The Really Big One

A little-known fault line in the Pacific Ocean could cause “the worst natural disaster in the history of North America” by 2065, earthquake experts predict. The Cascadia subduction zone, which runs offshore from Vancouver, Canada, to Northern California, is where the Juan de Fuca oceanic plate is sliding underneath the North American tectonic plate. At some point, there will be a “backstop,” which will cause the continental plate to “rebound like a spring.” If only part of the subduction zone rebounds, an earthquake with a magnitude of between 8.0 and 8.6 will occur – comparable to the 2011 earthquake and tsunami in Japan. If the entire fault line opens up, however, the resulting earthquake will reach a magnitude of up to 9.2. Because of the rupture, the continental shelf beneath the ocean “will drop by as much as six feet and rebound 30 to 100 feet to the west,” triggering a massive tsunami that will affect about 140,000 square miles along the Pacific Northwest coast. The US Federal Emergency Management Agency (FEMA) estimates that a Cascadia earthquake and tsunami will kill almost 13,000 people (the figure could rise if it hits during main tourist season), injure another 27,000 and displace nearly a million more. It will hit the Pacific Northwest particularly hard due to the lack of an early-warning system and other seismic safeguards.

“We now know that the odds of the big Cascadia earthquake happening in the next fifty years are roughly one in three. The odds of the very big one are roughly one in ten. Even those numbers do not fully reflect the danger – or, more to the point, how unprepared the Pacific Northwest is to face it.”

Scientists found out about the Cascadia subduction zone only recently. In the 1980s, researchers studying the growth-ring patterns of dead trees in a “ghost forest” near the Washington Coast concluded that the trees all died suddenly between 1699 and 1700. They died after being inundated with salt water when the ground beneath them dropped because of an earthquake. Meanwhile, historic records from the northeast coast of Japan document that an “orphan tsunami” – a powerful wave that hit the coast without an earthquake preceding it – occurred in 1700. Scientists now believe that on January 26, 1700, a Cascadia earthquake with a magnitude of 9.0 caused the two seemingly unrelated events. Seismologists have calculated that the average amount of time between earthquakes at the Cascadia subduction zone is 243 years – making the next great Cascadia earthquake long overdue. Paleoseismologist and Cascadia fault-line expert Chris Goldfinger estimates that there is a “one-in-three” chance that a Cascadia earthquake will occur by 2065. He puts the odds of a full-margin rupture at about “one in ten.”