Microseisms (from the Greek micro for "very small" and seism for "shaking") are faint, continuous tremors of the Earth caused by natural processes. Most seismologists consider them to be annoying background noise that interfere with the detection of earthquakes, underground explosions, or oil exploration sources. However, microseisms sometimes become interesting in their own right.

As hurricane Katrina plowed its way over the Gulf of Mexico during the last week in August, its high winds whipped the sea surface into a frenzy of huge waves. The low atmospheric pressure in the eye of the storm raised the sea surface and as the storm moved onto the shallow continental shelf the shoreward winds east of the eye piled additional water onto the coast. All of these processes generated microseisms that traveled as sound vibrations through Earth's solid crust to the shore, and were recorded at seismograph stations.

The images on this page illustrate this story, as told by one seismic station at Livingston, Louisiana, operated by the LI.LTL (LIGO-SCSN) operated jointly by the LIGO Livingston Observatory of Caltech and the Southern California Seismic Network. LIGO is supported by the United States National Science Foundation. Each "helicorder" record shows the ground motion in a 12 hour period, with time increasing in 15 minute lines (each of a separate color) from top to bottom. The microseisms grow relentlessly as Katrina approaches slowly for at least 2 days. On August 28 the microseisms are so strong that the lines are overwriting each other. Finally on the 29th of August Katrina slams ashore at just after 6 a.m. Central Daylight Time. At about 6:10 the seismic station loses communications with the network.

The polygons on the map designate the time period for each helicorder. You can click on the map to see each helicorder or click on the small graphic at the right to see the full size.