Every time an earthquake, somewhere, is big enough to dominate headlines, everyone here in this northwestern corner of the United States starts wondering what it means for us.

Warning map from NOAA National Tsunami Warning Ctr, for a tsunami generated by the March 11, 2011, Honshu earthquake (yellow star is the epicenter, with the Americas to the right).

This has happened frequently enough (e.g. Haiti, Chile, New Zealand) that I’m thinking it would be useful to create a basic FAQ for earthquakes. Lots of FAQ’s exist, written by scientists, or emergency preparedness teams. But I haven’t come across one that’s written with the fears and fascinations of the public in mind. I guess I’d start with a basic fact:

Not all earthquakes are of the same kind.

Just like there are different kinds of volcanic eruptions (picture Mt. St. Helens blasting its top, vs. Hawaii’s Kilauea oozing lava as tourists stand nearby) — there are different types of quakes. Many types. Too many, so let’s start with a basic division: Subduction zone quakes vs. crustal quakes. What you see in Japan is the result of a subduction quake. New Zealand suffered from a crustal quake. Subduction quakes are typically off-shore, and these are the most common sources of tsunamis. Crustal quakes can run directly under a city, such as Seattle or Los Angeles, and in the aftermath, you sometimes can see a big section of earth thrust six or eight feet up from the ground next to it. The region around Seattle gets both kinds. Readers: Any other suggestions for another Mini-Lesson about  Earthquakes? Re. the subduction quake, much has been written by other news sources (not all of it accurate, some of it overly alarmist). I wrote this following piece for KPLU: ——————— You may have heard Washington has an earthquake fault similar to the one that devastated Japan.  While there are many fault-lines criss-crossing western Washington, the only one that bears a strong similarity is under the ocean, parallel to our coast-line.  It’s called the Cascadia subduction zone. Story starts on January 26, 1700 The Cascadia fault zone lies about 80 miles off-shore, where one of earth’s plates slides under another. It’s been silent for three centuries.  But, there’s strong evidence that in 1700 it let loose an earthquake slightly bigger than the one that hit Japan last week — sending a huge tsunami toward the Washington and Oregon coasts.

“It was probably a nightmare of five or six minutes of shaking, followed by a wall of water 30 to 40 feet high,” says oceanographer and tsunami expert Frank Gonzalez of the University of Washington, imagining what people here might have experienced. “It had to be a horrendous experience.”

Gonzalez, who had a long career studying tsunamis at NOAA, helped design the warning system of computers and ocean buoys that predicted where and when last Friday’s tsunami would hit the Pacific coast. All the estimates and descriptions for a future devastating tsunami in the Pacific Northwest are based on that one three centuries ago. The Cascadia fault could rupture again, at any time, although it also could sit silent for another century or two. Most estimates say it’s on a 300-500 year recurrence cycle. The details – who’s most at risk According to computer models, our urban areas in Seattle, Tacoma, and Everett (along with Portland and Vancouver, B.C.) would see buildings damaged, roads buckled and some bridges collapsed. But, as in Japan, where few people died from the earthquake itself, much worse damage would be found on the coast. The ensuing tsunamis would obliterate Ocean Shores, Westport, and the Long Beach peninsula, perhaps in less than a half-hour.  Towns on the Oregon coast would be even worse off, because the fault runs closer to their shoreline, says Tim Walsh, chief geologist of the Washington Department of Natural Resources (DNR) hazards program. Seaside, Ore. would likely get hit the worst. One study estimated about 50,000 people might die across the region. Consider the images of Japanese towns completely wiped away by the waves:

“Earthquakes like this are reminders, and that they would happen in Japan where we have close ties, brings it all closer to home, somehow,” says Brian Atwater of the US Geological Survey and the University of Washington.

By the time an ocean tsunami hit Puget Sound, after passing through the strait of Juan de Fuca, it might be ten feet high, rather than 30-50 feet in Japan. The brunt of that wave would hit Whidbey Island, although many low-lying shorelines could see flooding, according to hazard maps created by DNR. Japan’s experience, plus Chile’s and Papua New Guinea’s offer lessons The fault that ruptured off of northern Japan hadn’t had such a big quake for over a thousand years.  In the year 869, a very similar set of tsunamis swept across the Sendai region, according to recent research. Brian Atwater and colleagues wrote a classic essay called “Surviving a Tsunami,” based on three big ones prior to 1999 (and updated in 2005). It includes these sub-headings:

• Many Will Survive the Earthquake
• Heed Natural Warnings
• Heed Official Warnings
• Expect Many Waves
• Head for High Ground and Stay There
• Abandon Belongings
• Don’t Count on the Roads
• Go to an Upper Floor or Roof of a Building
• Climb a Tree
• Climb onto Something that Floats
• Expect the Waves to Leave Debris
• Expect Quakes to Lower Coastal Land
• Expect Company

The biggest new lesson so far from Japan is that debris – that means cars, busses, even houses – swept up in water moving 30 miles per hour can do more damage than anyone realized. Japan also has invested more than any other country in structures — such as evacuation towers, flood-gates on rivers, and sea-walls. It’s still too early to say which of those helped. It’s possible those defenses work for more common earthquakes.  But, this quake was the fourth or fifth strongest ever recorded. In an email, one scientist in Japan says she heard the tsunami washed people off the roof of an office building that was more than 40-feet tall. How can you prepare for that?

“You are talking about one of the forces of nature that packs more force per pound than any other,” says Gonzalez, referring to tsunamis. “You can minimize the fatalities, you can minimize the damage, but there are some things in nature where you can’t eliminate the danger.”