Earthquake-Induced Subsidence and Burial of Late Holocene Archaeological Sites, Northern Oregon Coast

Abstract

Fire hearths associated with prehistoric Native American occupation lie within the youngest buried lowland soil of the estuaries along the Salmon and Nehalem rivers on the northern Oregon coast. This buried soil is the result of sudden subsidence induced by a great earthquake about 300 years ago along the Cascadia subduction zone, which extends offshore along the North Pacific Coast from Vancouver Island to northern California. The earthquake 300 years ago was the latest in a series of subsidence events along the Cascadia subduction zone over the last several thousand years. Over the long term, subsidence and burial of prehistoric settlements as a result of Cascadia subduction zone earthquakes have almost certainly been an important factor contributing to the limited time depth of the archaeological record along this section of the North Pacific Coast.

Coastal subsidence in Oregon, USA, during the giant Cascadia earthquake of AD 1700

“…In the silty clay, foraminifera produced estimates of paleomarsh elevations between 0.70 m and 1.16 m MSL, which all have good matching analogues. The post-seismic (E post ) elevation from the sample (0.38 m depth) immediately above the tsunami sand (Briggs, 1994;Nelson et al, 1998) is 0.70 m ± 0.18 m. The minimum coseismic subsidence is 0.85 m. Grant (1994) and Minor and Grant (1996) suggested that Nehalem River experienced a change from a spruce forest to a Triglochin maritima colonized mudflat, producing 1 m to 2 m coseismic subsidence.…”

“…Grant (1994) and Minor and Grant (1996) found widespread stratigraphic and plant macrofossil evidence of subsidence during the AD 1700 earthquake exposed in bank sections over much of the lower estuary that traces landward for hundreds of meters. A sharp upper contact between the buried peaty horizon, including Native American fire hearths (Woodward, 1990), and an overlying muddy or sandy unit infer sudden subsidence during a megathrust earthquake (Grant, 1994;Minor and Grant, 1996). The mean age of eight radiocarbon-dated herbs rooted in the top of the buried peaty horizon at the outcrop at this site is AD 1666-1950 (179±15 14 C yrs BP; Nelson et al, 1995).…”

“…The contemporary data sampled in Hawkes et al (2010) lies across the river to the north (Figure 2c). Grant (1994) and Minor and Grant (1996) described a continuous buried peaty horizon with a sharp contact to an overlying muddy or sandy unit. The peat included Native American fire hearths at one site.…”

“…At Nestucca River estimates of coseismic subsidence from Darienzo (1991), Peterson and Darienzo (1988) and Darienzo et al (1994) m of coseismic subsidence during the AD 1700 earthquake. Grant (1994) and Minor and Grant (1996) used lithology and macrofossils to estimate that as little as 0.5 m or as much as 1.0 m of subsidence could have changed a high marsh to an unvegetated tidal flat in AD 1700.…”

“…We analyzed a vibracore taken on the south side of the river across from the contemporary training set of Hawkes et al (2010). Grant (1994) and Minor and Grant (1996) found widespread stratigraphic and plant macrofossil evidence of subsidence during the AD 1700 earthquake exposed in bank sections over much of the lower estuary that traces landward for hundreds of meters. A sharp upper contact between the buried peaty horizon, including Native American fire hearths (Woodward, 1990), and an overlying muddy or sandy unit infer sudden subsidence during a megathrust earthquake (Grant, 1994;Minor and Grant, 1996).…”