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Science 27 March 1998:
Vol. 279. no. 5359, pp. 2089 - 2091
DOI: 10.1126/science.279.5359.2089
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Reports

Earth's Background Free Oscillations

Naoki Suda, * Kazunari Nawa, Yoshio Fukao

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.

N. Suda and K. Nawa, Department of Earth and Planetary Sciences, Nagoya University, Nagoya, Japan.
Y. Fukao, Earthquake Research Institute, University of Tokyo, Tokyo, Japan.
*   To whom correspondence should be addressed. E-mail: suda{at}eps.nagoya-u.ac.jp

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