Breakthrough study to understand local earthquakes

Seismologists are celebrating the publication of a new study that will provide powerful new insights into one of the three primary seismic fault systems that put Vashon at risk.

The study looks, for the first time, deep beneath the surface to map the subterranean details of the Cascadia Subduction Zone Fault that extends 600 miles off the coast of northern California, Oregon, Washington and British Columbia.

In 1700, the Cascadia Event caused one of the most powerful earthquakes and tsunamis known, estimated at magnitude 9. It is the basis for the “Big Earthquake” that provided the scenario for the major regional Cascadia Earthquake Rising exercises in 2016 and 2022.

The 1700 tsunami reached across the Pacific Ocean and the resulting damage was recorded in Japanese history at the time. Experts have warned that this event could happen again at any time, although there is no way to predict whether it will happen tomorrow or in another century.

Vashon seismic hazard context

In addition to Cascadia, there are two other systems that could be disastrous for Vashon. It crosses the Tacoma Vashon fault zone in the middle of the island. The Seattle Fault Zone crosses Puget Sound at the southern end of Bainbridge Island.

Although a Cascadia event could generate a much stronger earthquake than a Tacoma or Seattle area earthquake, the Cascadia subduction zone is located off the coast about 170 miles from us, so the energy from the shaking will have dissipated somewhat by the time it reaches us. .

Tremor maps released by the United States Geological Survey (USGS) in recent years estimate that a magnitude 9 Cascadia event would look like a magnitude 7 earthquake by the time shaking reaches Vashon, and shaking from a Cascadia event could last for several minutes. The Tacoma and Seattle fault systems can generate magnitude 5 to 7 earthquakes with severe consequences on Vashon because they lie directly below us, not far away. They are also shallower, which can exacerbate the effects of vibration.

All three fault systems could generate catastrophic tsunamis on Vashon. A Washington Department of Natural Resources model released two years ago predicted that the Cascadia Fault would initially cause only a ripple in Puget Sound, but water from that small wave would accumulate in Commencement Bay south of us and flow back across the sound within a few hours. of the initial earthquake, causing tsunami waves at Vashon Beach as high as nine to 16 feet.

Earthquakes on the Seattle and Tacoma fault systems can also cause devastating tsunamis on Vashon Beach, and the waves will arrive almost without warning immediately after the earthquake.

Two preliminary findings

The new study makes two preliminary observations that have attracted a lot of attention from scientists analyzing earthquake hazards, and these findings are likely to be the focus of additional intense study for years to come.

Multi-segment rift theory: The data seems to confirm a theory that the north-south Cascadia rift was divided into three to five segments by the intersection of east-west faults. This is important, because most previous models treated the Cascadia subduction zone as one continuous fault.

For example, the scenario for the regional emergency exercises Cascadia Rising 2016 and Cascadia Rising 2022 were “full” scenarios that assumed a single large earthquake along the entire 600-mile fault zone. Instead, scientists will now study the possibility of just one segment rupturing, or multiple segments rupturing in a series of earthquakes. This can make a big difference in earthquake risk assessment.

Smooth fault theory: The section off the coast of Washington appears to be very smooth and flat compared to other parts along the Cascadia subduction zone.

“Contrary to what would be expected, a smoother fault at the boundary between the two plates has the potential to produce larger earthquakes than a rougher fault,” says Dr. Harold Tobin, co-author of the study and professor of geophysics at the University of Washington. “When this part of Washington goes, it will be particularly dramatic. We can conclude that it represents the bulk of the risk for future seismic rupture.”

What is the Cascadia subduction zone?

The word subduction means the collision of one plate of the Earth's surface with another and moves under the overriding plate. In this case, the Juan de Fuca oceanic plate to the west slides down and pushes the edge of the North American continental plate to the east upward.

When plates rub together, deformation and friction cause stresses to build up. Ultimately, the sudden release of this pent-up energy could generate an earthquake that could be similar in size to the one that occurred in 1700.

Why is the new study important?

Scientists will be exploring and interpreting this huge new set of data for years to come. Experts expect that the new information will allow the production of more accurate and detailed computer models of potential damage resulting from ground shaking and tsunami waves. Future new earthquake models could therefore support the development of significant improvements in our earthquake risk mitigation and preparedness.

Possibilities include more accurate vibration maps to guide improved building codes, computer models to help engineers improve building resilience, and more realistic emergency response plans.

How was the new study conducted?

The data was collected three years ago, but it took a long time to compile the huge amount of new information, come to some preliminary conclusions, and prepare the study for publication in the scientific journal Science Advances. (You can learn more by visiting the website online.)

The dense new data set was collected 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 fired acoustic pulses toward the seafloor and collected the echoes to map Cascadia fault systems many miles below the Earth's surface. Some have compared the tool to an ultrasound image taken at your doctor's office.

“They've done a tremendous amount of work, but the data set has much greater depth. New things will emerge from it for decades to come,” says Dr. Chris Goldfinger, a professor of marine geology at Oregon State University and one of the leading experts on Cascadia.

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