Constraining the aleatoric variability in rupture location over the ~100 m-scale based on a formalized analysis of past paleoseismic trenching studies

Ramon Arrowsmith, Chelsea P. Scott, Christopher M. Madugo, & Albert R. Kottke

Published September 8, 2024, SCEC Contribution #13919, 2024 SCEC Annual Meeting Poster #104 (PDF)

Paleoseismic trench investigations provide a direct indication of fault location and the distribution of faulting across the fault zone. This analysis addresses aleatoric variability in rupture characteristics (of relevance to fault displacement hazard analysis: e.g., location, localization, slip, deformation style) from event to event and as possibly controlled by the context such has the recurrence interval, slip rate and site materials (e.g., bedrock, hillslope, fan, sag fill.). We have developed a systematized approach for documenting results from paleoseismic trenching studies. Our Matlab-based tool is designed to enable efficient and objective review and data accumulation. This work builds on the Chen and Dawson USGS Final Technical report (2014).

The workflow to document fracture location from paleoseismic trenches has seven main steps: 1) collect and geolocate trench logs. 2) Record metadata for site/log: operator, slip sense, fault, site, trench, event, references, notes. 3) Record: Geologic unit, tectonic landform(s), landform to trench distance, angle from fault trench to landform. 4) From the trench log document lithofacies, slip, and maximum age. 5) Perform fracture analysis on trench image: set scale, digitize upper fracture tips, label primary fracture (largest displacement) and secondary fractures, and label fractures of most recent event (MRE). From those data, 6) Compute geometric parameters (trench width, fault zone width, trench half width and fault zone half width (two distances to edge of trench or factures from the principal fault); number of fractures, max distance from MRE fractures to any fracture). And 7) Record information in master table and export the fracture tip positions.

In our exemplary strike-slip case study at the Bidart site along the San Andreas Fault in the Carrizo Plain, central California, we examined the fracture patterns in ten fault crossing trench exposures from trench lengths up to 40 m. Fault location is exceptionally well indicated by tectonic landforms including aligned offset channels and small pop ups and sags, along the paleo-moletrack. The materials surrounding the fault are bedded sands and gravels. We find that the fault zone width is 10-40 meters as defined by 15 to 45 fractures defined by up to 5 major fault zones cutting units with a maximum age of 1200 calAD. The most recent rupture width is 12 meters generally centered on the cumulative rupture zone.

Key Words
fault displacement hazard

Citation
Arrowsmith, R., Scott, C. P., Madugo, C. M., & Kottke, A. R. (2024, 09). Constraining the aleatoric variability in rupture location over the ~100 m-scale based on a formalized analysis of past paleoseismic trenching studies. Poster Presentation at 2024 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology