Abstract
A new history of great earthquakes (and their tsunamis) for the central and southern Cascadia subduction zone shows more frequent (17 in the past 6700 yr) megathrust ruptures than previous coastal chronologies. The history is based on along-strike correlations of Bayesian age models derived from evaluation of 554 radiocarbon ages that date earthquake evidence at 14 coastal sites. We reconstruct a history that accounts for all dated stratigraphic evidence with the fewest possible ruptures by evaluating the sequence of age models for earthquake or tsunami contacts at each site, comparing the degree of temporal overlap of correlated site age models, considering evidence for closely spaced earthquakes at four sites, and hypothesizing only maximum-length megathrust ruptures. For the past 6700 yr, recurrence for all earthquakes is 370–420 yr. But correlations suggest that ruptures at ∼1.5 ka and ∼1.1 ka were of limited extent (<400 km). if="" so,="" post-3-ka="" recurrence="" for="" ruptures="" extending="" throughout="" central="" and="" southern="" cascadia="" is="" 510–540 yr.="" but="" the="" range="" in="" the="" times="" between="" earthquakes="" is="" large:="" two="" instances="" may="" be="" ∼50 yr,="" whereas="" the="" longest="" are="" ∼550="" and="" ∼850 yr.="" the="" closely="" spaced="" ruptures="" about="" 1.6="" ka="" may="" illustrate="" a="" pattern="" common="" at="" subduction="" zones="" of="" a="" long="" gap="" ending="" with="" a="" great="" earthquake="" rupturing="" much="" of="" the="" subduction="" zone,="" shortly="" followed="" by="" a="" rupture="" of="" more="" limited="" extent.="" the="" ruptures="" of="" limited="" extent="" support="" the="" continued="" inclusion="" of="" magnitude-8="" earthquakes,="" with="" longer="" ruptures="" near="" magnitude="" 9,="" in="" assessments="">
Keywords
Bayesian age models, Earthquake and tsunami hazards, Earthquake contacts, Maximum rupture model, Megathrust earthquake recurrence, Paleoseismology, Radiocarbon dating