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.
Highlights
Characterizing the seismic hazard posed by major faults partly relies on understanding the history of prehistorical surfacerupturing earthquakes
We lack an ability to quantitatively predict the form and facies of colluvial wedges under varying environmental conditions, such as climate or lithology. We desire this predictive power so that we can develop knowledge toward understanding broader questions such as: 1) under what environmental conditions do you preserve a post-earthquake colluvial wedge; 2) are 40 there conditions when a fault-scarp-generating earthquake does not produce a wedge, 3) How do these environmental conditions influence wedge morphology and internal stratigraphy; 4) are there geomorphic conditions that produce stratigraphy that can be misinterpreted as more than one earthquake event, and 5) what sort of time delay is expected between earthquake event and wedge formation? Here we propose a theoretical model of colluvial-wedge formation and stratigraphy that can provide the basis for quantitatively exploring these questions and improving our understanding of seismic hazards
We find that the 150 minimum processes needed to produce colluvial wedges are mobile regolith production, sometimes referred to as soil production in the geomorphology literature (e.g. Heimsath et al, 1997), mobile regolith disturbance, roughly equivalent to ‘soil diffusivity’, and gravitational collapse
Summary
Characterizing the seismic hazard posed by major faults partly relies on understanding the history of prehistorical surfacerupturing earthquakes. To obtain this history, we must constrain the timing of past earthquakes. The success of these dating methods often depends on how they are applied to sediments within a fault zone, regarding stratigraphic location. One such postearthquake deposit, the fault-scarp-derived colluvial wedge 30 (Malde, 1971; Swan et al, 1980; Schwartz and Coppersmith, 1984; McCalpin et al, 2009), is deposited immediately to thousands of years following fault rupture (e.g., Wallace, 1977). The paleoseismic analysis and interpretation of colluvial wedges directly feeds into seismic hazard assessments that in turn affect lives and livelihoods
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