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Beyond COPD: Assumptions, applications, and new approaches for interpreting the geomorphic record of strike slip

Nadine G. Reitman, Yann Klinger, Rich Briggs, & Ryan Gold

Submitted September 11, 2022, SCEC Contribution #12182, 2022 SCEC Annual Meeting Poster #089

The cumulative offset probability density (COPD) method is the primary way that researchers interpret paleoslip distributions from offset geomorphic landforms along strike-slip faults. The COPD method assumes earthquakes have constant slip along a fault or fault section, average slip is represented by the most common (modal) measurement values, and the offset landforms included in the dataset accurately record earthquake slip and haven’t been significantly modified by geomorphic processes. These assumptions are challenged by earthquakes that have high slip variability along fault strike (e.g., 2019 Ridgecrest) and the pervasive effects of geomorphic evolution and channel aliasing on offset landforms. Here, we evaluate the core assumptions and application of the COPD method by applying 1D and 2D COPD approaches to (1) idealized slip distribution data with and without noise, (2) slip distribution data from 40 recent ruptures, and (3) offset measurement data from 26 faults with geomorphic evidence of multiple earthquakes. We evaluate if modal measurement values correlate with average slip and if recent ruptures can be represented by a single COPD peak. We find that the 1D COPD approach oversimplifies the spatial variability exhibited in recent ruptures, peaks do not accurately represent theoretical earthquake slip distributions, and 2D approaches better represent spatial and temporal complexity in multiple earthquake datasets. Finally, we preview a preliminary method for interpreting slip history from the geomorphic record of offset landforms based on a 2D COPD and inspired by the “stringing pearls” approach (Biasi & Weldon 2009) for paleoseismic sites.

Reitman, N. G., Klinger, Y., Briggs, R., & Gold, R. (2022, 09). Beyond COPD: Assumptions, applications, and new approaches for interpreting the geomorphic record of strike slip. Poster Presentation at 2022 SCEC Annual Meeting.

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Earthquake Geology