The Cascadia earthquake: what it would mean for you, and what to do

No one wrote down the 1700 earthquake in the Pacific Northwest. Its date comes from three kinds of evidence that agree with each other. Along the Washington coast, the land dropped during the quake and seawater killed coastal forests. Tree-ring dating of these drowned "ghost forests", such as the dead cedars at Copalis, shows the trees died in the winter of 1699 to 1700. In Japan, officials and villagers recorded a damaging tsunami that came with no earthquake felt there. Researchers linked this "orphan tsunami" to Cascadia in the 1990s. Oral histories of the region's coastal tribes also describe the event.

DOGAMI, Oregon's geology agency, puts the time at about 9 pm and the size at about magnitude 9.0. The coast sank several feet. USGS says the geologic record shows at least 19 great earthquakes (magnitude 8 or more) along most of the coastline in the past 10,000 years. Oregon Emergency Management counts 43 earthquakes on the fault in that period. The higher count also includes smaller breaks that ruptured only part of the fault.

Nobody can predict the day. The figures below are odds over 50 years, worked out from how often past earthquakes happened. They differ by source and method, so each one is shown with the agency or study behind it.

Sources also give different intervals between great quakes. The USGS national hazard model uses about 500 years for full-fault breaks. Washington DNR gives about 500 years from offshore records, or about 400 if onshore evidence is added, and says the gaps have ranged from about 50 to 850 years. DOGAMI gives about 480 to 505 years for full breaks and about 220 years for partial breaks in the south. These are averages, not countdowns, and 1700 was more than 320 years ago.

⚠ In Puget Sound, a deep or shallow earthquake is much more likely in your lifetime than Cascadia. The same shaking advice applies to all of them, so preparing for Cascadia also prepares you for these.

Isolation: Oregon Emergency Management warns that landslides, liquefaction and broken bridges could cut coastal towns off from each other and from inland help. It calls these cut-off areas "islands", and it has published "Cascadia Island" maps for the coast from Astoria to Brookings.

The same plan estimated 1,250 to more than 10,000 deaths in Oregon from the earthquake and tsunami together, and more than $30 billion in losses. It also flags fuel as a weak point. Most of Oregon's liquid fuel comes from Washington and is stored along a six-mile stretch of the lower Willamette River, on riverside soil that can liquefy. For the Washington coast, the state's magnitude 9 scenario estimates 50,000 to more than 65,000 tsunami casualties in the first hour, out of more than 175,000 residents and visitors in the inundation zone. That scenario assumes a summer night at 2 am and people leaving 10 to 20 minutes after the shaking starts. These are planning estimates, not forecasts.

ShakeAlert is the USGS earthquake early warning system for Washington, Oregon and California. It can send a phone alert seconds before strong shaking reaches you. Washington EMD cites research that for most Pacific Northwest earthquakes the warning will be 10 seconds or less. It also notes the Washington sensor network was about 65% complete, and that some people may get no alert in a magnitude 9. DOGAMI says that even with more offshore sensors, a Cascadia warning may be only a few seconds. That is still enough to Drop, Cover and Hold On, and for trains to slow and valves to close. PreparedBC says British Columbia's own early warning can give seconds to tens of seconds of notice for quakes around magnitude 5 or greater.

⚠ Never wait for an alert to act. If you feel shaking, protect yourself right away. The alert is a bonus, not the trigger.

A tsunami from Cascadia would form just offshore, so the earthquake itself is your warning. Washington EMD says waves could reach the outer coast 10 to 20 minutes after a major quake. DOGAMI gives 10 to 20 minutes for the beach in its fact sheet, and 15 to 20 minutes on its tsunami FAQ. A tsunami from far away, such as from Alaska, takes 4 or more hours to arrive, according to DOGAMI. That usually leaves time for an official warning. Inner waters are also at risk: Washington DNR says the Seattle Fault and other crustal faults can cause tsunamis too, and every tsunami death in the state's history has come from a landslide-triggered tsunami.

British Columbia splits its coast into five tsunami notification zones, A to E. The west coast of Vancouver Island (Zone C) faces the open Pacific and the Cascadia fault. Zone D is the Juan de Fuca Strait to Greater Victoria. Zone E is the Strait of Georgia, Greater Vancouver and Johnstone Strait. PreparedBC's advice is the same: after shaking near the shore, go to high ground without waiting for a warning, and never go to the beach to watch the waves.

In 2013 the Oregon Resilience Plan recommended moving public advice from the old 72 hours to at least two weeks, possibly more. Oregon now asks every household to "Be 2 Weeks Ready". Oregon Emergency Management says that after Cascadia, people could be without services and help for at least two weeks, and possibly longer. Washington EMD also recommends two weeks of food, water and supplies, and Washington DNR says to plan on being on your own for at least two weeks. PreparedBC asks for at least 3 days and recommends 1 to 2 weeks, warning that services may be disrupted for weeks.