AN INCOMPLETE HISTORY OF LATE HOLOCENE GREAT EARTHQUAKES AT THE NEHALEM RIVER ESTUARY, CENTRAL CASCADIA SUBDUCTION ZONE

Consensus remains elusive about the rupture lengths, magnitudes, and frequency of megathrust earthquakes of the past few thousands of years at the Cascadia subduction zone. Along-strike correlation of coastal earthquake evidence has relied on the position of stratigraphic contacts produced by sudden (coseismic) subsidence and tsunami deposits within tidal wetland stratigraphic sequences and maximum-limiting ¹⁴C ages with errors of decades to hundreds of years. Numbers and times of lesser great earthquakes (~M8-8.8) are especially uncertain. Reconstruction of a more complete earthquake history requires comparison of precisely dated evidence at many sites along the subduction zone.

Here we report a history of great earthquakes and accompanying tsunamis along the shores of the lower Nehalem River in northern Oregon (lat 45.7°N). Despite the abundant sediment supply in this estuarine lowland, we found evidence of only four earthquakes in the past 3000 years. Fluvial sediment underlies many wetlands in the lower valley and many marshes formed in the past few centuries. Fossil foraminifera (60 samples) from a 7-cm-diameter vibracore in older tidal sediment yield transfer function estimates of coseismic subsidence for the three youngest earthquake contacts that vary from 0 to 0.7±0.3 m, and diatom analyses (160 samples) confirm the rapid changes in tidal environments across the contacts. Despite many new precise ¹⁴C ages for contacts at Nehalem and other Oregon estuaries, correlating earthquake evidence among sites remains uncertain. Comparison of OxCal-calculated age models for earthquake contacts at Nehalem with age models for other sites suggests three closely spaced earthquakes during the interval 700-1200 cal yr BP, only two of which have been identified at Nehalem (~850 and ~1100 cal yr BP). The absence of evidence for a third earthquake at Nehalem, and our difficulties in mapping subsidence contacts for pre-AD-1700 earthquakes in the lower estuary, suggest similarly incomplete records of earthquakes at other Cascadia coastal sites. Such incompleteness is likely the result of minimal subsidence (<0.2 m) during lesser earthquakes, as apparently recorded by one contact at Nehalem, and of differences in the abilities of individual sites for recording and preserving stratigraphic evidence of earthquakes and tsunamis.