Stratigraphic Evidence of Cascadia Earthquake Induced Relative Sea Level Change at Netarts Bay, Oregon

I present new dendrochronologic age constraints from a buried tree slab collected below the marsh surface at Netarts Bay, Oregon. A ghost forest is apparent at low tide on the central-eastern margin of Netarts Bay. 120 meters inland from the bay front, incision into marsh exposures reveal buried trees protruding 1.43 meters below the marsh surface and extending from the 4th layer of buried marsh soils overlain by tsunami sands. I interpret this stratigraphic sequence as evidence for tectonic subsidence and abrupt paleotsunami inundation. Prior research at the southern marsh at Netarts Bay has correlated Cascadia Subduction Zone (CSZ) ruptures to the stratigraphic record with sediment cores that have a pattern of marsh soils overlain by a sharp contact to sand, transitioning to mud and then marsh soils again, bulk sample radiocarbon dating, and diatom analysis. Sediment cores collected at the ghost forest site show the same stratigraphic pattern indicative of CSZ related subsidence. I correlated ghost forest trees exposed along the bay with trees protruding from incised marsh stratigraphy with both differential GPS and lidar elevation measurements for several ghost forest trees. I collected a cross section slab from a buried ghost forest tree, sanded the slab, counted growth rings at a consistent decadal interval, and submitted select samples for C14 accelerator mass spectrometry age dating. The species of the tree is an Oregon Ash (Fraxinus latifolia) which has no salinity tolerance and can grow close to sea level. I have wiggle matched C14 ages and the known interval between tree ring samples to the C14 calibration curve using OxCal 4.3, to find that the tree died 286-392 CE at a 95.4% confidence interval, and 290-313 CE at a 68.2% confidence interval. I compared the dendrochronologically constrained death of the trees with other on-shore and off-shore paleoseismic studies that constrained the timing of prehistoric CSZ ruptures, indicating that this was the 5th prehistoric earthquake back from present. Introduction Globally, prehistoric records of subduction zone earthquakes are determined by evaluating stratigraphic evidence of uplift and subsidence along the margin in the over-riding plate within a subduction zone. The subducting plate is locked by friction against the over-riding plate in the interseismic period between earthquakes. Accumulated strain is released in a coseismic earthquake when friction is overcome and slip occurs releasing the strain, resulting in a subduction zone earthquake (Yeats, 2004). In the interseismic period, strain accumulates causing the over-riding crust to bend to accommodate the strain over the length of the interseismic period (Yeats, 2004). Along the trench, the edge of the over-riding plate gets pulled downward along the plate interface, while on-shore the crust experiences uplift. A point in between these two regions of deformation acts as a hinge as they deform up and down simultaneously (Darienzo & Peterson, 1990; Yeats, 2004). In a coseismic earthquake on-shore subsidence occurs instantaneously, an example of elastic rebound, as the strain is released and interseismic uplifted regions subside and the crust resumes its pre-strain shape. The toe of the over-riding plate is uplifted during the earthquake as subsidence occurs on the other side of the hinge point (Yeats, 2004). Stratigraphic evidence of these types of earthquakes ranges from uplifted corals in Indonesia along the Sumatra subduction zone (Meltzner et al., 2012), to ghost forests in the Pacific Northwest along the Cascadia Subduction Zone (Atwater et al., 1991). Due to the vertical crustal movement associated with subduction zone earthquakes, tsunamis are caused by the displacement of water from the over-riding plate’s toe uplifting along the margin during the rupture. Figure 1: Map of Cascadia Subduction Zone with a star at Netarts Bay (Darienzo & Peterson, 1990). The Cascadia Subduction Zone (CSZ) lies off-shore of the Pacific Northwest from Northern California to British Columbia, Canada (Figure 1), and is shown as a thrust fault with triangles on the over-riding plate in Figure 1. Coastal regions along the CSZ experience interseismic uplift and instantaneous coseismic subsidence and have well established stratigraphic earthquake and tsunami records for 19 to 20 full length subduction zone earthquakes (Goldfinger et al., 2012; Goldfinger et al., 2016). The purpose of this project is to evaluate sequences of intertidal marsh and tsunami stratigraphy and an apparent ghost forest identified at Netarts Bay (Figure 2) to understand relative seal level drop associated with the 5th earthquake back in the Cascadia paleoseismic records. Prior research in Netarts Bay was conducted along a marsh at the south end of the bay (Peterson & Darienzo, 1988; Darienzo, 1991). Sediment cores along the shoreline reveal a repeating several hundred-year cycle of relative sea level change. The cores have a pattern of peat overlain by a sharp contact to sand, transitioning to mud and then peat again, which has been interpreted as evidence for tectonic subsidence and abrupt paleotsunami inundation (Peterson & Darienzo, 1988; Darienzo & Peterson, 1990; Darienzo, 1991; Darienzo & Peterson, 1995). On the central-eastern margin of the bay a ghost forest is apparent during low tides, elevation data for these trees and stratigraphic correlations from exposed stratigraphy reveal that these deceased trees extend from a layer of marsh soils overlain by tsunami sands. The death of these trees corresponds with coastal subsidence dropping a mature forest into the intertidal zone. This project uses carbon-14 (C14) wiggle matching from tree growth ring samples to find a precise age for timing of when the trees died. I have compared the dendrochronologically constrained death of the trees with the timing of the paleoseismic history of CSZ ruptures to identify it as the 5th prehistoric earthquake. Background Offshore along the Pacific Northwest the Juan de Fuca (JdF) oceanic plate dives beneath the North American plate forming the Cascadia Subduction Zone. In the cold upper crust, the subducting JdF plate is locked by friction with the over-riding North American plate and the strain accumulates in the crust and causes uplift along the Pacific Northwest coast up to about 50 kilometers inland (Witter, 2011). When enough strain has accumulated and friction at the contact between the plates is overcome, the fault slips and the strain is released as a coseismic event occurs, and the regions that experienced uplift in the interseismic period subsides up to 2 m in Oregon, and up to 1 m for Netarts Bay (Darienzo & Peterson, 1990; Witter et al., 2011). The CSZ last ruptured 319 years ago and has an average recurrence interval of 430 years for the northern segment, and 340 years in central Oregon (Goldfinger et al., 2016). During each rupture, elastic rebound causes the uplifted coastline to subside submerging anything along the shore (Darienzo & Peterson, 1990; Peterson et al., 1993; Yeats, 2004). Grassy wetlands in Netarts Bay and other locations along the coast have been subjected to the uplift and subsidence cycle, which is recorded in the stratigraphy when wetlands are buried by tsunami sand and succeeded by intertidal mud (Peterson & Darienzo, 1988; Peterson et al., 1993). The wetlands die once buried becoming the peat layers seen in sediment cores that contain roots and small sticks (Peterson & Darienzo, 1988; Darienzo & Peterson, 1990). This organic material can be radiocarbon dated to provide the maximum age of the burial event caused by the earthquake. Organics from the uppermost buried peat layer identified in sediment cores taken at the south end of Netarts Bay yielded C14 ages of 290-520 calendar years before present (cal. BP) (Peterson & Darienzo, 1988; Peterson et al., 1993). The C14 ages overlap in time and correspond with the last full-length rupture of the CSZ 319 years ago (Peterson & Darienzo, 1988; Peterson et al., 1993; Goldfinger et al., 2016). The 4th peat layer down the stratigraphic core was dated to 1354-1769 cal. BP and the 5th peat layer to 1540-1869 cal. BP (Darienzo, 1991), representing Source: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AeroGRID, IGN, and the GIS User Community 123°56'0"W 123°56'0"W 123°58'0"W 123°58'0"W