Implications for fault segmentation and rupture connectivity from high spatial-resolution geomorphic data, rupture forecasting, and Boundary Element Method modeling

Devin McPhillips, Andre Mere, & Scott T. Marshall

Published September 8, 2024, SCEC Contribution #13720, 2024 SCEC Annual Meeting Poster #093

Paleoseismologists aspire to reconstruct prehistoric earthquake ruptures with length and displacement resolved at the sub-kilometer scale, approaching the resolution of modern rupture data, but the geologic record rarely supports such detail. Here, we show that hillslopes and stream channels in the range front of the San Gabriel Mountains reveal kilometer-scale spatial patterns in long-term reverse-slip rate along the adjacent Sierra Madre and Cucamonga faults. We calibrate a stream channel incision model and a bedrock hillslope erosion model with Be-10 catchment-average erosion rates; both models yield uplift rates that are consistent with vertical separation rates measured on fault-offset alluvial fans. The calculated uplift rates show arcuate along-strike patterns, with the lowest values near the ends of the respective faults and highest (3x) values near the centers of the faults. To aid interpretation, we compare these results with long-term slip-rate predictions from Boundary Element Method (BEM) modeling and the 2023 National Seismic Hazard Model (NSHM). BEM results also show arcuate patterns in reverse-slip rate, but the position of the minimum value between the Sierra Madre and Cucamonga faults is different than that calculated from geomorphic data. Arcuate patterns in BEM reverse-slip rates reflect structural fault segmentation, where displacements necessarily taper to near zero at fault tips. The arcuate patterns in geomorphology-derived reverse-slip rates suggest that the two faults are indeed separate structures with distinct terminations. The different locations of this slip-rate minimum in the BEM and geomorphology results arises from the “Sierra Madre-Cucamonga fault connector” in the BEM model geometry, which was derived from the SCEC Community Fault Model: our results suggest this structure should be connected to the Sierra Madre fault, but not the Cucamonga fault, opposite to its current configuration. The NSHM includes an ensemble of possible earthquake ruptures, including connected ruptures of adjacent faults. We use this ensemble to explore the implications of the high-resolution slip-rate profile for rupture connectivity. We test competing hypotheses: either the faults rupture independently, or displacement drops to near zero at fault tips even during joint ruptures, as observed in recent earthquakes (e.g., 2016 M 7.8 Kaikoura). This work highlights new applications for geomorphic data to earthquake process and hazard.

Key Words
geomorphology, segmentation, connectivity, community fault model

Citation
McPhillips, D., Mere, A., & Marshall, S. T. (2024, 09). Implications for fault segmentation and rupture connectivity from high spatial-resolution geomorphic data, rupture forecasting, and Boundary Element Method modeling . Poster Presentation at 2024 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology