October 21, 2013
Want to see what happens to the ground in the United States when an earthquake snaps the crust elsewhere in the world? The waves ripple outward through the continent oscillating each county and city in turn.
This video shows real data from seismometers deployed across the country. Each dot represents a seismometer. Each instrument’s motion is displayed here as alternating red (for upward motion) and blue (for downward motion). Individually they would just look like flashing dots (representing those characteristic squiggles), but collectively they show real seismic waves emanating from the Gulf of California as they ripple outward beneath our feet. Don’t worry, you didn’t feel this because the motions are minute, and actually a fair bit slower than the playback speed.
This nifty animation is possible because of a rolling deployment of seismometers, the USArray (I’ve posted about this before), which is part of the Universe’s #1 Epic Project. Normally, the array takes up only a fraction of the continent–thanks to some reduction in government funding–but for this special animation they used a brilliant technique that combines data from similar, nearly repeating earthquakes.
Ground Motion Visualizations like this are routinely produced by IRIS, a consortium of universities and research institutions that curate and disseminate seismological data. This one is super special because it exploits the uncanny repetition of large earthquakes in this Gulf of California location. In some lucky coincidence there were similar earthquakes during enough different phases of the seismic array’s deployment that they managed to capture in every part of the U.S. wave propagation from the same spot. Neato!
This shows us not only the source location and basic propagation of elastic seismic waves, but it reveals the detailed structure of the continent (the ultimate objective of the array). Perturbations in the even circular propagation of those waves are due to changes in lithology, density, and temperature beneath different portions of the continent, revealing a lot of information that’s hard to get at from the surface.
On a more technical note, my other favorite thing about this animation is the visible nodal plane between different quadrants of the double-couple focal mechanism. The waves switch polarity across a northeast-oriented line that corresponds to the auxiliary nodal plane, which pleasingly happens to strike straight through the country. You can trace it right back to the “beach-ball” displayed on the map. Mmmm, strike-slip. If you have no idea what I’m talking about there’s a neat little primer here. Or just skip it.