Excitation of Earth's free oscillations by atmospheric motions
Abstract
Spheroidal Earth’s free oscillations 0Sn, where n is the number of oscillation nodes along a meridian between the poles, are observed during seismically quiet days. The seismometer measurements of these background Earth’s free oscillations with n = 12—65, located at frequencies of 2—7 mHz (periods ~8—2 min), show that the Earth’s free oscillations are caused by atmospheric processes. The following mechanism for excitation of the Earth’s free oscillations is proposed. During strong storms high waves generate oceanic internal gravity waves, which excite, in turn, Earth’s free oscillations acting on the seabed. The present study is devoted to the search for the excitation source of background Earth’s free oscillations 0S2, which is located at the frequency of about 0.3 mHz (period of about 54 min) and is the lowest frequency Earth’s free oscillations. We have used the continuous measurements of the vertical (Z) output signal of STS-2 seismometer at Collm, Germany (51.3°N, 13.0°E), for the full year of 2002. The spectral analysis was applied over the 5-day window sliding along the annual series of the seismometer measurements with 1-day step. The values of power spectral density at the frequencies of the 0S2 multiplet and the frequencies close to them are matched with the values of the Arctic oscillation index, which characterizes the intensity of dynamic perturbation of the atmosphere in the Northern hemisphere. It is found that the correlation coefficients between these values are positive and statistically valid. So, it is shown for the first time that the background Earth’s free oscillations 0S2 can be excited by dynamic processes in the atmosphere, just as it is for highfrequency Earth’s free oscillations.
About the Authors
S. I. Ermolenko
Saint-Petersburg State University
Russian Federation
Russian Federation
G. M. Shved
Saint-Petersburg State University
Russian Federation
Russian Federation
References
1. Жарков В. Н. Внутреннее строение Земли и планет. М.: Наука ГРФМЛ, 1983. 416 с.
2. Rosat S. et al. High-resolution analysis of the gravest seismic normal modes after the 2004 Mw = 9 Sumatra earthquake using superconducting gravimeter data // Geophys. Res. Lett. 2005. V. 32. L13304. doi:10.1029/2005GL023128.
3. Линьков Е. М., Петрова Л. Н., Зурошвили Д. Д. Сейсмогравитационные колебания Земли и связанные с ними возмущения атмосферы // Доклады АН СССР. 1989. Т. 306. С. 314—317.
4. Швед Г. М., Ермоленко С. И., Хоффманн П. Регистрация собственных колебаний атмосферы в диапазоне периодов 1-5 часов // Изв. РАН. Физика атмосферы и океана. 2015. Т. 51. С. 562—569.
5. Wielandt E. Seismometry // International handbook of earthquake and engineering seismology / (Eds.) Lee W.H.K., Kanamori H., Jennings P.C., Kisslinger C. Academic Press. London and San Diego, 2002. P. 283—304.
6. Climate Prediction Center (CPC), Daily Arctic Oscillation Index: ftp://ftp.cpc.ncep.noaa.gov/cwlinks/norm.daily.ao.index.b500101.current.ascii (дата обращения: 27.04.2016).
7. Duchon C. E. Lanczos filtering in one and two dimensions // J. Appl. Meteorol. 1979. V. 18. P. 1016—1022.
8. Press W. H. et al. Numerical recipes in FORTRAN 77: The art of scientific computing. New York: Cambridge Univ. Press, 1997. P. 569—577.
9. Scargle J. D. Studies in astronomical time series analysis. II. Statistical aspects of spectral analysis of unevenly space data // Astrophys. J. 1982. V. 263. P. 835—853.
10. Пустыльник Е. И. Статистические методы анализа и обработки наблюдений. М.: Наука ГРФМЛ, 1968. 288 с.
11. Nawa K. et al. Incessant excitation of the Earth’s free oscillations // Earth Planets Space. 1998. V. 50. P. 3—8; Reply V. 50. P. 887—892. 1998.
12. Tanimoto T. et al. Earth’s continuous oscillations observed on seismically quiet days // Geophys. Res. Lett. 1998. V. 25. P. 1553—1556.
13. Tanimoto T., Um J. Cause of continuous oscillations of the Earth // J. Geophys. Res. 1999. V. 104(B). P. 28,723—28,739.
14. Nishida K., Kobayashi N., Fukao Y. Resonant oscillations between the solid earth and the atmosphere // Science 2000. V. 287. P. 2244—2246.
15. Tanimoto T. Excitation of normal modes by atmospheric turbulence: Source of long period noise // Geophys. J. Int. 1999. 136. P. 395—402.
16. Ekstrom G. Time domain analysis of Earth’s long-period background seismic radiation // J. Geophys. Res. 2001. V. 106. P. 26,483—26,494.
17. Nishida K., Kobayashi N. Statistical features of Earth’s continuous free oscillations // J. Geophys. Res. 1999. V. 104(B). P. 28,741—28,750.
18. Fukao Y. et al. A theory of the Earth’s background free oscillations // J. Geophys. Res. 2002. V. 107(B9). 2206. doi:10.1029/2001JB000153.
19. Rhie A., Romanowicz B. Excitations of the earth’s incessant free oscillation by atmosphere/ocean/solid Earth coupling // Nature. 2004. V. 431. P. 552—556.
20. Nishida K., Fukao Y. Source distribution of Earth’s background free oscillations // J. Geophys. Res. 2007. V. 112. B06306. doi:10.1029/2006JB004720.
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For citations:
Ermolenko S.I., Shved G.M. Excitation of Earth's free oscillations by atmospheric motions. Fundamental and Applied Hydrophysics. 2016;9(4):3-6. (In Russ.)
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