Earthquake scientists are celebrating the publication of a new study that will provide powerful new insights into one of the three primary earthquake fault systems that put Vashon at risk.
The study, for the first time, looks deep under the surface to map subterranean details of the Cascadia Subduction Zone fault that runs for 600 miles off the coast of Northern California, Oregon, Washington, and British Columbia.
In 1700, a Cascadia event caused one of the strongest known earthquakes and tsunamis, an estimated magnitude 9 catastrophe. It’s the basis for the “Big One” that has provided the scenario for major regional Cascadia Rising earthquake exercises in 2016 and 2022.
The 1700 tsunami wave reached across the Pacific and damage from it was recorded in Japanese histories of the time. Experts have warned that there could be a repeat event at any time, although there’s no way to predict if it will happen tomorrow or in another century.
Seismic Risk Context for Vashon
In addition to Cascadia, two other fault systems could be catastrophic for Vashon. The Tacoma fault zone crosses Vashon at mid-island. The Seattle fault zone crosses Puget Sound at the southern end of Bainbridge Island.
Although the Cascadia event could generate a much stronger quake than the Tacoma or Seattle fault zones, the Cascadia subduction zone sits off the coast about 170 miles from us, so the energy from shaking would dissipate somewhat by the time it reached us.
U.S. Geological Survey (USGS) shake maps produced in recent years estimate that a magnitude 9 Cascadia event would feel like a magnitude 7 quake by the time the shaking reached Vashon, and the shaking from a Cascadia event could last for several minutes. The Tacoma and Seattle fault systems could generate magnitude 5 to 7 quakes with severe results on Vashon because they are directly under us, instead of far away. They are also shallower, and that could exacerbate the shake effects.
All three of the fault systems could generate catastrophic tsunamis on Vashon. A Washington Department of Natural Resources model released two years ago predicted the Cascadia fault would initially cause only a ripple in Puget Sound, but water from that small wave would build up in Commencement Bay south of us and slosh back through the Sound within a few hours of the initial quake, causing tsunami waves on the Vashon shoreline as high as nine to 16 feet.
Quakes on the Seattle and Tacoma fault systems could also cause damaging tsunami waves on the Vashon shoreline, and the waves would arrive with very little warning almost immediately after the quake.
Two Initial Findings
The new study offers two preliminary observations that have been attracting a lot of attention from the scientists who analyze earthquake risk, and those findings will likely be the focus of further intensive study for years to come.
Multi-Segment Fault Theory: The data seems to confirm a theory that the north-south Cascadia fault has been split into three to five segments by intersecting east-west faults. That’s important, because most previous models treated the Cascadia Subduction Zone as one continuous fault.
For example, the scenario for both the Cascadia Rising 2016 and Cascadia Rising 2022 regional emergency exercises were “full rip” scenarios that postulated a single megaquake along the entire 600-mile fault zone. Instead, scientists will now study the possibility that only one segment could rupture, or multiple segments could rupture in a series of quakes. That could make a significant difference in earthquake hazard assessment.
Smooth Fault Theory: The segment directly off the coast of Washington appears to be very smooth and flat compared to other segments along the Cascadia Subduction Zone.
“Counterintuitively, a fault that is smoother at the boundary between two plates has the potential for larger earthquakes than a fault that is rougher,” according to Dr. Harold Tobin, a study co-author and professor of geophysics at the University of Washington. “When that Washington segment goes, it will be especially dramatic. We can infer it represents the largest part of the hazard of a future earthquake rupture.”
What Is the Cascadia Subduction Zone?
The word subduction means one plate of the earth’s surface is colliding with another and moving underneath the overriding plate. In this case, the oceanic Juan de Fuca plate on the west is sliding below and forcing upwards the edge of the continental North American plate to the east.
As the two plates scrape together, deformation and friction cause stresses to build. Eventually, the sudden release of this pent-up energy can generate an earthquake that could be similar in size to the one in 1700.
Why the New Study Matters
Scientists will be exploring and interpreting this massive new set of data for years to come. Experts expect the new information will make it possible to produce more accurate and detailed computer models of potential damage from ground shaking and tsunami waves. Future new earthquake models could thus support the development of significant mitigation and preparedness improvements for our earthquake hazards.
Possibilities include more accurate shake maps to guide improved building codes, computer models to help engineers improve building resilience, and more realistic emergency response plans.
How the New Study Was Conducted
The data was gathered three years ago, but it has taken this long to compile the massive amount of new information, produce some initial conclusions, and ready the study for publication in the scientific journal Science Advances. (You can learn more by visiting online.)
The dense new data set was gathered by the study team working aboard a ship towing a “streamer,” a nine-mile-long array of thousands of underwater recording devices. The study team shot sound pulses toward the sea floor and collected the echoes to develop a map of the Cascadia fault systems many miles below the earth’s surface. Some have compared the tool to an ultrasound image taken in your doctor’s office.
“They’ve done a tremendous amount of work, but the dataset has much more depth to it. New things will come out of it for decades to come,” according to Dr. Chris Goldfinger, a professor of marine geology at Oregon State University and one of the ranking experts on Cascadia.
