Transcript Subduction Processes in Cascadia

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
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.
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 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.
• 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”.
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
USGS
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
Peat mud/ peat/sand
contacts in buried
soils draw a picture
of the past
At the left: Peat-mud/Peatsand 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.
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!
Conclusion
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;
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.
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.
• Not just a 9+ magnitude earthquake, but a rupture zone encompassing at least the entire coast line of Oregon. These
References
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.
• An extraordinary amount of evidence puts megathrust in our backyard.
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?