Saturday, September 21, 2013

With Every Fault is there an Earthquake?

There is nothing that motivates a teacher more than a student asking a serious question. Perhaps even more motivating is a teacher who, like us, wants to help a student.

Q: My 5th grade student Awva asked this question. Thank you!

With Every Fault is there an Earthquake? 
Amy S

A: With every fault there once WAS an earthquake - that's how the rock broke in the first place. With most faults there are many earthquakes over time. 

However, there are active faults (like the San Andreas) and there are (usually temporarily) inactive faults. There are faults mapped in Precambrian times that seem to have been completely inactive for over 600 million years. However, a fault implies a zone of crustal weakness, and it is not at all uncommon for a fault to be re-mobilized by a later tectonic event. Because the zone is already weakened, it is preferentially broken open again - it's easier to break than the surrounding rocks. I've seen billion-year-old faults in Venezuela that were activated again as the Atlantic Ocean opened between North America and Europe the first time, again when it closed, and then yet again when the modern Atlantic reopened a second time.

To make things even more complicated, however, there are faults that have "slow creep" or "silent" earthquakes. The continental-margin subduction fault off the coast of Vancouver Island, Washington State, and Oregon is an example. These events are movements of the underlying oceanic slab beneath the continental crust. Shallower parts of these faults are fluid-lubricated, and motion on them is so slow that they are detected only by continuously-recording GPS instruments - they are not felt. These silent earthquakes can take more than a day to move two fault surfaces past each other, instead of the milliseconds that would be involved for a more typical earthquake. Thus the strain is slowwwwwly released. However, the strain on rocks deeper down is then increased - this is where the surfaces are locked because fluids no longer reach them.

When one of these huge subduction earthquakes happen, it's a real attention-getter. The Great Tohoku earthquake of 2011 was a magnitude 9 event. It caused massive damage, and the Fukushima nuclear reactor complex is just getting more dangerous every week as time goes on since the original tsunami destroyed the cooling system.


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