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.

[1]  J. Southon,et al.  Radiocarbon Dating of Pollen by Accelerator Mass Spectrometry , 1989, Quaternary Research.

[2]  John B. Adams,et al.  Geodetic evidence for active landward tilting of the Oregon and Washington Coastal Ranges , 1982 .

[3]  H. Nelson.  Late Pleistocene and Holocene depositional trends, processes, and history of Astoria deep-sea fan, Northeast Pacific , 1976 .

[4]  S. P. Nishenko,et al.  A generic recurrence interval distribution for earthquake forecasting , 1987 .

[5]  O. Pilkey.  Basin plains; Giant sedimentation events , 1988 .

[6]  L. Kulm,et al.  Some observations of near‐bottom currents in deep‐sea channels , 1972 .

[7]  R. Hyndman,et al.  Canada's Active Western Margin: The Case for Subduction , 1976 .

[8]  John Adams,et al.  Active deformation of the Pacific Northwest Continental Margin , 1984 .

[9]  W. Barnard.  The Washington continental slope: Quaternary tectonics and sedimentation , 1978 .

[10]  Paula J. Reimer,et al.  A Computer Program for Radiocarbon Age Calibration , 1986, Radiocarbon.

[11]  R. Fryxell.  Mazama and Glacier Peak Volcanic Ash Layers: Relative Ages , 1965, Science.

[12]  J. R. Duncan.  Late Pleistocene and postglacial sedimentation and stratigraphy of deep-sea environments off Oregon , 1968 .

[13]  Emery I. Balazs,et al.  Geodetic evidence for aseismic subduction of the Juan de Fuca Plate , 1979 .

[14]  B. F. Atwater,et al.  Evidence for Great Holocene Earthquakes Along the Outer Coast of Washington State , 1987, Science.

[15]  Gary B. Griggs,et al.  Cascadia channel : the anatomy of a deep-sea channel , 1968 .

[16]  C. Bacon.  Eruptive history of Mount Mazama and Crater Lake Caldera, Cascade Range, U.S.A. , 1983 .

[17]  R. Sternberg.  Transport and accumulation of river-derived sediment on the Washington continental shelf, USA , 1986, Journal of the Geological Society.

[18]  Bruce C. Heezen,et al.  Turbidity currents and submarine slumps, and the 1929 Grand Banks [Newfoundland] earthquake , 1952 .

[19]  G. Griggs,et al.  Deep-sea sedimentation and sediment-fauna interaction in Cascadia Channel and on Cascadia Abyssal Plain☆ , 1969 .

[20]  G. Griggs,et al.  Sedimentation in Cascadia Deep-Sea Channel , 1970 .

[21]  S. Hartzell,et al.  Earthquake Hazards on the Cascadia Subduction Zone , 1987, Science.

[22]  Thomas H. Heaton,et al.  Source characteristics of hypothetical subduction earthquakes in the northwestern United States , 1986 .

[23]  D. DeMaster,et al.  Accumulation of modern sediment in Quinault submarine canyon , 1986 .

[24]  Garry C. Rogers,et al.  An assessment of the megathrust earthquake potential of the Cascadia subduction zone , 1988 .

[25]  Donald O. West,et al.  Coastline uplift in Oregon and Washington and the nature of Cascadia subduction-zone tectonics , 1988 .

[26]  Thomas H. Heaton,et al.  Seismic potential associated with subduction in the northwestern United States , 1984 .