SOUNDS OF SEISMOLOGY:
Geodynamics, plate motion and mantle convection


Earthquakes are tiny discrete events in the slow and vast dynamics of plate tectonics and mantle convection. However, to us, earthquakes are huge events, far more powerful than anything humans could create. In this section, we progress from the global scale downward. Earthquakes occur as plates move past each other. Over millions of years, these motions are smooth, but over the short geologic time scales (of our lifetimes), the movements are rough, jerky and violent. These two movies illustrate plate motion and mantle convection. The coupling between the two is here left to your imagination (but is one of the Big Questions in earth science).


This movie is part of an extraordinary long-term effort by Chris Scotese, to assemble geologic data that constraints plate motions, positions and timing through earth history. This particular movie represents the last 750 million years of plate motions, supercontinent breakup and assembly: http://www.scotese.com/

(reload page to play this movie again-- it is an animated gif)
And the mantle convects (similar to boiling), but very slowly. The following movie shows the upwelling plumes (in black) and sinking cold slabs (in white) that drive plate motions. The movie duration represents several hundred million years of mantle convection (from Dave Yuen's group at the University of Minnesota):
http://marina.geo.umn.edu/




One of the central observations that lead to plate tectonic theory is that most earthquakes occur on plate boundaries, as shown here for Central and North America:

http://svs.gsfc.nasa.gov/vis/a000000/a000100/a000155/



When we zoom in on the San Andreas Fault in California, we can see that these boundaries are defined on the surface by networks of faults that extend, oh, about 10-20 km deep into the crust, and then give way to more ductile faults and shear zones. Much research is focused on the nature of these transitions and how they interact in space and time.




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