Along the west coast of North America, the Cascadia subduction zone stretches more than 1,000 kilometers from Vancouver Island to Cape Mendocino, Calif. It produced a magnitude 9 megathrust earthquake about 300 years ago, one of the biggest quakes in world history.
Scientists know that Cascadia will produce another earthquake at some point in the future; the question is how soon. The odds of it happening in the next 50 years are 1 in 3. The Federal Emergency Management Agency projects that Cascadia’s next megathrust earthquake will cause thousands of deaths and injuries and leave millions in need of shelter, food, and water.
To better understand subduction zones, scientists often study the thermal environments of material that has been pushed up onto the surface during past earthquakes. This buildup of material, called an accretionary wedge, might consist of rock, soil, sand, shells, or any other kind of debris. These wedges also sport subtly different average temperatures at various depths, compared to material located off the wedge.
In a recent study, Salmi et al. examined the thermal environment of the Cascadia subduction zone’s accretionary wedge, which stretches for about 97 kilometers along the coast of the state of Washington. Their goal was to find out more about the physical changes of fluids and solids within the wedge in the hopes that the knowledge can help them better anticipate future earthquakes.
Using data collected on a cruise by the R/V Marcus G. Langseth, the researchers found significant variations in temperature within this section of the Cascadia subduction zone, as well as signs of gas hydrates (ice-like deposits that form from natural gas at the bottom of the ocean) throughout the region. They also detected that most fluids from the deep move upward through the accretionary wedge instead of through the crust, which is different than in most other subduction zones. This change in fluid pathway prevents the plate from cooling and reduces the area where an earthquake might rupture along the two plates: completely within the accretionary wedge, rather than under the continental plate.
This is the first study to concentrate on the southern Washington margin alone, rather than the subduction zone as a whole, revealing the influence of fluid distribution on local, small-scale temperature variability. This insight opens the door to further research into how local temperature variability might interact with other factors, like stress or fault roughness, to affect earthquake hazards. Overall, this study provides a revised method for probing the thermal environment of an accretionary wedge, a crucial link to the cause of ruptures in Earth’s crust that can lead to earthquakes and tsunamis.
By understanding these mechanisms more fully, scientists can tell us more about how to prepare for the smallest of tremors and the largest of megaquakes. (Journal of Geophysical Research: Solid Earth, https://doi.org/10.1002/2016JB013839, 2017)
—Sarah Witman, Freelance Writer