruptured the GHF at this location.
Intellectual Merit This project helps constrain the earthquake history and behavior of the San Andreas Fault system in the northern Coachella Valley and San Gorgonio Pass regions. The findings of this project are aligned with objectives outlined for the San Gorgonio Pass Special Fault Study Area.
Broader Impacts This project provided a graduate thesis project for Jose Cardona (CSUN MSc in 2016), and his field assistants graduate student Brittany Huerta (CSUN MSc in 2017), and undergraduate students Bryan Castillo (now a graduate student working with Sally McGill at CSUSB), Bryan Sanchez, and David Quezada. All five students (two grads and three undergrads) are from underrepresented minority groups.
Exemplary Figure Figure 2. Yellow stars indicate trench site located on Caltrans right-­‐of-­‐way property adjacent to Interstate10. Blue line indicates likely position of Whitewater River when it deposited gravels found at base of trench (Figures 3 and 4). To left, map view of the Garnet Hill and Banning strands of the San Andreas Fault. East Whitewater Hill is a doubly plunging fold growing at a compressive stopover in the Garnet Hill fault trace. Note that a pair of 1 km-long sections of Interstate 10 lie directly upon the inferred trace of the fault. to right, an oblique aerial view looking to the WNW shows the same compressive stopover. Qa, recent alluvium; Qoa, older, deformed alluvial fan surface characterized by a well-developed pavement and brick red soil horizon; Qca and Qcb, alluvial gravels that underlie Qoa and most likely correlated with gravels of the middle Pleistocene Cabezon fanglomerate.