Dispersion of Alluvial Fan Scarp Ages and Epistemic Uncertainty of Cumulative Vertical Separation, Cucamonga Fault, Southern California

Date: 4/24/2019

Time: 06:00 PM

Room: Grand Ballroom

Characterizing cumulative fault displacement is an essential component of geologic slip rate and seismic hazard assessment. However, cumulative fault displacement, measured as vertical separation, is over-dispersed along the strike of the Cucamonga Fault and perhaps other such thrust faults. This observation suggests that the measurement of geologic slip rates may be severely limited by epistemic uncertainty. One probable source of dispersion on the Cucamonga Fault is the range of alluvial fan fault scarp ages: we expect that the avulsing and/or braided channels of an active alluvial fan can yield a patchwork surface with some areas altered recently by erosion or deposition while other areas remain stable. If alluvial fan surfaces are abandoned gradually, rather than instantaneously, then late-stage fan modification could yield considerable dispersion of vertical separations by destroying scarps, even within one geological map unit with one nominal surface age. In order to better understand the role of scarp age variability, we measure 310 morphological ages of fault scarps by solving a linear diffusion equation. Within single map units, ages grow younger with increasing distance from the mountain front, which is consistent with previous interpretations. Within one map unit, we calibrate a linear model relating morphological age to vertical separation at Day Canyon, Rancho Cucamonga, California. This model can explain at least half of the observed dispersion within the same map unit elsewhere along the Cucamonga Fault. Our results suggest that (1) scarp age variability is the source of a considerable fraction of epistemic uncertainty in cumulative thrust displacement and (2) this uncertainty may be resolved with accurate and precise geochronology, perhaps including a combination of radiometric and morphological ages.

Presenting Author: Devin McPhillips

Authors