If you happened to see two research vessels off the coast of Grays Harbor this month and wondered what was going on, would you believe earthquake research was being conducted? It turns out the coast off Grays Harbor isn’t just known for its good fishing grounds, it also has a fault that is the centerpiece of several research studies.
Not all earthquakes are equal. There are the long-lasting, magnitude 5-6 earthquakes that occur regularly beneath the Puget Sound, and then there are the sudden megathrust magnitude 9 earthquakes capable of producing the tsunami that in 1700 crossed the Pacific Ocean and destroyed Japanese coastal villages. Along the Washington Coast is a fault capable of producing a megathrust earthquake.
The fault in our backyard is located within the Cascadia Subduction Zone, an area spanning more than 680 miles, from Mendocino, Calif. to northern Vancouver Island. A fault is the contact zone where two tectonic plates collide and scrape against each other. For those of us living on the Washington margin, it’s the younger and heavier Juan de Fuca plate diving or subducting beneath the older and lighter North American plate. Other similar subduction zones are found south of the Alaskan Aleutian Islands and off the coasts of Chile, Indonesia, Japan, and New Zealand. These zones have also experienced megathrust earthquakes, causing extensive damage and producing tsunamis.
Each year the Juan de Fuca and North American plates collide at the rate of about 4 centimeters (about 2 inches) per year. While this slow rate of movement might seem insignificant, if movement becomes halted or “locked,” energy builds between the plates. Locking can last for hundreds of years until the stored energy is abruptly released, resulting in an earthquake. This earthquake may occur along a small portion of the fault or be a megathrust earthquake that spans the entire fault. In the Cascadia Subduction Zone the geological record shows that megathrust earthquakes occur approximately every 500 years, said Professor Paul Johnson in the UW School of Oceanography.
The westward portion of the locked zone occurs about 60 miles west of the Washington Coast. This is based upon the observation that while the low magnitude 5-6 earthquakes (Episodic Tremor and Slip events ETS) are occurring in the deeper portion of the contact zone beneath the Puget Sound, there is an absence of the Episodic Tremor and Slip events occurring in the shallower area. It is unknown how far east this locked portion extends. It could be under the Washington continental shelf or the Olympic Mountains. And it’s within the locked area where the epicenter of the megathrust earthquake will occur. This is why scientists are interested in determining the extent of the locked portion.
Yet before scientists can answer the question of “Why the plates are locked,” they must first identify the factors that may cause the locking. On July 8, scientists aboard the research vessel Langseth returned to Astoria after spending nearly a month in the northeast Pacific Ocean. Chief Scientist Suzanne Carbotte of Lamont-Doherty Earth Observatory of Columbia University, led the cruise where the focus was studying seawater absorption or hydration of the rocks forming the crust of the Juan de Fuca plate. It turns out the amount of water absorbed into the rocks forming the oceanic plate affects the friction between the two colliding plates. How is Professor Carbotte determining whether seawater is being absorbed? By using sound waves.
Towed behind the R/V Langseth was an array of 36 airguns that, when fired simultaneously, send pulses of sound penetrating not only through the Juan de Fuca plate but into the upper portion of the mantle upon which the plate floats. These sound waves reflect off the mantle and are received as echoes by the hydrophones placed in the five miles of streamers trailing behind the ship. The recorded echoes also provide information on how far inland the “locked” portion of the plate extends. Placed in the coastal mountains of Oregon and Washington are land-based seismometers also recording the sound wave pulses produced by the airguns. When these echoes are processed through computer software, an image of the Juan de Fuca plate is produced. This image enables scientists to study the structure of the Juan de Fuca plate and identify where seawater can seep beneath the sediments covering the plate and then alter the composition of the rocks that compose the plate.
The amount of seawater absorbed by the Juan de Fuca plate before being subducted appears to play a role in causing the Episodic Tremor and Slip events occurring beneath the Puget Sound region. Professor Carbotte explained in an email that “These [ETS] are believed to be linked to water being expelled from the crust and mantle under the high pressures and temperatures found deep in the subduction zone.” This means that when the Juan de Fuca plate carries more seawater into the subduction zone, the result is in more low magnitude earthquakes that releases strain between the plates – a good scenario.
The nearly month long cruise was “uneventful, which is what we hope[d] for,” Professor Carbotte wrote in an email. “Before the cruise, we coordinated with the Quinault Nation about working in their fishing grounds area for about two days, but in the end we did not enter their primary fishing area.”
After returning to Astoria to drop off Professor Carbotte and her students, the R/V Langseth headed out for another cruise. From July 12 – 23, the vessel cruised along the coast off Grays Harbor. This time the study’s focus was to create images of the Juan de Fuca and North American plate contact zone beneath the Washington margin. Oceanography graduate student Marie Salmi was one of two University of Washington young earth science investigators aboard this cruise. A competitive application process determined the 15 cruise participates. These young scientists are students at academic institutions throughout the United States and are studying marine geophysics and subduction zones. Marie was eager to participate in the cruise, saying she likes “being part of working in the field. Part of me likes being out on the water.”
Yet being out in the field has its challenges and, as Professor Johnson in the UW School of Oceanography observed in an email, “Oceanography is not for the faint-hearted.” Though Professor Johnson is also a co-principal investigator (another name for researcher) on this study, he sat this cruise out, which might have been fortunate. He received updates that 12 – 15-foot swells caused most of the students to retreat to their bunks, the vessel operated on reduced power due to one engine being damaged, there was a “full ship electrical power outage that trashed all on-board computers” and those five miles of hydrophone streamers became tangled on the first day. But Marie took these incidents in stride, writing in email after the cruise, “One thing I was particularly impressed with was the real world of marine seismology was anything but simple. We had waves, swells, gaps in the data, wind that would push the streamer off line but with all that was against us, we were able to account for all the differences and produce wonderful images.”
In spite of these challenges, the cruise was successful and the early images are already being reviewed. Because our understanding of the processes driving subduction zones is limited, these images will only prompt further questions and avenues of exploration, but we will be closer to understanding the earthquakes that shake our region.
Andrea Watts is a graduate student in the University of Washington’s School of Environmental and Forest Sciences. As part of her graduate work, she interned with the Washington Sea Grant office. This article idea was borne from a conversation with H. Paul Johnson, a professor in the UW’s School of Oceanography who is conducting research on the Cascadia Subduction Zone.