Preprints
https://doi.org/10.5194/esurf-2021-70
https://doi.org/10.5194/esurf-2021-70

  06 Oct 2021

06 Oct 2021

Review status: this preprint is currently under review for the journal ESurf.

A geomorphic-process-based cellular automata model of colluvial wedge morphology and stratigraphy

Harrison Gray1, Christopher DuRoss2, Sylvia Nicovich3, and Ryan Gold2 Harrison Gray et al.
  • 1US Geological Survey, Geosciences and Environmental Change Science Center, Lakewood CO 80225
  • 2US Geological Survey, Natural Hazards Science Center, Golden, CO
  • 3US Bureau of Reclamation, Technical Services Center, Lakewood CO 80225

Abstract. The development of colluvial wedges at the base of fault scarps following normal-faulting earthquakes serves as a sedimentary record of paleoearthquakes and is thus crucial in assessing seismic hazard. Although there is a large body of observations of colluvial wedge development, connecting this knowledge to the physics of sediment transport can open new frontiers in our understanding. To explore theoretical colluvial wedge evolution, we develop a cellular automata model driven by the production and disturbance (e.g. bioturbative reworking) of mobile regolith and fault scarp collapse. We consider both 90° and 60° dipping faults and allow the colluvial wedges to develop over 2,000 model years. By tracking sediment transport time, velocity, and provenance, we classify cells into analogs for the debris and wash sedimentary facies commonly described in paleoseismic studies. High values of mobile regolith production and disturbance rates produce relatively larger and more wash facies dominated wedges, whereas lower values produced relatively smaller, debris facies dominated wedges. Higher lateral collapse rates lead to more debris facies relative to wash facies. Many of the modelled colluvial wedges fully developed within 2000 model years after the earthquake with many being much faster when process rates are high. Finally, for scenarios with the same amount of vertical displacement, different size colluvial wedges developed depending on the rates of geomorphic processes and fault dip. A change in these variables, say by environmental change such as precipitation rates, could theoretically result in different colluvial wedge facies assemblages for the same characteristic earthquake rupture scenario. Finally, the stochastic nature of collapse events, when coupled with high disturbance, illustrate that multiple phases of colluvial deposition are theoretically possible for a single earthquake event.

Harrison Gray et al.

Status: open (until 25 Nov 2021)

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Harrison Gray et al.

Harrison Gray et al.

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Short summary
Some types of big earthquakes create small cliffs or "fault-scarps" around a ~1–3 meters in height, where sediments can pile up and create deposits we call "Colluvial Wedges." Geologists will look at colluvial wedges and use them to understand how often big earthquakes occur. Here we made a computer simulation to find out if the way we think colluvial wedges form works with physics. We found that it does in theory, but there are conditions where it may be more complicated than we expected.