40 Fun Facts About Cascadia Subduction Zone

Which country's records of a mystery tsunami let scientists date the last great Cascadia earthquake?

Japan logged waves that arrived with no shaking felt first, and working backwards from them put the rupture at about nine o'clock on a single evening in 1700.

What are the stands of dead grey trunks called that mark where the Cascadia coast dropped into the tides?

Ghost forests stand at more than thirty spots on the Oregon and Washington coast, where salt killed the trees but also held off the fungi that would rot them.

Which mythical creature battles a whale in Pacific Northwest stories tied to the 1700 Cascadia earthquake?

Thunderbird seizes the whale and drops it from the sky in tellings shared by several coastal peoples, a struggle that shakes the mountains and flattens trees.

Which magazine ran 'The Really Big One', the Pulitzer-winning article on the Cascadia earthquake threat?

The New Yorker piece earned staff writer Kathryn Schulz the 2016 Pulitzer Prize for Feature Writing and a National Magazine Award.

Which California fault meets the Cascadia subduction zone at a triple junction off Cape Mendocino?

San Andreas quakes in the north may even be set off by Cascadia: traces of past shocks suggest most big ones of the last 3,000 years followed a Cascadia rupture.

Which Cascade volcano, fed by magma from the Cascadia subduction zone, blasted out sideways in 1980?

Mount St. Helens lost about 1,300 feet of summit when a magnitude 5.1 quake set off a huge landslide, and no Cascade volcano has been more active in 10,000 years.

The Cascadia subduction zone's northern end lies off which landmass, home to the city of Victoria?

Vancouver Island towns such as Tofino and Ucluelet could drop a metre or two in a great quake, a 2004 study found.

In which season did the last great Cascadia earthquake strike, a detail preserved in Huu-ay-aht oral history?

Winter is when the Huu-ay-aht account sets the wave, on an evening just after villagers had gone to sleep; only one hillside village on Pachena Bay survived.

For how long is the coast expected to shake in a magnitude 9 Cascadia earthquake?

About 5 minutes is the low end of Oregon's official estimate of five to seven, with the shaking weakening the farther inland you are.

The strongest earthquake ever recorded, a 1960 subduction quake like Cascadia's, struck which country?

Chile's Valdivia earthquake reached about magnitude 9.5, and geophysicists who likened quiet Cascadia to that fault in the 1980s were met with scepticism.

Which small plate, named after a Greek sailor in Spain's service, is sliding beneath North America at Cascadia?

Juan de Fuca was born Ioannis Phokas, and the plate carrying his name is one of Earth's smallest, creeping toward the continent at about 4 cm a year.

What name do geologists give the kind of earthquake the locked Cascadia fault produces when it finally slips?

Megathrust quakes account for every earthquake of magnitude 9 or more since 1900, because only subduction faults break across a large enough area.

The Cascadia subduction zone belongs to which 40,000 km belt of volcanoes and earthquakes around the Pacific?

Ring of Fire faults elsewhere are far noisier, which is why seismically quiet Cascadia was wrongly thought harmless until the 1980s.

Which volcano, the tallest peak in the Cascade Range, is counted among the most dangerous in the world?

Mount Rainier carries more glacier ice than any other peak in the lower 48 states, so an eruption could send mudflows through valleys where about 80,000 people live.

What force keeps the shallow part of the Cascadia fault locked for centuries while stress builds?

Friction holds the two plates together down to about 30 km, and when the stored stress finally beats it the rocks lurch past each other in a giant quake.

Which tree, still standing dead in Washington tidal marshes, dated the last great Cascadia earthquake?

Red cedar was the fallback: nearby spruce trunks were too rotted to read, but the cedars' growth rings ran up to 1699, months before the earthquake.

What nickname did scholars give the 1700 tsunami that reached a far shore with no earthquake felt beforehand?

Orphan suited a wave whose parent earthquake nobody on that shore had felt; the parent turned out to be a rupture on the far side of the Pacific.

What kind of building in Westport, Washington, opened in 2016 with the first US tsunami refuge on its roof?

A school: Ocosta Elementary put its refuge on top of the gym, 53 feet above sea level, because the only way off the low peninsula is a single-lane highway.

What substance, driven out of the sinking plate beneath Cascadia, melts the mantle and feeds the volcanoes above?

Water bound up in the slab's minerals is released by heat and pressure, and it sharply lowers the melting point of the hot rock overhead.

A full Cascadia rupture is often compared with the fault break behind the deadly 2004 tsunami in which ocean?

Indian Ocean shores were struck in 2004 after a fault break of about 1,300 km; Cascadia's fault runs about 1,000 km, and its tsunami could reach about 100 feet on the nearest coast.

