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Cover Page Pacific Ring of Fire Pacific Northwest Earthquake Silent Subduction? Indigenous Oral History Ground Motion Animation Tsunami Wave After 1700 Earthquake Geologic Evidence GPS and Seismic Stations Knowledge Gap
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  • Image Attribution: Janet Watt, USGS
  • Image Attribution: Figure 3. Swai’xwe mask from mainland British Columbia from Ludwin, Ruth S et al., 2005.
  • Image Attribution: USGS
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    The Unfolding Mystery of Cascadia’s Giant Earthquakes

    The Cascadia Subduction Zone "megathrust" fault separates the North American tectonic plate from the Juan de Fuca, Gorda, and Explorer plates (the three move together as one), which are continuously being pushed toward one another by forces within the Earth. This giant fault stretches over 600 miles from Cape Mendocino, California to Vancouver Island British Columbia and laterally from hundreds of km offshore to depths beneath the coastline. The fault resists this relentless convergent motion, which thrusts the offshore plates beneath North America (i.e., "subducts"), building stresses for hundreds of years until it breaks suddenly in giant earthquakes. This Geo-Narrative presents the evolving scientific understanding of these megathrust earthquakes.

    This project was funded by National Science Foundation award #190024
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    Pacific Ring of Fire

    Global M7+ Earthquakes: 1900 - 1900
    Data Source: United States Geological Survey (USGS)

    The CSZ forms part of the “Ring of Fire” - chains of active volcanoes that rim much of the Pacific Ocean basin as the subducting plates that comprise it melt when they subduct. Just as in Cascadia, megathrust earthquakes occur within these subduction zones.

    “Subduction zones all look similar. Wherever you go if there's a subduction zone, you see a trench and then about 200-300 km away you see a volcano. They all just look similar, and that is what makes subduction research very interesting. So then the hardest part is to try to identify what are the site-specific issues that not only applies to a single subduction zone but to all of them; the global implications. If you solve that problem, and that is a big problem, you want to address fundamental problems that generalize to all subduction zones. It is just a wonderful research field.”

    Kelin Wang

    Research Scientist at Natural Resources Canada

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    Pacific Northwest Earthquakes

    1900 2020
    Data Source: United States Geological Survey (USGS)

    Cascadia is unlike all other subduction zones because only a few tiny earthquakes have occurred on the megathrust fault in historic to modern times. Nearly all the earthquakes have occurred within the overriding North American plate, within or along the boundaries of the subducting plates before they begin their descent beneath North America. From megathrust earthquakes elsewhere and other information from Cascadia, scientists anticipate how motion (slip) will occur in the next giant megathrust earthquake.

    “It's not going to be 10 meters of slip everywhere on a 1000-kilometer long fault. It's going to be very large in some areas, very small in other areas; it's two rock interfaces sliding past each other. Some places it sticks, other places it slips a lot farther, and so the actual slip pattern is smooth at a long enough scale but there's a lot of variation along the length and even the depth of the subduction zone.”

    Randy Leveque

    Professor of Applied Mathematics at University of Washington

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    Silent Subduction?

    “I had an opportunity for funding from the USGS Powell Center, to do some work in Cascadia and I was so excited to have the opportunity to work on Cascadia Subduction Zone science in general. But especially the Cascadia subduction zone, which is in some ways I think, one of the more interesting subduction zones, just because we haven't had a big earthquake on it in recorded memory. It's been a lot of fun, it's always been interesting, and it's been great to have the opportunity to get involved.”

    Maureen Walton

    Research Geologist at United States Geological Survey

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    Indigenous Oral History

    “I was really just sent into the libraries, but what was impressive was how much material there was. You go up and down the coast and these stories are just everywhere, and they're just incontrovertible as evidence of some recurrent events. And that's the point we tried to make in the article [Finding Fault (2007)] is that indigenous societies, however the individual indigenous communities decided to understand this experience, that understanding was based on millennia, whereas settler societies' understanding of this place is based on decades and maybe a couple of centuries, if you're being generous. It's really decades as opposed to millennia, I really started thinking "Wow, this is an event that settler society simply has not experienced, at least not in this place."”

