Incremental slip rates of the Puente-Hills blind-thrust fault based on analysis of back-limb folding: Implications for seismic hazards in the Los Angeles region and structural evolution of fault-related folds

Abstract:

We combine high-resolution seismic reflection data, optically stimulated luminescence (OSL) ages, and water-well data to determine the slip history over the last 30 ka of the Puente Hills blind-thrust fault (PHT) beneath metropolitan Los Angeles. Specifically, we used the geometry of several prominent reflectors folded in the back-limb growth triangle above the central, Santa Fe Springs segment of the PHT, together with new mapping of the uppermost Pleistocene Gaspur aquifer based on water-well data, to define the uplift history of the fold. These uplift measurements are converted to thrust displacements on the PHT using a novel structural method that accounts for the precise geometry of the fault surface. Combining these data with OSL ages from a continuously cored, 65-m-deep borehole, we calculate PHT incremental slip rates spanning four time intervals over the past c. 30 ka. These back-limb slip rates complement fore-limb incremental slip rate constraints determined by Leon et al. (2009) at a site 6.5 km to the south, on the opposite side of the hanging-wall fold above the central segment of the PHT. These two data sets allow direct comparison of fore-limb and back-limb slip rates determined for the same structure, facilitating accurate determination of the proportion of thrust displacement that is consumed in the development of the hanging-wall fold. Moreover, these new data provide more detailed constraints on both average slip rate of the PHT, a major seismic threat to the highly urbanized Los Angeles metropolitan region, as well as the variability of slip rate through time on this large thrust fault. The rate data will also facilitate comparisons with similar rate data from other faults within the southern California plate boundary fault network.