Ash from which collapsed Oregon volcano, now holding Crater Lake, marks time in Cascadia's seafloor quake cores?

Mount Mazama's eruption left an easily seen ash band in every core, and an early study counted 13 earthquake layers stacked above it.

Cascadia's small sinking plates are remnants of which vast ancient plate, now mostly gone beneath North America?

Farallon crust has been sinking under the continent for about 200 million years, and other surviving scraps include the Cocos and Nazca plates farther south.

Which technology revealed in 1999 that the deep Cascadia fault can slip silently, with no earthquake felt?

GPS stations caught the ground briefly reversing direction across a wide area, the trace of about 2 cm of silent slip on the deep fault, equal to a magnitude 6.7 earthquake.

What do seismologists call the faint rumbling that joins slow slip on the Cascadia fault about every 14 months?

Tremor is so weak that only very sensitive seismometers pick it up, and Canadian scientists who spotted the repeating pattern named it episodic tremor and slip.

Which California mountain, the southernmost active volcano of the Cascade arc, erupted from 1914 to 1917?

Lassen Peak's biggest blast, in May 1915, threw ash 280 miles to the east, and the eruptions prompted a national park around it the following year.

Which Washington coastal people, known to Twilight readers, tell of a flood that swept canoes inland to Hood Canal?

Quileute tradition fits a great tsunami, and in 2012 the tribe secured federal land to move its village to higher ground.

Cascade peaks such as Shasta and Baker, built above the Cascadia subduction zone, are which kind of volcano?

Stratovolcanoes are steep cones of stacked lava and ash; their thick, slow-flowing lava and explosive eruptions set them apart from broad shield volcanoes.

Which Washington range is built largely of seafloor rock scraped off the plate sinking at Cascadia?

Olympic Mountains rock began as ocean floor jammed against the continent, and an English captain named the highest peak in 1788 after the home of the Greek gods.

Pythia's Oasis, a warm seafloor spring on the Cascadia fault, is named after the priestess of which Greek oracle?

Delphi's Pythia was said to prophesy in a trance brought on by vapours seeping from an opening in the rock; her namesake spring gushes fluid from 2.5 miles down.

A 1964 tsunami that killed people in Oregon and California came from an earthquake centred in which US state?

Alaska's Good Friday earthquake, the strongest recorded in North America, sent waves down the coast yet left no layer in Cascadia's seafloor record.

Which 2001 earthquake struck inside the sinking slab beneath Puget Sound and injured about 400 people?

Nisqually's quake followed deep slab earthquakes under the same region in 1949 and 1965, making three damaging shocks in 52 years.

Which Oregon beach community's 2,000-year-old Sitka spruce stumps were laid bare by storms in 1997–98?

Neskowin's stumps are widely read as the mark of a sudden drop in the land during an earlier Cascadia quake, though some geologists argue that creeping dunes buried the trees instead.

Which American geologist uncovered the buried marshes that first proved great Cascadia earthquakes in the 1980s?

Brian Atwater found his first clue at Neah Bay in 1986 with a small hand hoe: marsh plants sealed under sand after the ground suddenly dropped.

The Cascadia subduction zone has no visible deep-sea trench because the trench is completely buried by what?

Sediment 3 to 4 km thick fills the trench, so the seabed shows almost no dip where the plate dives under the continent.

Which undersea avalanches, set off by shaking, left the seafloor layers used to count past Cascadia earthquakes?

Turbidity currents from separate canyons merged into single layers, showing they ran at the same moment, as only a great earthquake would make them.

While the Cascadia fault stays locked between great earthquakes, what is happening to the land along the coast?

Slowly rising land is the locked plates squeezing the coast upward; when the fault lets go, that ground drops in moments, which is what drowned the coastal trees.

Which US agency runs ShakeAlert, the earthquake early warning system for California, Oregon and Washington?

USGS alerts went statewide in California in 2019 and in Oregon and Washington in 2021, completing the West Coast rollout.

The Cascade Range, whose volcanoes rise above the Cascadia fault, was named for rapids on which river?

Columbia River rapids in the gorge were the last obstacle for Lewis and Clark; the peaks above became the 'mountains by the cascades', and the rapids now lie under a reservoir.

For how long does Oregon tell residents to be ready to live without services or help after a Cascadia earthquake?

2 weeks is the minimum Oregon plans for, because landslides and broken bridges are expected to cut coastal communities off from one another.

Which small plate at Cascadia's southern end, off northern California, is being crumpled inside its own boundaries?

Gorda is squeezed by its two giant neighbours, so earthquakes often strike in its interior, and its sinking edge feeds Mount Shasta.