Los Angeles and San Francisco are not the only west coast cities sitting on shaky real estate. North of these usual seismic suspects, lies Vancouver, British Columbia, which, according to a new study published in Nature, may be in greater danger of experiencing a massive earthquake than anyone had previously imagined.
The study, funded by the Earth Institute at Columbia University, shows that an offshore fault line is longer and closer to land than scientists had thought, and that the activity along this fault poses the possibility that a megaquake, greater than 9 on the Richter scale, could cause catastrophic damage on the Canadian mainland, according to Dr. Mladen Nedimovic, a seismologist Lamont-Doherty Earth Observatory (a partner of the Earth Institute).
"Obviously the closer you are to the rupture, the greater the shaking will be," Nedimovic said. "The data does suggest that the danger is greater, but how much greater, I did not even try to estimate."
Nedimovic and his team focused their research on the northern Cascadia margin, an area off the Pacific coast where one tectonic plate slides over another, forming what is known as a subduction zone.
The hazards presented by subduction zone were not unknown when they began their work, he said. But no one realized how close it came to shore.
Nedimovic looked at a section of the margin known as a megathrust, a region of serious seismic pressure, capable of generating massive earthquakes every few hundred years. In this case, seismologists have determined that this particular megathrust fires off a quake with a magnitude of 9 or greater every 200 to 800 years. Since the last megaquake happened in the 1700s, the northern Cascadia region has already entered a potentially dangerous seismic period.
"Nobody really knows when that would happen," he said, "but we are certain that it will happen."
Nedimovic and his colleagues used a technique called reflection imaging – which employs powerful bursts of sound – in order to reach their conclusions. While the method itself is nothing particularly new, what was new was the way that Nedimovic put together the data, including two years of his own measurements, as well as energy industry data "to fill in the gaps."
"Basically, all the oil and gas that we find – all the deposits – are found using this method," he said. "With the reflection method, you can actually image the structure. You can create much greater detail with much greater accuracy."
Other methods showed that the megathrust on the Cascadia margin was about 36 miles long. Nedimovic's survey, however, showed it to be 20 miles longer than that – 20 miles directly east, closer to civilization.
This finding may have profound implications on disaster planning in that area, Nedimovic said – or at least he hopes it does, considering the level of danger that region will face in the future (even though he admits that could be hundreds of years in the future).
Alaska, Chile and Japan are the next on Nedimovic's agenda, since there are megathrusts in each of these places that show similar characteristics to the Cascadia subduction zone.
"Deep seismic reflection images from Alaska, Chile, and Japan show a similar broad reflection band above the megathrust in the region of stable sliding and thin thrust reflections further seaward where the megathrust is locked, suggesting that reflection imaging may be a globally important predictive tool for determining the maximum expected rupture area in great subduction earthquakes," said Nedimovic. "Mega earthquakes have been instrumentally recorded for all three regions making them potential targets for a future investigation to confirm the reflection method and improve characterization of megathrust seismic hazards in the study area."
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