SCEC Award Number 16263 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title Quantifying Active Deformation through San Gorgonio Pass on the Northern Strands of the 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, 4a, 4c SCEC Groups FARM, Geology, SoSAFE
Report Due Date 03/15/2017 Date Report Submitted 04/04/2017
Project Abstract
 We investigate the kinematics and activity of faults that transfer slip through the northern San Gorgonio Pass. Based on our fieldwork in 2014 through 2016, we find that Mission Creek fault is active through the pass, while the Mill Creek fault is inactive through the area of highest topography. Subsidiary fault kinematics from Mill Creek canyon through upper Raywood Flat are consistent with distributed transfer of slip from the Mill Creek fault to the Mission Creek fault in the vicinity of the Galena Peak fault. The western Mill Creek fault, Galena Peak fault, and Mission Creek fault serve as alternative potential rupture paths to the geometrically complex and mechanically inefficient San Gorgonio Pass thrust. However none of these faults shows continuous morphologic evidence for surface rupture like that observed along the San Gorgonio Pass thrust. In 2016 we completed geochronology, structural-kinematic measurements, and mapping efforts along the Mill Creek and Mission Creek faults. Newly acquired airborne lidar data shows that debris flow-deposits, now dated as mid-Holocene and Latest Pleistocene, do not show offset by the Mill Creek fault, confirming its inactivity. A scarp formed by dextral slip along the westernmost Mission Creek fault offsets coarse mid to late Holocene boulder alluvial fan deposits ranging in age from 1.5 to 4.1 kyr, and indicate a preliminary slip rate of ~4mm/yr.
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 document illuminates that the northern strands of the San Andreas fault are a viable pathway for fault rupture through the San Gorgonio Pass. Our efforts mapping faults and damage zones at the surface compliments the mapping of seismic sources in the subsurface to determine the fault geometries and potential rupture paths at depth. New slip-rate information helps to quantify slip partitioning through this complex area that reflect the cumulative effects of recent earthquake surface ruptures.
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 and Mission Creek fault are active through portions of the Pass. 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 debris-flow deposits offset by the Mission Creek fault at Banning Canyon. 10Be ages of boulders indicate two populations, consistent with our field observations, separated by an interpreted boundary between an older and younger set of deposits (blue line). Note strongly deflected, northeast strike of this dextral fault. We estimate the younger deposit to be 1.5 kyr based on the cumulative distribution of boulder exposure ages, and the older to be 4.1 kyr. A 6m offset was directly observed for an offset debris-flow levee within the younger deposit (hatched lines are levees). The older deposit displays a ~2m scarp, and must exhibit at least 12m of offset based on the predominance of right slip here indicated by scarps on the adjacent ridge line. Overall these age and map relationships indicate a slip rate of ~4 mm/yr along the westernmost Mission Creek fault.