Paleoseismicity of the Cascadia Subduction Zone: Evidence from turbidites off the Oregon‐Washington Margin

Cores from Cascadia deep‐sea channel contain sequences of turbidites that can be correlated and dated by the first occurrence of volcanic glass from the Mount Mazama eruption (6845±50 radiocarbon yrBP). Turbidity currents from the tributaries appear to have occurred synchronously to form single deposits in the main channel, there being only 13 turbidite deposits in the lower main channel since the Mazama eruption, instead of the twice as many expected if the tributaries had behaved independently. In addition to the Cascadia Channel, 13 post‐Mazama turbidites have been deposited in the Astoria Canyon and at two sites off Cape Blanco, sample locations that span 580 km of the Oregon‐Washington margin. Pelagic intervals deposited between the turbidites suggest that in each place the turbidity currents occurred fairly regularly, every 590±170 years on average. The best explanation of the spatial and temporal extent of the data is that the turbidity currents were triggered by 13 great earthquakes on the Cascadia subduction zone. The variability of turbidite timing is similar to that for great earthquake cycles. The thickness of the topmost pelagic layer suggests the last event was 300±60 years ago (from three places along the margin), but this number may be a biased underestimate. It is, however, consistent with the youngest sudden‐subsidence event on the Washington coast. The turbidite data demonstrate that the near‐term hazard of a great earthquake on the Cascadia subduction zone is of the order of 2–10% in the next 50 years.