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Paper No. 134-2
Presentation Time: 9:00 AM-6:30 PM
STRATIGRAPHIC AND MICROFOSSIL EVIDENCE FOR THREE CASCADIA SUBDUCTION ZONE EARTHQUAKES AT JACOBY CREEK, ARCATA BAY, CALIFORNIA, USA
Arcata Bay (northern Humboldt Bay) overlies a locked portion of the southern Cascadia subduction zone, ~60 km north of the Mendocino Triple Junction. Over the past three decades, researchers have reported stratigraphic evidence of sudden relative sea-level rise during subduction-zone earthquakes beneath the salt marshes of Humboldt Bay. However, estimates of such coseismic subsidence, with errors of >±0.5 m, minimally constrain earthquake rupture models. At Jacoby Creek, a small coastal drainage that flows into northern Arcata Bay, we observed three sharp contacts between salt marsh peat and intertidal mud dating from the past ~2000 yrs. Radiocarbon ages, of identifiable plant macrofossils from these buried peats, are 195±15 yr BP, 1280±20 yr BP, and 1710±20 yr BP, which provide maximum age estimates for the buried soil sequences. A coseismic subsidence origin is supported by (1) sharp mud-over-peat contacts that are laterally continuous over 5 km, (2) evidence for long-lasting submergence indicated by changes in fossil foraminifera and diatom assemblages across sharp contacts, and (3) radiocarbon ages, of plant macrofossils taken from buried peat deposits, that agree with other southern Cascadia earthquake chronologies (radiocarbon ages from other buried peat and tsunami deposits). The Jacoby Creek earthquake record is similar to other stratigraphic records at Humboldt Bay, but without the complications introduced by coincident megathrust and upper-plate subsidence events observed in South Bay. We use quantitative transfer function analysis of fossil foraminifera and diatom assemblages at Jacoby Creek to present improved estimates of coseismic subsidence for Arcata Bay. Additionally, we present a refined earthquake chronology, including maximum and minimum ages, for the Jacoby Creek site based on radiometric analyses that suggest the occurrence of Cascadia earthquakes in ~250 yr BP, ~1300 yr BP, and ~1700 yr BP.