2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 134-1
Presentation Time: 9:00 AM-6:30 PM


NELSON, Alan R., Geologic Hazards Science Center, U.S. Geological Survey, 1711 Illinois St, Golden, CO 80401, ENGELHART, Simon E., Department of Geosciences, University of Rhode Island, Woodward Hall, 9 East Alumni Avenue, Kingston, RI 02881, WITTER, Robert C., U S Geological Survey, Alaska Science Center, 4210 University Drive, Anchorage, AK 99508 and DUROSS, Christopher B., Geologic Hazards Science Center, U.S. Geological Survey, 1711 Illinois St., Golden, CO 80401, anelson@usgs.gov

After three decades of debate, consensus remains elusive about the rupture lengths and frequency of megathrust earthquakes at the Cascadia subduction zone. Radiocarbon ages for earthquake evidence from coastal wetland stratigraphic sequences generally overlap broad 14C-age intervals for the most widely correlated marine turbidites, triggered by shaking from the greatest (~M9) earthquakes rupturing much of the 1200-km-long subduction zone. But the times and number of lesser great earthquakes (~M8-8.8), which may have ruptured only a few hundred kilometers of the megathrust, are uncertain. Along-strike correlation of coastal earthquake evidence has largely relied on position in a stratigraphic sequence and maximum-limiting 14C ages with errors of decades to hundreds of years.

We consider 180 (72 unpublished) AMS 14C ages from the 12 best-dated sequences along 350 km of the Oregon coast to test along-strike correlations of stratigraphic contacts inferred to mark subsidence during great earthquakes and(or) inundation by their accompanying tsunamis during the past 2000 years. At each coastal site we selected the highest quality (least ambiguous stratigraphic context) plant macrofossils providing the closest maximum- and minimum-limiting ages for each contact predating the ~M9 AD 1700 earthquake. Comparison of OxCal-calculated age models for contacts at each site suggests three closely spaced earthquakes from 700-1200 cal yr BP, an interval previously considered to include coastal evidence for only two earthquakes. If accurate, the comparison implies incomplete stratigraphic records for some great earthquakes at some sites. Product means (with 2s uncertainties) of OxCal age probability distribution functions yield great earthquakes at 798±55(6 sites), 933±59(3 sites), 1124±59(6 sites), 1264±26(5 sites), and 1546±23(11 sites) cal yr BP.