This is an illustration of plate subduction. On a horror scale of Adam Sandler films, this is only Just Go with It.
This is an illustration of plate subduction. On a horror scale of Adam Sandler films, this is only Just Go with It. Vitoriano Junior / Shutterstock.com

If your palms aren't sweaty already with thoughts of the teetering viaduct, please know that there isn't an earthquake that will hammer us. Seattle (delightfully, terrifyingly) can be struck by three different kinds of earthquake—each with a distinct character, risk to life and property, and aftermath.

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Living Seattleites have only experienced the mildest of the three.

Interplate/Deep Earthquake

Representative Example: The February 2001 Nisqually quake.

Character: This is the earthquake you might have lived through in 2001. Most recent construction in the Seattle area is designed to survive (or gracefully fail from) this sort of earthquake. Older brick buildings and crumbling-concrete-based transportation infrastructure are most likely to fail.

How often: Every 30-50 years or so. Last was in 2001.

What will happen if you're on the viaduct/western end of the 520 bridge if another quake like this strikes: Probable death/major injury.

Degree of Horror (Adam Sandler Movie Equivalent): You Don't Mess with the Zohan.

Silver lining? Seattle fared reasonably well from the 2001 earthquake. We'd probably do okay during the next one as well.

Subduction Zone/Megathrust Earthquake

Representative Example: The March 2011 Tohoku/Fukushima earthquake

Character: Truly massive movement of tectonic plates beneath the ocean floor off the Washington (and Oregon and Northern California) Coast, resulting in many minutes of intense shaking, followed by a massive Pacific-wide tsunami. The damage to Seattle—particularly low-lying landfill areas—will be extreme, and accompanied by widespread destruction of the entire region's infrastructure.

How often: Every 500 years, or so. Last was in about 1700 (based on Japanese records of the resultant massive tsunami).

What will happen if you're on the viaduct/western end of the 520 bridge if another quake like this strikes: Probable death/major injury.

Degree of Horror (Adam Sandler Movie Equivalent): Just Go with It.

Silver Lining? If we can get the Earthquake Early Alert system working properly here, we'd probably have a few minutes of advance notice here in Seattle before the shaking starts, to prepare.

The Japanese early earthquake alert system is amazing. Seriously, it's amazing.

Shallow/Crustal/Blind Thrust Fault aka the "Seattle Fault"

Representative Example: The 1995 Kobe earthquake.

Character: The most destructive earthquake for Seattle—almost certain to result in massive destruction to buildings in the city, landslides, a local (but large) tsunami in Elliot Bay, with the forces of the quake magnified by the geology of the Seattle basin. We are only now really beginning to understand what this fault—roughly paralleling I-90 through the city—is capable of.

The fault is theorized to produce two distinct types of quakes. The most recent from this fault rose the half of the city south of it by about three meters (9-10 feet). Consider how much energy it takes to move half a city up 10 feet—all in a few minutes.

How often: Last was in about 900 (based on geologic records and Native American oral histories). We're probably overdue for another.

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What will happen if you're on the viaduct/western end of the 520 bridge if another quake like this strikes: Probable death/major injury.

Degree of Horror (Adam Sandler Movie Equivalent): Jack and Jill

Silver Lining? Not much. You know Harborview (the region's only level 1 trauma center) is built nearly on top of this fault?

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