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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.

"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.

In addition, organizations in British Columbia and Canadian governmental agencies also linked their earthquake preparedness exercises to coincide with FEMA's four-day event. While Oregon has planned for this worst-case scenario in the past 10 years during other exercises, FEMA's Cascadia Rising was held on a much larger scale.

Estimates of when the next major earthquake will occur range from 10 to 15 percent within the next 50 years, Gomberg said, adding it could happen tomorrow, or within several hundred years. When the area finally does rupture, it could slowly tear apart along the 600-mile stretch, which could cause several minutes of shaking and displace a massive amount of seawater.

Within approximately 20 minutes of a major earthquake, a colossal wall of seawater would hit the shoreline, Gomberg said. Unfortunately, depending on the time of year and location, some people will not be able to avoid the deadly wave, she added.

According to Cory Grogan, public information representative for the Oregon Office of Emergency Management, the main concern for preparedness is how many are at risk, directly or indirectly, from a Cascadia subduction zone earthquake and tsunami event.

Grogan agreed with Gomberg, stating that given that a portion of the population along coastal Oregon is located within the identified tsunami inundation zone, the timing of the event would have an impact on public safety and could vary based on a variety of factors.

"In general terms, planning estimates place the number of fatalities and injuries statewide at 5,200 and 15,500, [sourced from the Cascadia Rising Scenario], respectively as a result of both the earthquake and tsunami," Grogan said.

"In a broad sense, the entire state will be impacted from a catastrophic earthquake, either directly or indirectly, as infrastructure and networks fail, transportation routes are impacted, fuel becomes unavailable, businesses go defunct, and commodities become scarce. So, total impact to people could equal the current population plus transit visitors."

The largest Oregon coastal cities that would be directly impacted by a tsunami include Astoria, Seaside, Lincoln City, Newport, North Bend, Coos Bay, Florence, Bandon, Gold Beach and Brookings. Additional smaller coastal cities, towns and communities would also be impacted, he added.

"Any significant subduction zone quake would have immediate impact on coastal and valley areas of the state," Grogan said. "It is estimated that, depending upon how the subduction zone fractures, warning time for a coastal tsunami would be between 15 and 30 minutes."

Fortunately, the cities of Vancouver, Seattle and Portland do not sit directly on the coastline, but the threat of shaking still poses a risk to older structures not designed to withstand earthquakes, Gomberg said.

While technology continues to advance scientists' understanding of the Earth's plate movements and its rich, geologic history, at this point in time, there are no warning systems for early seismic detection.

Tsunami warning systems are in place, and evacuation routes have been mapped in coastal regions, Gomberg said. However, earthquakes strike without advanced warning and the amount of time to react in coastal areas will be limited.

"One of the primary goals of Cascadia Rising is to train and test this whole community approach to complex disaster operations together as a joint team," FEMA reported, adding that recent subduction zone earthquakes around the world like the 2011 Tohoku quake "underscore the catastrophic impacts we will face when the next [Cascadia subduction zone] earthquake and tsunami occurs in our region."

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