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Meet The 'Really Big One': The Northwest's Megaquake

July 16, 2015

Maybe you learned about it in high school, heard it on OPB, saw it in newspapers or maybe you have a subscription to The New Yorker. Or maybe all this earthquake talk is new to you.

Seismologists predict that the Northwest has a 37 percent chance of experiencing a Cascadia Subduction Zone earthquake happening in the next 50 years. It will be so disruptive, it will change the Pacific Northwest forever.

But how prepared will residents be?

That depends on what we all do now, before it strikes. You can prepare for it. And the first step to preparedness is knowledge. So, let's meet "The Really Big One."

How We Know The Megaquake Is Coming

Over its 10,000-year rupture history, earthquakes around magnitude 9.0 occurred along the length of the Cascadia Subduction Zone 19 times — about every 526 years. The southern section of the CSZ has seen 19 additional quakes of 8.0 or higher.

Along this southern section, the average recurrence is every 234 years. With the last major event placed at 315 years ago, we’re due.

Read more: How scientists know a magnitude 8.0 or 9.0 earthquake hit here 315 years ago.

Umm, What's A Subduction Zone Earthquake Again? ¯\_(ツ)_/¯

A subduction zone is a large area where two plates of the Earth’s crust meet and one is forced under the other. The plates sliding past each other cause extreme amounts of force to build up as friction restricts the movement. When the amount of force exceeds the friction holding it back, the plates slide past each other, causing the ground to rumble. 

This causes the largest type of earthquake on the planet.

Learn more: Why these 9.0 earthquakes happen.

How Powerful Is A 9.0 Earthquake?

A magnitude 9.0 earthquake is measured in moment magnitude, which is the measurement of the exponential increase of energy as the magnitude increases.

Here's another way to look at it: If a magnitude 3.9 earthquake equals 1 grain of sand, then a magnitude 9.0 would equal 100 million grains, or about 572 pounds.

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The South Island, New Zealand earthquake occurred as part of the aftershock sequence of the 7.0 magnitude earthquake in 2010. It involved oblique-thrust faulting at the easternmost limit of previous aftershocks, and like the mainshock itself, is broadly associated with regional plate boundary deformation as the Pacific and Australia plates interact.

Steve Taylor (Ray White)