Sandy Shook US Like an Earthquake

The primary background seismic hum of the Earth is called the microseism and is caused by winds in the ocean generating standing waves. The microseism is composed mostly of Rayleigh waves (a type of seismic wave that propagates on the surface) and is strongest at periods of around 7 seconds. The data in this animation have been filtered around this period in order to highlight the changing strength of the microseism. This movie shows hour long median amplitudes at each station spanning three weeks of data (2012-10-14 to 2012-11-04). Each station typically records many different channels of data. The four panels shown plot 1) vertical ground motion 2) differential air pressure 3) North-South ground motion and 4) East-West ground motion.

The apparent wavefront going from Florida and spreading North and East on 10/26 starting at 13:00 EDT is likely showing changing isoseismic contours indicative the time-varying strength of the microseism being affected by Sandy. The excitation of the stations (changing sizes of the symbols) is unlikely a wavefront passing since Rayleigh waves propagate at velocities of around 3 km/sec. The NE propagating contour has an apparent velocity that is much too slow, only a few hundred km/hour. So this may represent the growing intensity of the storm whose seismic noise "reaches" greater distances. During the three weeks plotted, there were many large earthquakes whose amplitudes were orders of magnitudes stronger than the noise levels being highlighted. Their effects have purposefully been suppressed to show the storm's effect on background noise levels.

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