Quantifying uncertainty in cumulative surface slip along the Cucamonga Fault, a crustal thrust fault in southern California

Devin McPhillips, & Katherine M. Scharer

Published August 8, 2018, SCEC Contribution #8339, 2018 SCEC Annual Meeting Poster #255

Studies of historic earthquake surface ruptures show that displacements along strike are spatially variable. As a result, latest Quaternary slip rates developed from a spatially restricted set of cumulative displacement measurements may not accurately represent fault velocity. Moreover, dispersion that persists over multiple ruptures may stem from complex deformation processes, including fault segmentation, distributed deformation, blind faulting, and rupture nucleation and propagation. Here, we undertake a detailed examination of the dispersion of slip along the Cucamonga Fault. The Cucamonga Fault is part of a network of faults that have generated damaging historical earthquakes in the Los Angeles, California region, and numerous scarps along its ~25 km length are well expressed on alluvial fans. These fans were deposited with conical geomorphic surfaces at intervals throughout late Quaternary time and periodically broken by surface-rupturing earthquakes. We make 310 measurements of vertical separations across the scarps using lidar data. We also re-interpret cosmogenic radionuclide data in order to better understand the alluvial fan surface ages. We show that the along-strike dispersion of the vertical separations cannot be explained by our best estimates of analytical uncertainties alone. Additional, epistemic uncertainties are required. We find that the magnitude of the required epistemic uncertainty is typically larger than analytical uncertainty by a factor of 3 and typically about 20% of the maximum vertical separation. This result holds at several spatial scales, from the full fault length (25 km) to ~2 km sections. We examine three potential sources of epistemic uncertainty and find that neither surface age uncertainty, fault dip, nor anthropogenic landscape alteration are likely sufficient to explain the over-dispersion of the data. As a result, we infer that deformation processes are responsible for a significant fraction of the over-dispersion. We calculate a range of plausible dip-slip rates between 0.4 and 2.6 mm/yr. In light of our results, we suggest that future thrust-fault slip-rate studies adopt an epistemic uncertainty of ~20% for vertical separation measurements, unless there are sufficient data to demonstrate otherwise. Whenever possible, slip rate studies will benefit from additional reporting of the spatial distribution of strain.

Key Words
cumulative slip, slip rate, fault scarp, uncertainty

Citation
McPhillips, D., & Scharer, K. M. (2018, 08). Quantifying uncertainty in cumulative surface slip along the Cucamonga Fault, a crustal thrust fault in southern California. Poster Presentation at 2018 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology