There has not been a major earthquake in the Cascadia subduction zone since 1700. But every year or so, there is a month-long ‘slow slip’ of tectonic plates.
It is well known that, off the coast of Vancouver Island, the massive undersea Juan de Fuca tectonic plate is slowly sliding under the larger North American plate, putting the west coast of Canada at grave risk of a megathrust earthquake and resulting tsunami.
What is lesser known about this process is the massive amount of energy that is unaccounted for, and might be building silently toward catastrophe. Newly published research based on artificial intelligence claims to have found it in a constant, low, background tremor, and, with it, a new way of predicting when the next big one might hit.
There has not been a major earthquake in what is known as the Cascadia subduction zone since about 9 p.m. on Jan. 26, 1700, long before Europeans first landed on Vancouver Island. The event is moderately well known to historians, however, for two reasons.
One is the oral histories and archeological remains of the various First Nations whose villages were swamped by a tsunami. Storytellers began to speak of dwarfs in the mountains who danced around a drum, causing the earth to shake and waters to rise. Others described a sea battle between a thunderbird and a whale that caused “a shaking, jumping up and trembling of the earth beneath, and a rolling up of the great waters,” according to research by Alan McMillan, who studies the archeology and ethnography of Vancouver Island First Nations at Simon Fraser University.
The other reason is that the tsunami also went the other way, westward to Japan, making landfall the next day, killing hundreds and sinking boats.
Ever since, Cascadia has been relatively quiet. But every year or so, there is a roughly month-long “slow slip,” when the North American plate lurches southwesterly over the Juan de Fuca plate.
This slip can account for as much as half of the total relative motion of the tectonic plates, as measured by displacement on the surface. It is usually accompanied by bursts of tremors, but the physics of “slow slip” is not well understood. It only occurs in some regions of the fault line between the plates, for example, while others appear to be “locked,” and only move in megathrust events.
This slip has been observed in advance of major earthquakes, which suggests it might be part of the process that causes them, according to a new paper by Bertrand Rouet-Leduc and Claudia Hulbert of the Los Alamos National Laboratory in New Mexico.
There is also a strange discrepancy between the amount of surface movement as measured by global positioning systems and the amount of energy measured by seismographs in tremors.
Somewhere in this process, a vast amount of energy has gone missing.
The key to finding it was to apply the tools of supervised machine learning, a kind of artificial intelligence that can separate a real signal from background noise. In a newly published paper, the researchers claim to have found that the Cascadia subduction zone is “continuously broadcasting a low-amplitude, tremor-like signal,” and that this low amplitude hum from the depths of the Earth “may account for most of this missing energy.”
Recognizing what this hum reveals about fault line physics might lead to something like an early warning system that “may prove useful in determining if and how a slow slip may couple to or evolve into a major earthquake,” the authors write in the journal Nature Geoscience.
They used seismic data from the Canadian National Seismograph Network and global positioning data from stations in the Western Canada Deformation Array, processed by the United States Geological Survey.
They found the Juan de Fuca plate is moving under the North American plate at around four centimetres a year as the North American plate slips over it. The highest slip speed for the North America plate, moving in the other direction, was also around four centimetres a year, which means that the transition zone between the plates — coastal British Columbia, basically — was slipping at around eight centimetres a year.
“The continuous tremor-like signal we identify tracks the slow slip rate, apparently at all times, and so provides real-time access to the physical state of the slowly slipping portion of the megathrust,” the authors write. “As the slow earthquakes transfer stress to the adjacent locked region where megaquakes originate, careful monitoring of this tremor-like signal may provide new information on the locked zone, with the potential to improve earthquake hazard assessment in Cascadia.”