Beach Response to Subsidence Following a Cascadia Subduction Zone Earthquake Along the Washington-Oregon Coast

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.

Download Full-text

Discordant 14C Ages from Buried Tidal-Marsh Soils in the Cascadia Subduction Zone, Southern Oregon Coast

Quaternary Research ◽

10.1016/0033-5894(92)90031-d ◽

1992 ◽

Vol 38 (1) ◽

pp. 74-90 ◽

Cited By ~ 42

Author(s):

Alan R. Nelson

Keyword(s):

Subduction Zone ◽

Tidal Marsh ◽

Late Holocene ◽

Cascadia Subduction Zone ◽

Buried Soils ◽

Oregon Coast ◽

Coos Bay ◽

Accelerator Mass Spectrometer ◽

The Times ◽

Stratigraphic Sequences

AbstractPeaty, tidal-marsh soils interbedded with estuarine mud in late Holocene stratigraphic sequences near Coos Bay, Oregon, may have been submerged and buried during great (M > 8) subduction earthquakes, smaller localized earthquakes, or by nontectonic processes. Radiocarbon dating might help distinguish among these alternatives by showing that soils at different sites were submerged at different times along this part of the Cascadia subduction zone. But comparison of conventional 14C ages for different materials from the same buried soils shows that they contain materials that differ in age by many hundreds of years. Errors in calibrated soil ages represent about the same length of time as recurrence times for submergence events (150–500 yr)—this similarity precludes using conventional 14C ages to distinguish buried soils along the southern Oregon coast. Accelerator mass spectrometer 14C ages of carefully selected macrofossils from the tops of peaty soils should provide more precise estimates of the times of submergence events.

Download Full-text

Impacts of Simulated M9 Cascadia Subduction Zone Motions on Idealized Systems

Earthquake Spectra ◽

10.1193/052418eqs123m ◽

2019 ◽

Vol 35 (3) ◽

pp. 1261-1287 ◽

Cited By ~ 9

Author(s):

Nasser A. Marafi ◽

Marc O. Eberhard ◽

Jeffrey W. Berman ◽

Erin A. Wirth ◽

Arthur D. Frankel

Keyword(s):

Subduction Zone ◽

Ground Motions ◽

High Strength ◽

Sedimentary Basins ◽

Cascadia Subduction Zone ◽

Seismic Resistance ◽

Spectral Acceleration ◽

Single Degree Of Freedom ◽

Conditional Mean ◽

Subduction Zone Earthquake

Ground motions have been simulated for a magnitude 9 (M9) Cascadia Subduction Zone earthquake, which will affect the Puget Lowland region, including cities underlain by the Seattle, Everett, and Tacoma sedimentary basins. The current national seismic maps do not account for the effects of these basins on the risk-targeted Maximum Considered Earthquake (MCER). The simulated motions for Seattle had large spectral accelerations (at a period of 2 s, 43% of simulated M9 motions exceeded the MCER), damaging spectral shapes (particularly at periods near 1 s), and long durations (5%–95% significant durations near 110 s). For periods of 1 s or longer, the resulting deformation demands and collapse likelihood for four sets of single-degree-of-freedom systems exceeded the corresponding values for motions consistent with the conditional mean spectra at the MCER intensity (MCER). The regional variation of damage was estimated by combining probabilistic characterizations of the seismic resistance of structures and of the effective spectral acceleration, Sa,eff, which accounts for the effects of spectral acceleration, spectral shape, and ground-motion duration. For high-strength, low-ductility systems located above deep basins ( Z2.5 > 6 km), the likelihood of collapse during an M9 earthquake averaged 13% and 18% at 1.0 s and 2.0 s periods, respectively. For low-strength, high-ductility systems, the corresponding likelihoods of collapse averaged 18% and 7%.

Download Full-text

Impacts of an M9 Cascadia Subduction Zone Earthquake and Seattle Basin on Performance of RC Core Wall Buildings

Journal of Structural Engineering ◽

10.1061/(asce)st.1943-541x.0002490 ◽

2020 ◽

Vol 146 (2) ◽

pp. 04019201 ◽

Cited By ~ 2

Author(s):

Nasser A. Marafi ◽

Andrew J. Makdisi ◽

Marc O. Eberhard ◽

Jeffrey W. Berman

Keyword(s):

Subduction Zone ◽

Cascadia Subduction Zone ◽

Subduction Zone Earthquake

Download Full-text

Rainfall triggers more deep-seated landslides than Cascadia earthquakes in the Oregon Coast Range, USA

Science Advances ◽

10.1126/sciadv.aba6790 ◽

2020 ◽

Vol 6 (38) ◽

pp. eaba6790

Author(s):

S. R. LaHusen ◽

A. R. Duvall ◽

A. M. Booth ◽

A. Grant ◽

B. A. Mishkin ◽

...

Keyword(s):

Subduction Zone ◽

Pacific Northwest ◽

Cascadia Subduction Zone ◽

Mean Annual Precipitation ◽

Coast Range ◽

Ground Acceleration ◽

Oregon Coast Range ◽

Oregon Coast ◽

Subduction Zone Earthquakes ◽

Large Earthquakes

The coastal Pacific Northwest USA hosts thousands of deep-seated landslides. Historic landslides have primarily been triggered by rainfall, but the region is also prone to large earthquakes on the 1100-km-long Cascadia Subduction Zone megathrust. Little is known about the number of landslides triggered by these earthquakes because the last magnitude 9 rupture occurred in 1700 CE. Here, we map 9938 deep-seated bedrock landslides in the Oregon Coast Range and use surface roughness dating to estimate that past earthquakes triggered fewer than half of the landslides in the past 1000 years. We find landslide frequency increases with mean annual precipitation but not with modeled peak ground acceleration or proximity to the megathrust. Our results agree with findings about other recent subduction zone earthquakes where relatively few deep-seated landslides were mapped and suggest that despite proximity to the megathrust, most deep-seated landslides in the Oregon Coast Range were triggered by rainfall.

Download Full-text