Off the coasts of southern British Columbia, Washington, Oregon, and northern California, there is a 600-mile-long band where the Pacific ocean floor slowly dives eastward beneath North America. This area, called the Cascadia Subduction Zone, contains giant thrust faults that cause tectonic plates to move in opposite directions in a very dangerous way. The plates can lock periodically, building up stress over a large area, eventually breaking free when they hit each other. The result was the world’s largest earthquake, shaking both the ocean floor and land and generating a tsunami more than 100 feet long. These deficiencies in Japan led to the Fukushima nuclear accident in 2011. Similar areas also exist in Alaska, Chile, and New Zealand. In Cascadia, major earthquakes are known to occur approximately every 500 years, or once every few hundred years. The last incident occurred in 1700.
Scientists have long sought to understand the subsurface structure and dynamics of the Cascadia subduction zone to determine which places are most vulnerable to earthquakes, their magnitude, and warning signs. There is no such thing as predicting an earthquake. Rather, scientists try to predict the probabilities of different scenarios, hoping to help authorities design building codes and warning systems that can minimize damage if something happens.
Newly Published Study We are committed to significantly advancing these efforts. A research vessel towing a series of state-of-the-art geophysical instruments along almost the entire section has conducted the first comprehensive survey of the many complex structures beneath the seafloor. This includes the geometry of the underlying oceanic plate and overlying sediments, and the composition of the overlying North American plate. The study was recently published in the journal Science Advances.
“The models currently used by public agencies are based on limited, old, low-quality data from the 1980s,” he said. Susan Cabotmarine geophysicist Lamont-Doherty Earth Observatory, which is part of the Columbia Climate School. “Megathrusts have a much more complex geometry than previously assumed. “This study provides a new framework for earthquake and tsunami risk assessment.”
Lamont’s research vessel will be launched in 2021 with funding from the U.S. National Science Foundation. Marcus G. Langseth. Researchers aboard the ship penetrated the seafloor with powerful sound waves, read the echoes and converted them into images, somewhat similar to how doctors create scans of the inside of the human body.
One key finding: The megathrust fault zone is not just one continuous structure, but is divided into at least four parts, each potentially insulated to some degree from the movement of the other parts. Scientists have long debated whether past events, including the 1700 earthquake, ruptured the entire region or just parts of it. This is an important question. This is because the longer the rupture, the larger the earthquake.
The data show that the segment is divided by buried features, including large faults sliding opposite each other perpendicular to the coast. This can help prevent jumping from one segment to the next. “We can’t say for sure that this means only a single part ruptured or everything ruptured at the same time,” said Harold Tobin, a geophysicist at the University of Washington and co-author of the study. “But this strengthens the evidence that there is a segmented tear.”
The image also hints at the cause of the split. The hard edge of the overriding North American continental plate is made up of many different types of rocks that formed at different times over tens of millions of years, some of which are denser than others. This diversity of continental rocks causes the incoming, more flexible oceanic plates to bend and twist to accommodate differences in pressure above them. In some places the segments dip at relatively steep angles, in others at shallow angles.
Researchers focused on one section in particular, starting on southern Vancouver Island along Washington state and ending almost at the Oregon border. Other parts of the subsurface are relatively rough, with oceanic features such as faults and subducted seamounts touching the upper plate. These features limit the size of earthquakes by allowing the upper plate to erode and limiting the distance an earthquake can propagate within that section. In contrast, the Vancouver-Washington route is very smooth. This means it is more likely to rupture at once along its entire length, making it potentially the most dangerous part.
Also, in this part, the ocean floor is subducting beneath the continental crust at a shallower angle than in other parts. Elsewhere, much of the earthquake-prone interface between plates is offshore, but here the study found that shallow subduction angles mean it probably extends just below Washington’s Olympic Peninsula. This may lead to increased land shaking. “More research is needed, but in places like Tacoma or Seattle, it could mean the difference between a warning and disaster,” Tobin said.
A consortium of state and federal agencies and academic institutions, funded by the U.S. Geological Survey, has already been reviewing the data since it became possible to piece together its meaning.
Tsunami risk “is still a work in progress,” said Kelin Wang, a research scientist with the Geological Survey of Canada, who was not involved in the study. Wang’s group is using this data to model features of the seafloor off Vancouver Island that could trigger tsunamis. (Typically, tsunamis occur when the deep ocean floor moves up or down during an earthquake, sending waves to the surface, concentrating their energy and gathering height as they reach shallower coastal waters.) Wang said his results are similar to those of others. He said it would be passed on to the group. He models the tsunami itself and then passes it on to another group that analyzes the risk on land.
Researchers say the actual assessment, which could influence building codes or other aspects of preparation, could be published as early as next year. “There is a much more complex issue here than previously inferred,” Carbotte said.
