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In commemoration of the last known Cascadia subduction zone earthquake, I present a summary background material here. Since last years commemoration, there have been a few additions. Most notably was a pair of articles that were published in the New Yorker magazine, written by Kathryn Shulz.
The first article summarized what we might expect from a CSZ earthquake and tsunami and the follow-up article discussed how to best prepare for this series of devastating events. The first article sparked a national debate after it was read by millions using an estimate based upon the recorded time the website was being used. This debate has resulted in a renewed interest in becoming more resilient. On this evening, years ago, the Cascadia subduction zone fault ruptured as a margin wide earthquake.
I here commemorate this birthday with some figures that are in two USGS open source professional papers. The Atwater et al.
The Goldfinger et al. The reference list is extensive and this is but a tiny snapshot of what we have learned about Cascadia subduction zone earthquakes. Brian Atwater and his colleagues have updated the Orphan Tsunami and produced a second edition available here for download and here for hard copy purchase I have a hard copy. Here is a map of the Cascadia subduction zone, modified from Nelson et al. Sites where evidence of past earthquakes paleoseismology are denoted by white dots. Where there is also evidence for past CSZ tsunami, there are black dots.
These paleoseismology sites are labeled e. Some submarine paleoseismology core sites are also shown as grey dots. The two main spreading ridges are not labeled, but the northern one is the Juan de Fuca ridge where oceanic crust is formed for the Juan de Fuca plate and the southern one is the Gorda rise where the oceanic crust is formed for the Gorda plate.
The project was funded by the National Science Foundation.
YT link for the embedded video below. This figure shows how a subduction zone deforms between interseismic and during coseismic earthquakes. We also can see how a subduction zone generates a tsunami. Here is a version of the CSZ cross section alone Plafker, Here is an animation produced by the folks at Caz Tech following the Sumatra-Andaman subduction zone earthquake. I have several posts about that earthquake here and here. One may learn more about this animation, as well as download this animation here.
This figure shows the regions that participate in this interseismic and coseismic deformation at Cascadia.
Black dots on the map show sites where evidence for coseismic subsidence has been found in coastal marshes, lakes, and estuaries. This shows how the CSZ is deforming vertically today Wang et al. This figure, also from Wang et al. Contours are in czs. Here is a graphic showing the sediment-stratigraphic evidence of earthquakes in Cascadia.
There are 3 panels on the left, showing times of 1 prior to earthquake, 2 several years following the earthquake, and 3 centuries after the earthquake. Before the earthquake, the ground is sufficiently above sea level that trees can grow without fear of being inundated with salt water.
During the earthquake, the ground subsides lowers so that the area is now inundated during high tides.
The salt water kills the trees and other plants. Tidal sediment like mud starts to be deposited above the pre-earthquake ground surface. This sediment has organisms within it that reflect the tidal environment. Eventually, the sediment builds up and the crust deforms interseismically until the ground surface is again above sea level. Now plants that can survive in this environment start growing again. There are stumps and tree snags that were rooted in the pre-earthquake soil that can be used to estimate the age of the earthquake using radiocarbon age determinations.
Here is a photo of the ghost forest, created from coseismic subsidence during the Jan. Here is a photo I took in Alaska, where there was a subduction zone earthquake in These tree snags were living trees prior to the earthquake and remain to remind us of the earthquake hazards along subduction zones.
This shows how a tsunami deposit may be preserved in the sediment stratigraphy following a subduction zone earthquake, like in Cascadia.
If there is a source of sediment to be transported by a tsunami, it will come along for the ride and possibly be deposited upon the pre-earthquake ground surface.
Following the earthquake, tidal sediment is deposited above the tsunami transported sediment. Sometimes plants that were growing prior to the earthquake get entombed within the tsunami deposit. Just before midnight on January 27, a tsunami struck the coasts of Japan without warning since no one in Japan felt the earthquake that must have caused it.
They learned that this geologically active region, the Cascadia Subduction Zone, not only hosts erupting volcanoes but also produces megathrust earthquakes capable of generating devastating, ocean-crossing tsunamis. By comparing the tree rings of dead trees with those still living they could tell when the last of these great earthquakes struck the region. The trees all died in the winter of when the coasts of northern California, Oregon, and Washington suddenly dropped m ft.
That much motion over such a large area requires a very large earthquake to explain it—perhaps as large as 9.
Pictures of CSX
Such an earthquake would have ruptured the earth along the entire length of the km mi -long fault of the Cascadia Subduction Zone and severe shaking could have lasted for 5 minutes or longer. The Pacific Tsunami Warning Center PTWC can create an animation of a historical tsunami like this one using the same too that they use for determining tsunami hazard in real time for any tsunami today: This animation shows these values through the simulated 248 of the waves and as they race around the globe one can also see the distance between successive wave crests wavelength as well as their height half-amplitude indicated by their color.
More importantly, the model also shows what happens when these tsunami waves strike land, the very information that PTWC needs to issue tsunami hazard guidance for impacted coastlines. From the beginning the animation shows all coastlines covered by colored points. These are initially a blue color like the undisturbed ocean to indicate normal sea level, but as the tsunami waves reach them they will change color to represent the height of the waves coming ashore, and often these values are higher than they were in the deeper waters offshore.
This is the timeline of prehistoric earthquakes as preserved in sediment 24800 in Grays Harbor and Willapa Bay, Washington. This timeline is based upon numerous radiocarbon age determinations for materials that died close to the time of the prehistoric earthquakes inferred from the sediment stratigraphy at locations along the Grays Harbor, Willapa Bay, and Columbia River estuary paleoseismic sites.
These cores contain submarine landslide deposits called turbidites. These turbidites are thought to have 24800 deposited as a result of strong ground shaking from large magnitude earthquakes. This map shows where the cores are located.
The correlations are the basis for the interpretation that these submarine landslides were triggered by Cascadia subduction zone earthquakes. This correlation figure demonstrates how well these turbidites have been correlated. This map shows the various possible prehistoric earthquake rupture regions patches for the past 10, years. I cz the plot above into another figure that includes all the recurrence intervals and segment lengths in a single figure.
This is modified from Goldfinger et al. Today is the th anniversary of the Cascadia earthquake of How do we know about Cascadia ‘s ancient earthquakes?
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Watch this video to find out! New post about The M9 Project and 3-D computer simulations on our blog! Viola Riebe, elder of vsz Hoh Indian Tribe, talks about an oral history of the Thunderbird and the whale — and how this most respected story is related to tsunami awareness.
Here is the Chaytor et al.
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Here is a map from Rollins and Stein, showing their interpretations of different historic earthquakes in the region. This was published in response to the Januray Gorda plate earthquake.
The faults are from Chaytor et al. The Blanco fracture zone is also an active transform plate boundary. Here is a map that shows the tectonic setting and some earthquakes related to the BFZ from April There are some animations on this web page showing seismicity with time along the BFZ, over the past 15 years.
Is this a reasonable statement: I summarized the regional tectonics of the Cascadia subduction zone recently here. I present a summary about the CSZ tectonics on a th year commemoration page here. I include a yt link and embedded video and an mp4 embedded video and download […]. Your email address will not be published. This site uses Akismet to reduce spam.
Learn how your comment data is processed. He was most recently teaching classes at Humboldt State University, Dept. Back to top Proudly powered by WordPress. A 6, year earthquake history in the region of the Sumatra-Andaman Earthquake Paper 2: Geology Student Colloquium Rotary Silverton: I should have said locally… there are extensive uplifted dunes in oregon….