An application of luminescence sediment tracing using a portable luminescence reader in two-dimensions to evaluate the colluvial wedge sedimentological model

The removal and regeneration of luminescence by sunlight and burial offers a new way to evaluate the transport paths of sediment. This luminescence sediment tracing can provide new data to evaluate various models in geomorphology and sedimentology. One such evaluation can be done with the Colluvial Wedge Model, which is an interpretive framework to understand the sequence of earthquake events in the depositional features known as colluvial wedges created at the base of vertical fault scarp exposures. The colluvial wedge model was heuristically developed from experiments and observations of modern fault scarps and underlies many of the estimates of earthquake timing, recurrence, and fault slip rate that form critical inputs to seismic hazard models. However, there appears to be no sediment tracing data to evaluate the model over the thousands of years in which colluvial wedges develop. Here, we adapt the colluvial wedge model into predictions of the spatial distribution of luminescence across a colluvial wedge in in tandem with predictions from the development of a physics-based model of colluvial wedge formation using a continuous-time cellular automata model. We compare these predicted colluvial-wedge luminescence distributions to 342 samples measured using a portable luminescence reader in a grid-like pattern across a natural exposure displaying classic colluvial wedge features. We interpolated the portable luminescence measurements to create a map of the spatial distribution of luminescence. The luminescence map, reveals complex depositional dynamics relating to the interplay between fast gravitationally-driven collapse and slower diffusive-like processes such as bioturbation and surface wash sedimentation. The luminescence map, coupled with results from the physics-based cellular automata model, reveals nuance and dynamics that can be incorporated into the colluvial wedge conceptual model. This evaluation suggests that paleoseismologists applying the colluvial wedge model must be mindful that colluvial wedges result from a complex interplay of depositional processes and that the interpretation of paleoseismic data, including geochronology, should consider these complex dynamics.