Earth's Background Free Oscillations

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Science  27 Mar 1998:
Vol. 279, Issue 5359, pp. 2089-2091
DOI: 10.1126/science.279.5359.2089

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Earth's free oscillations were considered to be transient phenomena occurring after large earthquakes. An analysis of records of the IDA (International Deployment of Accelerometers) gravimeter network shows that Earth is freely oscillating at an observable level even in seismically inactive periods. The observed oscillations are the fundamental spheroidal modes at frequencies between 2 and 7 millihertz. Numerical modeling indicates that these incessant excitations cannot be explained by stacked effects of a large number of small earthquakes. The observed “background” free oscillations represent some unknown dynamic process of Earth.

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