Abstract
This study aims to supplement the paleogeodetic database of past Cascadia subduction zone earthquakes and further our understanding of the Cascadia subduction zone seismic and tsunami hazards. I first address a previously identified spatial gap within the Cascadia paleogeodetic database in southern Cascadia by refining the timing and magnitude of Cascadia subduction zone earthquakes over the past 2000 years in northern Humboldt Bay, California (~44.8°N, -124.2°W). There, I mapped wetland stratigraphy consistent with past megathrust earthquakes across three marshes; Jacoby Creek, McDaniel Creek, and Mad River Slough. To improve the existing paleoseismic chronology at northern Humboldt Bay, I employed Bayesian age modeling based on 21 minimum and maximum limiting ages of short-lived plant macrofossils. These AMS ages found above and below subsidence contacts coupled to the construction of Bayesian age models provide the tightest age distributions for stratigraphic evidence of plate boundary earthquakes along the southern Cascadia coastline over the last 2 ka. These subduction zone earthquakes are dated to CE 1700, ~870 cal yrs BP, ~1125 cal yrs BP, and ~1600 cal yrs BP. I also provide estimates of coseismic subsidence of 0.90 ±0.46m for the 1700 earthquake, 0.39±0.33 m for the ~870 cal yr BP earthquake, 0.99±0.44 m ~1125 cal yr BP earthquake, and ≥0.86 m for the ~1600 cal yr BP earthquake using a validated foraminiferal-based Bayesian transfer function (BTF). To further improve our confidence in the BTF analysis required an evaluation of the stratigraphic and biostratigraphic variability preserved within the wetland stratigraphy across northern Humboldt Bay. Therefore, I compiled a large stratigraphic and biostratigraphic dataset that allowed for inter- and intra-site variability and replicability assessments of foraminiferal BTF coseismic subsidence estimates. I analyzed 26 sediment cores containing the four mud-over-peat contacts; nine for the 1700 contact (average of 0.63 ±0.36 m subsidence), five for the ~870 cal yr BP earthquake (average of 0.39 ±0.35 m), six for the ~1125 cal yr BP earthquake (average of 0.7±0.39 m) and six for the ~1600 cal yr BP earthquake.(≥0.86 m). The estimate for the 1600 cal yr BP earthquake is a minimum because, across the estuary, the contact formed above the upper limit of foraminiferal habitation. Intra-site variability of coseismic subsidence estimates reached a maximum of 0.59 m for the 1700 earthquake at McDaniel Creek. Inter-site averaged coseismic subsidence variability reached a maximum of 0.47 ±0.48 m between McDaniel Creek (0.80 ±0.41 m) and Mad River Slough (0.33 ±0.25
Highlights
Stratigraphy consisting of abrupt mud-over-peat and mud-over-forest soil contacts are the most common signature of coastal coseismic subsidence associated with megathrust deformation in temperate environments (Atwater, 1987; Combellick, 1991; Nelson 1992; Shennan et al, 1996, 1999; Sawai, 2001, 2002; Cisternas et al, 2005)
Our results suggest that northern Humboldt Bay has recorded four Cascadia subduction zone (CSZ) earthquakes over the past 2,000 years and that the magnitude of coseismic subsidence has varied over these earthquake deformation cycles
We studied stratigraphy beneath three tidal marshes that fringe the northern portion of Humboldt Bay, Mad River Slough, McDaniel Creek, and Jacoby Creek, which are protected and managed by U.S Fish & Wildlife Service Humboldt Bay National Wildlife Refuge and the City Arcata, California (Fig.1)
Summary
Stratigraphy consisting of abrupt mud-over-peat and mud-over-forest soil contacts are the most common signature of coastal coseismic subsidence associated with megathrust deformation in temperate environments (Atwater, 1987; Combellick, 1991; Nelson 1992; Shennan et al, 1996, 1999; Sawai, 2001, 2002; Cisternas et al, 2005). Coastal paleogeodetic research revolutionized our understanding of subduction zone hazards by identifying and mapping wetland stratigraphy consistent with megathrust rupture and estimating coseismic vertical deformation across subsidence contacts based on qualitative and semi-quantitative elevation assignments of paleo-depositional environments (Plafker, 1969; Atwater, 1992; Nelson 1992, 1996; Darienzo et al, 1994; Shennan et al, 1996; Atwater and Hemphill-Haley, 1997). Estimates of past megathrust-induced land subsidence (referred to as paleogeodetic estimates) can be derived from analysis of coastal stratigraphic records (e.g., Guilbault, 1995, 1996; Shennan et al, 1996; Hawkes et al, 2010, 2011) These estimates can inform hypothetical rupture scenarios used in seismic and tsunami hazard models (Hyndman and Wang, 1995; Wang et al, 2003; Leonard et al, 2010, Wang et al, 2013, Witter et al, 2013). It is critical to develop the highest-quality and most-widespread paleogeodetic datasets possible
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