SCEC Award Number 14135 View PDF
Proposal Category Collaborative Proposal (Special Fault Study Area)
Proposal Title Basement Deformation in the San Gorgonio Pass and its Implications for Large Earthquakes on the Southern San Andreas Fault
Investigator(s)
Name Organization
Michael Oskin University of California, Davis Judith Chester Texas A&M University
Other Participants Alex Morelan, Daniel Elizondo
SCEC Priorities 1a, 2d, 4a SCEC Groups SoSAFE, Geology, SDOT
Report Due Date 03/15/2015 Date Report Submitted N/A
Project Abstract
 We investigate the kinematics and activity of faults that transfer slip through the northern San Gorgonio Pass. Based on our fieldwork in the 2014 season, we find both the Mill Creek fault and the Mission Creek fault are active where they bound the high topography of Yucaipa Ridge. We also identified possible mechanisms for the unusually high rock uplift rate of the ridge. The Mill Creek and Mission Creek faults through north of San Gorgonio Pass serve as alternative rupture paths to the geometrically complex and mechanically inefficient San Gorgonio Pass thrust. Rapid uplift, and newly identified northward tilting of the Yucaipa Ridge block also contribute to the complicated kinematics of slip transfer through the Pass.
Intellectual Merit Rates of active faulting and uplift through the northern San Gorgonio Pass, as well as the distributed deformation of basement between major active strands of the San Andreas fault, provide important information for modeling earthquake rupture behavior through the San Gorgonio Pass Special Fault Study Area (SFSA). The distributed and complex deformation we have documented illuminates the difficulty in determining slip rates on the major strands of the San Andreas system through the San Gorgonio Pass region. Continued work mapping faults and damages zones at the surface will compliment the mapping of seismic sources in the subsurface to determine the fault geometries and potential rupture paths at depth.
Broader Impacts The San Gorgonio Pass region is the most complex and enigmatic section of the San Andreas fault. Our work confirms that the Mill Creek fault is active throughout the Pass region. Understanding how earthquake ruptures may pass through this area affects the probability assessment of strong ground motion over a wide area of southern California. In addition, this project is contributing to the education and research training of two graduate students, including one from an underrepresented group (D. Elizondo).
Exemplary Figure Figure 2: Map of eastern half of Yucaipa Ridge, northern San Gorgonio Pass area, highlighting data collected along the Mill Creek fault. Orange polygons are the current mapped extent of perched alluvial-fluvial deposit adjacent to Mill Creek. Dark blue points are the locations of fault-kinematic measurement stations. Large red points are locations of observed gravel-bedrock fault contacts. The yellow faults are from the existing geologic and active fault maps of the area. The red fault is our updated trace of the Mill Creek fault, with a restraining bend near headwaters of Mill Creek (see detail of fault kinematic interpretation in Figure 3). Inset figures: a) Photograph of the tilted terrace deposit, arrows delineate the steeply tilted bedrock contact. b) ~190m right-lateral offset of terrace gravel by Mill Creek fault in Upper Raywood flat. c) Mountain-down scarp along the Mission Creek fault near the base of Yucaipa Ridge.