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

[摘要] The development of colluvial wedges at the base of faultscarps following normal-faulting earthquakes serves as a sedimentary recordof paleoearthquakes and is thus crucial in assessing seismic hazard.Although there is a large body of observations of colluvial wedgedevelopment, connecting this knowledge to the physics of sediment transportcan open new frontiers in our understanding. To explore theoreticalcolluvial wedge evolution, we develop a cellular automata model driven bythe production and disturbance (e.g., bioturbative reworking) of mobileregolith and fault-scarp collapse. We consider both 90 and60 ∘ dipping faults and allow the colluvial wedges to develop over2000 model years. By tracking sediment transport time, velocity, andprovenance, we classify cells into analogs for the debris and washsedimentary facies commonly described in paleoseismic studies. High valuesof mobile regolith production and disturbance rates produce relativelylarger and more wash-facies-dominated wedges, whereas lower values producedrelatively smaller, debris-facies-dominated wedges. Higher lateral collapserates lead to more debris facies relative to wash facies. Many of themodeled colluvial wedges fully developed within 2000 model years after theearthquake, with many being much faster when process rates are high. Finally,for scenarios with the same amount of vertical displacement, differently sizedcolluvial wedges developed depending on the rates of geomorphic processesand fault dip. A change in these variables, say by environmental change suchas precipitation rates, could theoretically result in different colluvialwedge facies assemblages for the same characteristic earthquake rupturescenario. Finally, the stochastic nature of collapse events, when coupledwith high disturbance, illustrates that multiple phases of colluvialdeposition are theoretically possible for a single earthquake event.

[学科分类] 土壤学