Emerson M Lynch

and 4 more

The active deformation field in subduction forearcs provides critical information about the stress and strain state of the upper plate and its potential for seismogenesis. However, these properties are challenging to quantify in most subduction systems, and in the northern Cascadia forearc, few faults have been identified that can be used to reconstruct the upper plate deformation field. Here we investigate the slip history of the Beaufort Range fault (BRF) on Vancouver Island. This fault was proposed to host the 1946 M7.3 Vancouver Island earthquake, but no surface rupture or evidence of Quaternary activity has been documented, and the stress and strain conditions that promoted this event are poorly understood. We provide the first evidence that the BRF is active, using newly-collected lidar to map topographic scarps along the fault system and to reconstruct slip vectors from offset geomorphic markers. Quaternary deposits and landforms that show increasing magnitude of displacement with age provide evidence for at least three M ~6.5-7.5 earthquakes since ~15 ka, with the most recent event occurring <3-4 ka. Kinematic inversions of offset geomorphic markers show that the BRF accommodates right-lateral transtension along a steeply NE-dipping fault. This fault geometry and kinematics are similar to those modeled for the 1946 earthquake, suggesting that the BRF is a candidate source fault for this event. We find that the kinematics of the BRF are consistent over decadal to millennial timescales, suggesting that this portion of the northern Cascadia forearc has accommodated transtension over multiple earthquake cycles.

Paul Alessio

and 2 more

Overland flow self-organized into rills and eroded colluvium from steep hillslopes after a wildfire in the Santa Ynez Mountains. During 12-15 minutes of runoff, rill erosion generated a slurry that scoured bouldery alluvium from mountain canyons to form large debris flows that severely impacted the community of Montecito, CA. The timing, volume, and peak discharge of the debris flows, and their capacity to scour boulders from canyons, depended on the mechanism and generation rate of the granular fluid (slurry) responsible for the sediment transport. Field surveys and aerial imagery revealed dense networks of rills on bare, burned hillslopes, and levees of fine-grained sediment lining the rill margins indicated that flows were highly viscous and the water and sediment were already intimately mixed before entering stream channels. We mapped networks of rills and measured their cross-sectional geometry to quantify the influences of lithology, hillslope length, gradient, and planform on the sediment volumes released by the rills and their volumetric contribution to the canyon-scale debris flows in six watersheds. We interpreted the mechanism of slurry generation by rill erosion, as far as field evidence will allow, through the lens of field and laboratory experiments conducted by others under comparable conditions. We used the geometry of rills, and the duration of intense rainfall and modelled surface runoff, to develop an empirical model of the rate of slurry generation by rill erosion and to suggest a mechanism by which the necessary intensive mixing occurred.