Development of the Geomorphic Indicator Ranking System for pre-rupture fault mapping

Rachel N. Adam, Chelsea P. Scott, Ramon Arrowsmith, Malinda G. Zuckerman, Christopher M. Madugo, Rich D. Koehler, Darryl Reano, & Ozgur Kozaci

Published September 11, 2022, SCEC Contribution #12521, 2022 SCEC Annual Meeting Poster #111 (PDF)

Pre-rupture fault mapping in advance of major earthquakes is a tool used to characterize potential fault rupture hazard. Fault trace mapping is usually guided by geomorphic features that indicate prior tectonic disruption of the landscape.Geologists often identify geomorphic features such as scarps, triangular facets, lineations, etc. to aid in mapping. Judgment-based mapping can result in subjective variation between how student and professional mappers (or even different professional mappers) choose to interpret a landscape and produce a fault trace map.

We developed the Geomorphic Indicator Ranking System (GIR) with the goal to standardize the mapping of geomorphic features in the landscape and reduce epistemic uncertainty in desktop-based fault mapping. Geomorphic features receive a quantitative ‘score’ that ranges from 1 - 4 depending on the geomorphic strength for a fault (triangular facets and scarps are more indicative of faulting than lineaments). These features help the mapper place a certainty level (strong, distinct, weak, uncertain) on the mapped fault traces. We model our approach after Scharer et al. (2007) who assigned paleo-earthquake likelihood based on a systematic grading of the quality of the paleoseismic evidence.

We tested the GIR tool with student mappers in a pre-rupture fault mapping course co-taught by Arizona State University and University of Nevada, Reno in Spring 2022. After the course concluded, a group of professional geologists completed fault maps using the same approach. Our goal is to determine whether the GIR can be an effective framework to document the evidence that supports pre-rupture fault trace maps and to make the maps more consistent across mappers of different experience levels.

Further work includes comparing the final confidence ranking for the mapped fault traces determined strictly by the scaled GIR score versus the mapper’s geologic intuition. Meaning, the confidence ranking for a fault trace segment will differ depending on whether the mapper uses their best judgment to assign a confidence ranking or they use the quantitative approach (where the scores for the GIR features are counted and scaled to a 1-4 score that corresponds with fault confidence strength). This analysis will inform the efficacy of using the GIR, the discrepancies between the quantitative and qualitative approach to the mapping, and determine which geomorphic features indicate active faulting better than other features.

Key Words
Probabilistic Fault Displacement Hazard Assessment, Geomorphic Indicator Ranking

Citation
Adam, R. N., Scott, C. P., Arrowsmith, R., Zuckerman, M. G., Madugo, C. M., Koehler, R. D., Reano, D., & Kozaci, O. (2022, 09). Development of the Geomorphic Indicator Ranking System for pre-rupture fault mapping. Poster Presentation at 2022 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology