Abstract

1. Cascadia Subduction Processes and Earthquake Hazards by Janelle Anzalone Earth Science Major, ES473 Environmental Geology, Spring 2009 Abstract • Prior decades of scientific research document the potential for a great megathrust earthquake on the Oregon coast due to subduction of the Juan de Fuca Plate beneath North America. The initial research questions have progressed form “if” to “when and how big”? • This poster will present paleosiesmic and subsurface geologic data that suggest a strong possibility for a large 9 + magnitude Cascadia earthquake in the near future. • Evidence for regional earthquake activity includes neotectonic uplift, tsunami deposits, coastal submergence of wave-cut coastal benches, burial of previously vegetated lowlands and tidal flats, tree ring and carbon dating and sedimentary structures indicative of ground shaking. • All findings support an extensive and regular history of megathrust events, the last of which occurred in January 1700. Upon analysis of the data, it appears that we are in fact on track to experience a great earthquake “starting yesterday”. BFRs are another thing that come up from under the sand. Some of them show classic signs of emergence in the past, this rock seen at less than low tide is one of many at Hug Point near Canon Beach. This Venus Di Milo-looking stump is one of many sticking out of the beach at Neskowin, north of Lincoln City, Oregon. These trees have been dated between 300 and 2000 years old. A ghost forest at Neskowin Beach. Introduction • Understanding plate tectonics is key when contemplating the idea of an enormous earthquake. The lithospheric portion of this planet moves about on plates of crust. There are 7 major and as many minor plates moving around the globe constantly. They move away from each other in divergence, they slide past each other in transform slipping and they crash into each other at convergent plate boundaries. • The later is the case in the Pacific Northwest where the Juan De Fuca plate runs into the North American Plate. Because the Juan De Fuca is an oceanic plate and oceanic crust is thinner and denser than continental crust, the plate succumbs and subducts beneath the thicker and less dense North American plate. • The area of subduction in the case of the Pacific Northwest is called the Cascadia Subduction Zone. This relatively quite area is building up stress and accreting the coastal mountain range. It will eventually snap causing sudden uplift of the land followed by subsidence. Lowlands submerge into the ocean and sand generated by the consequential tsunami bury everything in its sizable path. • Events like this tend to leave evidence about and so is the topic of this poster Evidence • When lowlands subside they leave tale tell signs in the buried landscape. As time passes on grand scales some of these landscapes are unearthed through various forces of Mother Nature. • Stands of once standing forest are readily found along our coast. As they rematerialize from beneath the beaches scientists can study them to determine factors about past dynamics of our coast. • Tree ring analysis is very helpful in dating the events that submerged these buried forest. • Researchers also dig holes in the landscape to look at soil horizons for evidence in the form of peat, (undecomposed organic layers) and sand from tsunami deposition. These connections are called peat-mud contacts and peat- sand contacts. They represent a time when the two layers met with some untimely contact. These paleohorizons have been instrumental in developing a reoccurance profile of megathrust. • Associated with these ancient soils are tidal wetlands that record times when sea tides, muds ands silts went far inland. • Scientists find all of these things in linear and quantifiable layers of past events of uplift and subsidence. The only possible conclusion to be drawn is MEGATHRUST! USGS At the left: Peat-mud/Peat-sand contacts can be found all along the Oregon coast. When coastal lands drop suddenly, the plants are poisoned by the sea salt. They leave dark layers in the soil. At the right: Alsea marsh has been a major contributor of buried horizons. Tidal wetlands in mid-latitude, temperate are strongly affected by Rising Sea Levels. Peat mud/ peat/sand contacts in buried soils draw a picture of the past Conclusion • An extraordinary amount of evidence puts megathrust in our backyard. Much of the evidence collected by research supports a 9 + magnitude event early 1700. Said evidence also suggest an reoccurrence interval of megathrust every 500 years with the longest separation between events being 1000 years apart and the shortest being 150. • Not just a 9+ magnitude earthquake, but a rupture zone encompassing at least the entire coast line of Oregon. These events are recorded in the stratigraphy of marshes, tidal wetlands and buried forest of Earth’s past. • The Cascadia Subduction Zone is silent, but deadly. Some day in the near future megathrust will raise its sleeping head and stretch; once again changing the landscape on which we live forever. How well will you sleep tonight? References Brian Atwater, USGS at Dept. of Earth and Space Sciences, University of Washington, Seattle, Washington, Probable Local Precedent for Earthquakes of Magnitude 8 or 9 in the Pacific Northwest, 1991; Alan Nelson et al., USGS at Denver, Colorado, USGS Earthquakes, Alsea bay, Oregon Paleoseismology, Differences in great earthquakes rupture extent inferred form tsunami-laid sand and foraminiferal assemblages beneath tidal marshes at Alsea Bay, Oregon. Nelson et al., Identifying coseismic subsidence in tidal-wetland stratigraphic sequences at the Cascadia Subduction Zone of western North America, 1995. BeachConnection.net, Oregon Coast Connection, Stumps Found on Northern Oregon coast believed to be 80,000 years old.