Where the "Big One" of the Pacific Northwest is most likely to strike (2023)

The following essay is reproduced with permission fromThe conversation, an online publication with the latest research.Where the "Big One" of the Pacific Northwest is most likely to strike (1)

The Pacific Northwest is known for many things: its beer, its music, its big-legged mythical creatures. Most people don't associate it with earthquakes, but they should. is the house ofCascadia Mega Thrust Faultwhich runs 600 miles from Northern California to Vancouver Island, Canada and includes several major metropolitan areas including Seattle and Portland, Oregon.

This fault line has been relatively quiet recently. There weren't many widespread earthquakes during Cascadia's megaboom, certainly nothing to rival a catastrophic event like that of 1989.Loma-Prieta-Erdbebenalong active San Andreas, California. However, this does not mean that he will remain silent. Scientists know it has the potential for large earthquakes, as big asmagnitude 9.

Geophysicists have known for more than a decade that not all parts of the Cascadia mega-fault behave the same way. The northern and southern sections are much more seismically active than the central section, with frequent small earthquakes and ground deformation that are rarely noticed by local residents. But why do these variations exist and what causes them?

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OurInvestigationtry to answer these questionsCreating images of what is happening deep inside the earth, more than 100 kilometers below the fault. We have identified high-altitude regions below these active trends that we believe result in the observable differences along the Cascadia Fault.

Cascadia and the "Big Ones"

in the waterfallSubduction zoneIt is a region where two tectonic plates collide. HeJuan de Fuca, a small oceanic plate, is being pushed under the North American Plate on which the continental US sits.

Subduction systems, where one tectonic plate slides over another, can produce the world's largest earthquakes. An excellent example is the2011 Tohoku earthquakethat shook Japan.

Cascadia is seismically very quiet compared to other subduction zones, but not completely inactive. The investigation shows that the error has penetrated9.0 magnitude event in 1700. That's about 30 times stronger than the largest predicted earthquake in San Andreas. The researchers assume that within approx.300 to 500 years windowduring theanother great Cascadia event could happen.

Each year there are many minor events in North and South Cascadia that cause no harm or fanfare. However, in central Cascadia, below most of Oregon, there is very little seismicity. Why would the same bug behave differently in different regions?

Over the past decade, scientists have made several additional observations that highlight variations along the fault.

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one has to do with itlock plate, which tells us where the stress builds up along the fault. If the tectonic plates are locked, i.e. really stuck together and can't move relative to each other, the stress increases. Eventually, this stress can be rapidly released as an earthquake, the magnitude of which depends on the size of the ruptured fault section.

Geologists recently managed to unravel itHundreds of GPSMonitors at Cascadia to record the subtle ground deformations resulting from the slabs' inability to slide over one another. Like historic seismicity, plate blockage is more common in theNorthern and southern parts of Cascadia.

Geologists can now also observe hard-to-detect seismic sounds known asTremble. These events occur over a period of several minutes to weeks and last much longer than a typical earthquake. They do not cause large ground movements, although they can release significant amounts of energy. researcherI have just found outthese charactersNOlast 15 years, but permanent seismic stations helped build a robust catalog of events. The tremors also seem to become more concentrated over timeNorth and South Zoneof guilt

What would cause this situation if all of these measures made the area below Oregon relatively less active? To explain that, we had to look more than 100 kilometers deep into the earth's mantle.

Earth images with distant earthquakes

Doctors use electromagnetic waves to "see" internal structures like bones without having to open up a human patient to see them directly. geologistsearth imageAlso. Instead of X-rays, we use the seismic energy emitted by distant earthquakes of magnitude 6.0 or greater to help us "see" features we simply don't have physical access to. This energy travels through the structures of the earth as sound waves. When the rock is warmer or partially melted, even slightly, seismic waves decrease. By measuring the arrival times of seismic waves, we create 3D images that show the speed at which seismic waves propagate through specific parts of the earth.

To see these signals we need recordings from seismic monitoring stations. More sensors offer better resolution and a sharper image, but collecting more data can be problematic if half the area you're interested in is underwater. To meet this challenge, we were part of a team of scientists who deployed hundreds of seismometers on the ocean floor of the western United States over a four-year period beginning in 2011. This experiment, thatCascadia-Initiative, was the first to cover an entire tectonic plate with instruments at a distance of about 50 kilometers.

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What we find are two anomalous regionsbelow the fault where seismic waves propagate slower than expected. These anomalies are large, approximately 150 kilometers in diameter, and occur under the northern and southern portions of the fault. Keep in mind that researchers have already observed most of the activity here: seismicity, blocking, and tremors. Interestingly, the anomalies are absent below the central portion of the fault, below Oregon, where we are seeing a decrease in activity.

What exactly are these anomalies?

Tectonic plates float on top of the Earth's mantle. Where the mantle slowly rises over millions of years, the rock decompresses. Since it is almost 1500 degrees Celsius at 100 km depth with such high temperatures, it canmelt very easily.

These physical changes give more buoyancy to anomalous regions: hotter molten rock is less dense than cooler solid rock. It is this buoyancy that we believe influences the error behavior above. The hot, partially melted area pushes up whatever is on top, much like a helium balloon might be lifted against a foil covering it. We believe this increases the forces between the two plates, making them more tightly coupled and therefore more fully connected.

A general prediction of where, but not when.

Our results provide new insights into how this subduction zone, and possibly others, behave over geological timescales of millions of years. Unfortunately, our results cannot predict when the next large Cascadia megaquake will occur. This requires further investigation and close active monitoring of the subduction zone, both onshore and offshore, using seismic and GPS-like stations to capture short-term phenomena.

Our work suggests that a major event is more likely to start in the northern or southern sections of the fault, where plates are most blocked, and provides a possible reason why this might be the case.

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It remains important that the public and policy makers are kept informed of the potential risk involved.Coexistence with a subduction fault zoneand support programs such asEarly warning of earthquakeswho wish to expand our surveillance capabilities and mitigate casualties in the event of a major outbreak.

This article was originally published inThe conversation. read thisOriginal article.


    Miles Bodmer is a PhD student in Earth Sciences at the University of Oregon.

      Doug Toomey is Professor of Earth Sciences at the University of Oregon.

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