    Coll Thrush

    Professor of History at the University of British Columbia


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    Indigenous Oral History

    Reference: Ludwin, Ruth S, Smits, Gregory J, Carver, D, James, K, Jonientz-Trisler, C, McMillan, A. D, . . . Wray, J. (2007). Folklore and earthquakes: Native American oral traditions from Cascadia compared with written traditions from Japan. Geological Society Special Publication, 273(1), 67-94.
    Temprano, V. (2015). NativeLand.ca. Retrieved May 31, 2021, from https://native-land.ca/.
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    Ground Motion Animation Scene Placeholder

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    Tsunami Wave After the 1700 Cascadia Earthquake

    Tsunami Simulation

    “In a megathrust earthquake, tension is released in the continental plate, which slips past the subducting ocean plate. This leads to a large vertical displacement of the seafloor and all the water on top of it. The resulting tsunami propagates towards the nearby shore and also outward across the ocean. Scientists have estimated the seafloor deformation during the 26 January 1700 Cascadia earthquake, based on evidence along the West Coast and in Japan, and we have used this to model the resulting tsunami by solving the equations of fluid dynamics.”

    Randy Leveque

    Professor of Applied Mathematics at University of Washington



    This simulation is performed by the UW Tsunami Modeling Group using the GeoClaw software, based on a seafloor deformation model of the 1700 earthquake published in Gao et al. (2018).
    Reference: Gao, D., Wang, K., Insua, T. L., Sypus, M., Riedel, M., & Sun, T. (2018). Defining megathrust tsunami source scenarios for northernmost Cascadia. Natural Hazards, 94(1), 445–469. https://doi.org/10.1007/s11069-018-3397-6

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    Geologic Evidence for CSZ Megathrust Rupture



    Geologists looking at the ghostly remains of trees subsided and killed by the 1700 Cascadia earthquake, when the land suddenly subsided allowing salt water to flow into the marsh at high tide. By counting the rings in the trees, the year that the earthquake occurred can be dated.



    Geologists extracting a sample of the sediment layers from a coastal marsh. In the hundreds of years between earthquakes peat layers are deposited, and when the ground sinks suddenly following an earthquake mud is deposited on top of the peat, making a record of the earthquake.

    Reference: Walton, M. A. L. et al. Toward an Integrative Geological and Geophysical View of Cascadia Subduction Zone Earthquakes. Annu Rev Earth Pl Sc 49, 1–32 (2021).

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    The Geologic Evidence in Space and Time

    Each rectangle and color corresponds to an onshore record (e.g., a buried sands left by an ancient tsunami that washed ashore) or offshore record (e.g., near-coastal sediments from an earthquake shaking-generated submarine landslide found beneath the deep ocean) at a particular location (circles on the map), with the width showing the estimated age of the causative earthquake. Vertical grey bars show here the ages of earthquake records overlap from sites along nearly all of Cascadia, indicating the evidence may have formed at the same time - a giant earthquake is the most likely cause of such widespread impacts.

    Reference: Walton, M. A. L. et al. Toward an Integrative Geological and Geophysical View of Cascadia Subduction Zone Earthquakes. Annu Rev Earth Pl Sc 49, 1–32 (2021).

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    GPS Stations & Seismic Stations

    “...improvements in seismic monitoring has been a major technological development for sure...We are going to learn a lot when an earthquake happens. Earthquakes are now more precisely located, down to the smallest magnitude. That has led to the recognition of silent slip events... More and more papers are being published on how they relate to the subduction process. Silent slip events are recognized in other parts of the world as well. This is an advance that has been driven by technology, by improvements in seismic monitoring, and by GPS technology.”

    John Clague

    Professor Emeritus of Earth Sciences at Simon Fraser University

    “Geodesy has had a huge impact in our understanding of the Cascadia Subduction Zone. And I think with the increasing reliance on offshore geodesy, I think that has the potential to really help understand a lot more about what the subduction zone is doing. Because right now, a lot of our data sets come from the onshore data sets, but the closer we get to the trench with geodesy, the more unique our models that fit the data can become. So I think geodesy is quite a huge one.”

    Lydia Staisch

    Research Geologist at United States Geological Survey

    Reference: Walton, M. et al. A Database and Working Group for Cascadia Earthquake Research: Synthesizing Existing Knowledge to Answer Outstanding Questions. (2019).

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    The Knowledge Gap

    Besides the uncertainties and discrepancies in the geologic data that we present in this geonarrative, there are also many unsolved issues remaining in other aspects of the Cascadia Subduction Zone (CSZ) Science, such as undated earthquake events, incomplete geodetic coverage, and dubious assumptions (Walton et al., 2021). As Walton et al. stated, “due to the gaps in knowledge, there is currently no consensus on an appropriate recurrence model for the CSZ.”

    “It's important that scientists acknowledge uncertainties. And it is difficult in the public arena, because most people don't want to hear about uncertainty. They want a yes or no answer. You have to be sensitive to this fact when you're communicating. Be prepared to explain why, when you are arguing for public expenditures, you've got to tolerate some uncertainty. You still have to take action and you can’t just use uncertainty as an excuse for doing nothing.”

    John Clague

    Professor Emeritus of Earth Sciences at Simon Fraser University

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