Mar 14, 2011


Amazingly this post won't have anything directly to do with what is going on in Japan. There's enough coverage by people smarter than me on that subject. But the Sendai earthquake did get me interested in the physics of the event and I haven't seen much coverage on that. Basically the Sendai earthquake was what is known as a megathrust earthquake which only happen at subduction zones.  Subduction zones are those regions where two tectonic plates are moving towards each other. One goes over the top and one goes under. All 9.0+ earthquakes have occurred in subduction zones.

This video kind of shows the effect I'm talking about.

This all got me thinking... Hmmmm. Where else do subduction zones exist?

Turns out my old backyard. And potentially where my backyard might be in the future. The Pacific Northwest or specifically the Cascadia Subduction Zone. This picture does a good job of laying out the subduction zone in relation to places you know.

The legend points out red triangles for volcanoes (The Ring of Fire).  This is caused by the weakening inland of the plate by the other plate subducted under it.  The reddish obloid is the area of significant slip and the red line within is the actual boundary where the two plates meet.

Earthquake magnitude turns out to be proportional to the length of the fault line.  I'm not sure why this is so I'm taking it on faith.  The Cascadian Subduction Zone happens to be one of the biggest and therefore will produce extremely large earthquakes. The last earthquake in that region was in January of 1700 (The Cascadia Earthquake). These large earthquakes have occurred every 500 years on average over the last 3,500 years. All 7 earthquakes had evidence suggesting tsunamis. In fact since the area wasn't heavily populated much of the evidence comes from Japan and their tsunami records.

The last earthquake was between 8.7 and 9.2 in magnitude.  Big. The fault ripped pretty much across its entire length and shifted over each other by 20 meters.

Seattle will be somewhat protected because it's inland through many waterways.  But it won't be immune from significant damage when a big one hits.  Also the topology of the Pacific Northwest has extremely steep shores. The recent earthquake in Japan was in a location where the shore was quite level and flat causing waves that weren't massive in height but travelled inland in huge volume.  The steep shores of the Pacific Northwest should create more of a Hollywood tsunami.  Good job we have a condo on the water.  It'll be great viewing.  For a few seconds.

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