One of the best science stories you'll read all year is Kathryn Schulz's unnerving piece in The New Yorker about the Cascadia subduction zone, a little-known fault line along the US Pacific Northwest that has produced tremendous earthquakes in the distant past — and is all but certain to strike again someday. If another truly large earthquake ever hit, it could ravage buildings and bridges throughout Seattle, Tacoma, Portland, and Eugene — and, much more worryingly, produce a giant tsunami capable of killing thousands of people along the West Coast. As Schulz recounts, scientists didn't even realize earthquakes occurred along the Cascadia subduction zone until the 1980s. Roeloffs' bottom line was that we can't say precisely when the next gigantic earthquake will hit Cascadia — all the more reason we should start preparing now. We don't have any official records of past subduction zone earthquakes in Cascadia, so scientists have had to reconstruct history from indirect evidence. About 19 of those Cascadia earthquakes, however, were truly enormous events, running the full length of the fault — magnitude 9.0 or greater.
Only about 5 percent of earthquakes have smaller "foreshock" quakes that occur beforehand — so that's only mildly helpful. 1946 EarthquakeCanada's largest recorded earthquake on land, measuring 7.3 in magnitude, occurred west of Courtenay on Sunday morning, June 23, 1946. Landslide!When Slopes FailLandslides are common on the steep wet slopes of Vancouver Island. In order to do so, seismologists would need to find some sort of telltale sign that regularly pops up before an earthquake hits.
More recently, GPS and other instruments have allowed scientists to track plate motion more precisely, but it's not yet clear how all this data might be used to predict earthquakes.

The first wave arrived in Port Alberni about six hours after the earthquake, and four more major waves arrived over the next seven hours. Periodically, the two plates get stuck by friction, building up enormous strain over many hundreds of years until the rocks suddenly slip past each other, releasing that pent-up energy in seconds — and creating a staggeringly large earthquake.
Careful sleuthing by two scientists determined that a tsunami known to have slammed Japan in 1700 was actually caused by a giant earthquake in the Pacific Northwest. As best we can tell, the really enormous subduction zone earthquakes that run the full length of the Cascadia fault line — magnitude 9.0 or more — seem to occur every 500 to 600 years, on average.
In 2012, a team led by Chris Goldfinger of Oregon State University looked at marine sediment cores for signs of underwater landslides thought to be linked to earthquakes. Schulz cites a FEMA analysis suggesting that 1 million buildings in the Pacific Northwest could either collapse or be compromised in the event of a major earthquake.
This, after all, is where the vast majority of casualties are likely to happen — it was the cause of most of the deaths in Japan after the 2011 Tohoku earthquake. The January 1700 MegaquakeGreat subduction zone earthquakes release 30 to 100 times more energy than the 1946 earthquake, but fortunately occur only once every 500–600 years, on average. Tsunami!A destructive tsunami, triggered by the huge Alaska earthquake of March 1964, struck the west coast of Vancouver Island. They may be large or small, rapid or slow, natural or human-induced, but they generally require steep slopes and a trigger such as rain or an earthquake. And since then, Schulz writes, scientists have deduced that there have been roughly 41 earthquakes along the Cascadia subduction zone in the last 10,000 years.

Meanwhile, in a recent Reddit AMA chat, three earthquake experts explored various nuances about infrastructure throughout the Pacific Northwest.
The most recent of the great earthquakes, in January 1700, triggered a tsunami that destroyed First Nation villages on the west coast of Vancouver Island. 2001Today, few residents of Vancouver Island remember the 1946 central Vancouver Island earthquake, although most of us felt the Nisqually earthquake of February 28, 2001.The 2001 earthquake produced less shaking than the 1946 earthquake, partly because it was a smaller magnitude, and partly because its source was deeper in the Earth's crust. Wrangellia Terrane, which forms most of Vancouver Island, originated within what is now the Pacific Ocean and was transported northeasterly to collide with North America about 100 million years ago. When the next major earthquake strikes Vancouver Island, damage and loss of life is expected to be much greater. The force of this collision crumpled North American rocks as far to the east as the Rocky Mountains.Vancouver Island experiences frequent earthquakes because it is close to the western margin of the North America Plate.
Working conditions underground were harsh; about 700 men died mining coal on Vancouver Island.
The oceanic Juan de Fuca Plate dives down into the Earth's mantle beneath Vancouver Island, creating the stresses that are responsible for earthquakes. Damaging earthquakes on Vancouver Island can occur within the North America Plate, much deeper within the Juan de Fuca Plate, or at the boundary between these two plates.

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