By Nancy Steinberg
Like a parent of an often-rambunctious toddler, Anne Tréhu thinks it’s too quiet around here.
The deafening silence comes from the Cascadia Subduction Zone offshore of the U.S. West Coast, about 100 miles from Tréhu’s Corvallis office.
Subduction zones are places where the Earth’s tectonic plates collide, and one plate is forced beneath the other. When the descending plate makes a sudden move, a large earthquake can result; in fact, these zones are the source of the world’s largest, most damaging earthquakes.
The infamous Cascadia Subduction Zone, where the Juan de Fuca Plate dives underneath the North American Plate, currently seems to be “locked,” with no sign of swift movement since its last big upheaval (downheaval?) in the year 1700. Scientists’ data and indigenous peoples’ oral histories provide abundant evidence that a massive quake and tsunami happened that year (January 26 at 9:00 pm PST, to be exact) along the entire length of the plate boundary, from Cape Mendocino, California to Vancouver Island. Then the plate boundary fell mostly quiet. Given that massive Cascadia earthquakes seem to occur about every few hundred years, we know another big one is in our future, one that will devastate West Coast communities with both the shaking of the Earth and the resultant tsunami.
How can we learn more about the Cascadia Subduction Zone while it is silent? One important step is to create a detailed image of the whole subduction zone in order to reveal as much information as possible about its structure and composition. What types of rock make up each plate? Where are there pockets of gas trapped in the rock? Where are the transitions from one type of rock to another? Does crustal structure control the occurrence of the smaller earthquakes that occur in Cascadia?
One method of creating such an image uses sound. Tréhu has spent much of her career listening to sound echoing off the interior of the Earth, with an emphasis in the past three decades on the Cascadia Subduction Zone and a similar plate boundary off of Chile. In the summer of 2021, she helped lead a massive field campaign, called Cascadia2021, to create a new 3D image of Cascadia, collecting data from land and sea.
The method her team uses, called seismic tomography, is akin to a medical CT scan. A pulse of sound is emitted from a ship and directed downward, where it travels through and bounces off of features in the Earth’s mantle and crust. The speed at which that signal travels through the layers of the Earth depends on the type of rock the sound waves encounter, whether the materials are homogenous or have “discontinuities” in them, and the temperature of the materials. The bounced sound is picked up by receivers positioned by the researchers – some placed on the sea floor, called ocean bottom seismometers; some towed directly behind the boat in a kilometers-long string; and some located on shore. The receivers document how long it takes for the sound emitted by the research vessel to reach them, and those data contribute to 3D images of the entire subduction zone.
“One of the questions we were interested in is understanding why the Cascadia Subduction Zone has less seismic activity than other subduction zones around the globe. And then, given that, we want to understand what clues we can see in the structure that can give us information on what to expect in the future,” Tréhu says.
The extent and resolution of the image depends in part on the number and spacing of the receivers, thus the need for three sets of receivers on land and sea. It was no mean feat to get them all deployed simultaneously during Cascadia2021, the most complex project of its kind ever carried out along the Cascadia Subduction Zone.
First, the ocean bottom seismometers had to be placed by one ship on the ocean floor. Oregon State’s own R/V Oceanus, in one of its last cruises before being decommissioned, had the job of placing 50 seismometers on the ocean bottom along a southern stretch of the subduction zone over the course of two weeks. During the experiment, these receivers were retrieved and re-deployed further north, so the project ultimately extended from the Oregon-California border to Vancouver Island.
Around the same time, 15 pairs of students and volunteers fanned out across Oregon’s Coast Range and southwest Washington to install 755 temporary receivers, about the size of a coffee can, on national forest land, in parks, and people’s back yards.
“This coffee can-sized container includes the recorder, which has a battery life of about a month, and a little GPS clock in it,” Tréhu explains. “We bury them, but not too deep so that they can still receive the GPS signals.”
Once the receivers were in place, the ruckus began: The R/V Marcus Langseth, a huge research vessel operated by Columbia University and outfitted for just this purpose, started sending out loud acoustic signals using air guns. The booms traveled down into multiple layers of the Earth, were deflected, and then were picked up by the temporary receivers on land and on the ocean bottom, as well as by the strand of receivers being dragged by the Langseth itself. There are also permanently installed receivers, comprising the Northwest Seismic Network, that heard the Langseth’s calls.
After the six-week signal-sending phase of the project was done, Tréhu and her team re-collected the temporary receivers in the ocean and on land. The amount of data collected was massive – thousands of terabytes in all, equal to the storage capacity of more than 50,000 64GB iPhones.
The next steps will be to analyze the data by cross-referencing the timing of the acoustic signals’ reception at each seismometer and combining that data with modeling results to stitch together a 3D image of the subduction zone. While such images have been created using the land-and-sea approach before (similar projects took place in 1989, 1996 and 2012), this project has, by far, the largest footprint.
What kinds of information will this portrait of tectonic plates give us? Especially when coupled with information about where earthquakes along the subduction zone have happened in the past, scientists will be able to determine where smaller quakes are likely to happen. Based on the mineral composition at different places on the plates, they will also be able to predict where shaking, and therefore damage, will be the worst. They will examine the structure of smaller faults along the plate, which could control the behavior of the seafloor in a large quake and therefore the size of the resulting tsunami.
Although the holy grail of earthquake prediction is still a long way off, Tréhu hopes the field will get there one day. “By combining our high-resolution crustal imaging with modeling of subduction zone dynamics, we’re hoping to go from correlations to an understanding of the processes controlling seismic events,” she says.
In the meantime, all of us who live on the U.S. West Coast will hope that the silence continues.
Photo of R/V Marcus G. Langseth at the top of the page is by Jared Kluesner.