Gath, E.M., Runnerstrom, E.E., & Grant, L.B.
Active Deformation and Earthquake Potential of the Southern Los Angeles Basin, Orange County, California
Research supported by the U.S. Geological Survey
(USGS), Department of the Interior, under USGS award number 01HQGR0117. The
views and conclusions contained in this document are those of the authors and
should not be interpreted as necessarily representing the official policies,
either expressed or implied, of the U.S. Government.
excerpt from Final Technical Report (USGS award number 01HQGR0117) submitted to USGS Fall 2002.
The Santa Ana Mountains (SAM) are a 1.7 km high mountain range that form the southeastern boundary of the Los Angeles basin between Orange and Riverside counties in southern California. The SAM have three well developed erosional surfaces preserved on them, as well as a suite of four fluvial fill terraces preserved in Santiago Creek, which is a drainage trapped between the uplifting SAM and a parallel Loma Ridge. By correlation of the terraces with the marine eustatic sea level curve, we were able to estimate a 0.31 mm/yr uplift rate for the SAM and an emergence age of ~3.6 Ma. Using a new method for temporally quantifying the age of drainage basins, we were able to develop an alternative check of the age of the SAM. This methodology was first developed in the adjacent Puente Hills by developing a correlation between basin area and basin age. The basin age was calculated by measuring the right-lateral strike-slip displacement of each basin’s primary stream, and regressing it against the basin’s area. Because the Whittier fault’s slip rate is known (2.5 mm/yr), the age of the channel can be calculated by retrodeforming it. From this analysis, the Puente Hills have been rising 0.4 mm/yr since their emergence 700-1,200 ka. Santiago Creek formed ~2.4 Ma in conjunction with the initiation of the Loma Ridge structure, a parasitic structure that formed in response to compressional buckling of sedimentary strata on the flanks of the uplifting Santa Ana block. Hanging wall block faulting appears to have deflected Santiago Creek northerly ~1,200 m along five discrete block margin faults. The source of this strain is still undetermined, though it may be from the same north-vergent structures that are generating the San Joaquin Hills uplift. The termination of the Elsinore fault into the Chino and Whittier faults leaves at least 1-2 mm/yr of north-vergent strain unaccounted. We speculate that this strain is being transferred into uplift of the SAM, with complex interaction among other north-vergent structures in southern California.
The Santa Ana Mountains are prominent features in the landscape of the metropolitan Los Angeles basin. Comparable sized mountains and foothills in surrounding areas are known to be associated with active faults or folds. This project investigates possible tectonic deformation and earthquake potential associated with undiscovered faults in or near the Santa Ana Mountains by examining the patterns carved into the landscape by streams. Preliminary results suggest that the mountains are rising at a rate of approximately 0.3 mm/yr and may contain active faults.