A megaquake, tsunami would forever alter the Pacific Northwest coastline within 30 minutes

Stretching more than 600 miles along the coastline from Vancouver Island down to California's Cape Mendocino lies a sleeping, tectonic giant capable of causing massive devastation and drastically changing the face of the Pacific Northwest.
The Cascadia subduction zone rests beneath the waves approximately 100 km offshore where the oceanic Juan de Fuca Plate converges and slides under, or subducts beneath, the western edge of the North American continent.
The same fierce geologic forces which gave rise to the Cascade Range and cement the region's place in the Ring of Fire have also been responsible for massive earthquakes, volcanism and catastrophic tsunami events in the past.
In 2011, an earthquake generated a devastating tsunami that was observed across the Pacific and caused tremendous local devastation in Japan. (Photo/Shunichi Koshimura)
"The last great [Cascadia] earthquake occurred 300 years ago," Seattle-based USGS seismologist Joan Gomberg said, referencing the megathrust quake that rocked Cascadia in 1700 and sent tsunami waves across the Pacific, damaging villages in Japan.
Unlike other subduction zones, or megathrusts, which are the result of other convergent plate boundaries, the Cascadia seems to be strongly locked and rarely releases its housed energy in the form of smaller magnitude quakes. Instead, it tends to rupture on a larger scale, according to Gomberg.
"That's one difference as we don't seem to have the frequency [of smaller magnitude quakes]," she said. "We just go big."
The return interval of the Cascadia is approximately 500 years, but some megaquakes have occurred within as short a time as 200 years or upwards of 1,000 years, Gomberg added.
Evidence of its destructive impact in the region can be found in the geological makeup of the land as well as the remains of ghost forests, or areas of trees that fell with the land and were submerged by seawater, Gomberg said.
Other indicators of the tectonic plate movements include marked movement of the land, a technique in measuring its effects that has greatly improved with technological advancements like GPS, Gomberg said.
Since seismologists cannot predict when earthquakes will occur, a Cascadia rupture leading to an 8.0 or higher magnitude earthquake would strike the Pacific Northwest without warning, a potential disaster the Federal Emergency Management Agency (FEMA) addressed within a four-day exercise of unprecedented scale.
Last month between June 7-10, FEMA held its Cascadia Rising exercise, which included participants from the U.S. military, 50 counties, major cities, tribal nations, state and federal agencies, members of private business and other nongovernmental organizations across Washington, Oregon and Idaho.

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