Cascadia Megaquake - The Big One

The 1700 Cascadia earthquake occurred along the Cascadia subduction zone on January 26, 1700 with an estimated moment magnitude of 8.7–9.2. The megathrust earthquake involved the Juan de Fuca Plate from mid-Vancouver Island, south along the Pacific Northwest coast as far as northern California. The length of the fault rupture was about 1,000 kilometers (620 miles), with an average slip of 20 meters (66 ft). The earthquake caused a tsunami which struck the west coast of North America and the coast of Japan. The geological record reveals that "great earthquakes" (those with moment magnitude 8 or higher) occur in the Cascadia subduction zone about every 500 years on average, often accompanied by tsunamis.

As seen in the 1700 quake, the 2004 Indian Ocean earthquake, and the 2011 Tōhoku earthquake and tsunami, subduction zone earthquakes can cause large tsunamis, and many coastal areas in the region have prepared tsunami evacuation plans in anticipation of a possible future Cascadia earthquake. However, the major nearby cities, notably Seattle, Portland, Vancouver, Victoria, and Tacoma, which are located on inland waterways rather than on the coast, would be sheltered from the full brunt of a tsunami. These cities do have many vulnerable structures, especially bridges and unreinforced brick buildings; consequently, most of the damage to the cities would probably be from the earthquake itself. One expert asserts that buildings in Seattle are vastly inadequate even to withstand an event of the size of the 1906 San Francisco earthquake, let alone any more powerful one.

Kenneth Murphy, who directs FEMA's Region X, the division responsible for Oregon, Washington, Idaho, and Alaska, put it quite dramatically: "Our operating assumption is that everything west of Interstate 5 will be toast."

Recent findings conclude that the Cascadia subduction zone is more complex and volatile than previously believed. In 2010, geologists predicted a 37% chance of an M8.2+ event within 50 years, and a 10 to 15% chance that the entire Cascadia subduction zone will rupture with an M9+ event within the same time frame. Geologists have also determined the Pacific Northwest is not prepared for such a colossal quake. The tsunami produced could reach heights of 80 to 100 feet (24 m to 30 m). (Wikipedia) 

Cascadia Subduction Zone earthquake involves a magnitude 9.0 or worse earthquake in the Pacific Northwest that would be felt all the way down to Northern California. The Pacific Coast would have perhaps an hour of warning before the tsunami hit. As a reference point to how bad this earthquake will be, the recent earthquake in Japan that registered magnitude 6.2 lasted 20 seconds. The Cascadia quake would shake the earth for three to six minutes.

The devastation would be great: If it happened tomorrow, there would be perhaps more than 10,000 deaths and 30,000 injuries.

Besides the dead and injured, the damage to infrastructure would be enormous. One of the big threats during such a quake is liquefaction. Many critical structures stand on silt and sand that become unstable. The grainy soil will begin to act like liquid, and structures — such as bridges, ports, airports and industrial facilities — may shift position or sink.
Local residents will have 20 to 30 minutes to get to high ground and away from the effects of the inevitable tsunami, which will consist of multiple waves over several hours. Some areas will remain flooded even as the tsunami retreats. (GovTech)


The Cascadia Earthquake: America’s Worst Disaster? (Video, PBS SoCal)

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