Sediment routing across the southern Cascadia subduction margin with implications for earthquake magnitude and recurrence

Understanding the recurrence and magnitude of megathrust (≥M_w8) earthquakes is crucial for assessing hazard and risk. The existing recurrence record along the Cascadia subduction margin is inferred primarily from synchronous, abyssal turbidites found along the entire margin, which are assumed to have resulted from flows that initiated up-system from strong, regional shaking causing sediment transport­ down submarine canyons. Yet, the sources and sediment pathways of flows that deposited the abyssal turbidites are poorly constrained. This leaves uncertainty as to whether synchronous turbidites along the subduction zone all reflect megathrust earthquakes with large rupture areas that induce flows from shelf to trench, or rather record failures more proximal to the subduction front, which may indicate more local and lower-magnitude shaking. To better characterize source, transport pathways, and sediment storage in southern Cascadia, we investigate a variety of depositional environments in the low-gradient, mid-slope Eel River forearc basin, which appears to store sediment and restrict transfer from the shelf to deep sea throughout Holocene time. A suite of piston cores (~3-7 m in length) were collected at water depths of ~900-1300 m. Additionally, we anticipate collecting vibracores (≤1.7 m in length) using a remotely operated vehicle (ROV) to sample at greater water depths (>1500 m) and with higher precision. Cores collected to date within and at the head of Klamath Canyon, located down-slope of Eel River basin, lack Holocene turbidite deposits observed elsewhere in the basin. This suggests a lack of connectivity of the Eel River basin to this lower-slope canyon during the Holocene. Vibracores will be collected in transects across converging channel thalwegs within the neighboring Trinidad Canyon system to further test for synchronicity and extent of recent turbidite-generating events along the mid-lower slope. We will also sample the deformation front itself, to test whether or not the accretionary wedge could be a proximal source for abyssal turbidites. Further investigation of sediment remobilization, transport, and storage is fundamental for using the turbidite record to infer past megathrust rupture behavior.