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  3 0 First things first. Seattle need not worry about the San Andreas fault...it is way down in California and runs from Cape Mendocino south to near the Mexican border. Far away from us! Now, closer to home. A number of active fault zones are known or suspected. There is a (rather crude, but reasonable) map at: http://www.pnsn.org/INFO_GENERAL/puget_faults.html and to see it you can click here . Major actors include the Seattle Fault Zone that runs east-west just south of downtown, the Tacoma fault which runs through -- you guessed it, Iowa (no, just kidding--Tacoma), and the South Whidbey Island fault to the north of the Seattle Fault zone. And of course, do not neglect the subduction fault that lies buried 40 miles or so beneath us--but which is only seismically active to the west. (There's a nice image of the subduction zone here ). Along this fault the sea floor on the Juan de Fuca plate offshore is being driven beneath North America. Stress that arises from that process (known as "subduction") ends up driving all the earthquakes we experience. To learn a bit more about what those earthquake sources are, you can read up on them at: ht tp://www.pnsn.org/INFO_GENERAL/eqhazards.html   Paul Bodin answered in Earthquake! over 2 years ago | Report 3 comments by MyrnaMinkoff, Paul Bodin, and Carly Dykes… 0 0 Ok, follow-up question: What exactly is a fault line? I always thought they were the areas where one tectonic plate meets another...but then I think of tectonic plates as being huge landmass-sized hunks of rock, floating around on magma. I also usually think of faults as following various geographic features (think of Drake's Bay on Pt. Reyes). That's clearly not happening in the dozen fault lines around us. In the link above, the fault lines look more like ripples or folds. Can you explain a bit more about how they work? MyrnaMinkoff commented over 2 years ago | Report 0 0 Ah! Geological jargon. To be technical, a fault is a planar surface within Earth across which there is a relative displacement of the two sides. [A similar surface, but with no relative displacement, is called a "joint"]. Anyway, you are right that boundaries between tectonic plates are fault zones. And you are right that faults often follow geographic features--like rivers or coastlines; usually because the fault either grinds up the rock or there is a big contrast of rock types across the fault that leads to the formation of a prominent topographic feature. But faults can also form within rock masses. They are just zones of weakness that slip in responses to stresses in the rock. And plate tectonics is a sloppy business...not all the relative displacement is necessarily taken up by the main plate boundary fault. This results in residual stresses that drive slip on secondary faults (such as the Seattle fault), albeit at much slower rates. One model to think about to picture faults relieving stresses in solids is to mix yourself a nice gin and tonic. There--isn't that better? Ooops, forgot the demo part...now drop an ice cube or two in and in no time they'll be some loud pops. Those are ice quakes as faults in the ice cubes slip suddenly in order to relieve the thermal stresses you just created in the ice cube by dropping it into room temperature fluid! Paul Bodin commented over 2 years ago | Report 0 0 When would an Earthquake on the Seattle Fault be due? Carly Dykes commented over 2 years ago | Report Enter your comment or ask a new question New comments are not being accepted at this time.

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