Similarities in number, depth, sequence stratigraphy, and radiocarbon ages characterize buried peats of seven estuaries along 175 Ian of the northern Oregon coast. We use these peats to infer the extent of earthquake-induced subsidence, earthquake magnitudes, and average recurrence intervals for late Holocene earthquakes at the Cascadia Subduction Zone. Synchroneity of earthquake-induced subsidence from Alsea Bay to the Necanicum River over a coastal distance of 175 Ian is inferred most confidently for the most recent (first) event and the third through sixth events. In contrast, earthquake-induced subsidence for the second event was lacking in at least three of the seven estuaries. However, tsunamis generated by the second event deposited sands in the unsubsided estuaries. Therefore, the second event is also considered synchronous between Alsea and Necanicum. A segment boundary between Yaquina and Netarts is inferred for the second event. From these fmdings of synchroneity, we esti...

Coastal deposits at Tofino, Ucluelet, and Port Alberni in Vancouver Island along the Cascadia subduction zone were re-examined to improve the earthquake history of the southwest coast of Canada. We found sand sheets interbedded within peat and mud, suggesting deposition by strong flows in a low-energy environment. Based on limiting maximum and minimum ages derived from plant macrofossils, the age of one of the sand sheets below the tsunami deposits of the great Cascadia earthquake in 1700 CE was estimated to be 1330–1430 CE. Onshore paleoseismic evidence has been documented in Vancouver Island, northern Washington, and northern Oregon during this period. However, the newly constrained age is between those of coseismic subsidence Y and W events in southern Washington, which have been recognized as the 1700 CE and the penultimate Cascadia earthquakes, respectively. Moreover, the new age partly overlaps with the age of offshore paleoseismic evidence for T2, interpreted to have originat...

Peaty, tidal-marsh soils interbedded with estuarine mud in late Holocene stratigraphic sequences near Coos Bay, Oregon, may have been submerged and buried during great (M > 8) subduction earthquakes, smaller localized earthquakes, or by nontectonic processes. Radiocarbon dating might help distinguish among these alternatives by showing that soils at different sites were submerged at different times along this part of the Cascadia subduction zone. But comparison of conventional 14C ages for different materials from the same buried soils shows that they contain materials that differ in age by many hundreds of years. Errors in calibrated soil ages represent about the same length of time as recurrence times for submergence events (150–500 yr)—this similarity precludes using conventional 14C ages to distinguish buried soils along the southern Oregon coast. Accelerator mass spectrometer 14C ages of carefully selected macrofossils from the tops of peaty soils should provide more precise...