Oceanic and atmospheric activity around the globe produces persistent background signals, called microseism, that are visible on seismographs everywhere on the planet.

These displays are spectrograms that show the distribution of vertical ground velocity energy as a function of time and frequency between approximately 0.001 and 0.45 Hz.  The times indicated on the horizontal axis are Universal Time days.

Katrina was one of the most intense storms every observed during its period as a category 5 hurricane (its eye pressure of 902 millibars was the fourth lowest ever measured in the Atlantic). Huge waves produced in the Gulf of Mexico are probably the principal source of the associated low-frequency seismic signal.  The microseismic signal from Katrina was not only extremely strong in the regional vicinity, for example at station DWPF at Walt Disney World, Florida (top), but also appears to be weakly observable as far away as the ultra-quiet station QSPA, which is located the icecap near the South Pole (118 degrees away).  

Energy at lower frequencies than the microseism is routinely generated by earthquake activity, and several prominent earthquakes from around the world are also visible in the spectrograms.

Figure by Kent Anderson (IRIS Consortium and New Mexico Tech) and Rick Aster (New Mexico Tech).  DWPF and QSPA stations are part of the Global Seismographic Network (GSN).  The GSN is operated by the US Geological Survey, Albuquerque Seismological Laboratory, and the University of California, San Diego. Support for these networks is provided by the National Science Foundation (through the IRIS Consortium) and U.S. Geological Survey.  Data provided by IRIS/USGS and distributed through the IRIS Data Management System.