Analysis of ‘background’ free oscillations and how to improve resolution by subtracting the atmospheric pressure signal

Abstract It is well established that the Earth oscillates continuously with its fundamental modes excited even on seismically quiet days but the source of this ‘background’ excitation is unclear. The source could be internal (for example, tectonic motions) or external (for example, atmospheric turbulence). The GEOSCOPE broad-band seismic network provides a long-term, high quality, seismic data set to better observe these oscillations. Nine years of data from the quietest GEOSCOPE station (WUS) show background oscillations as well as evidence of a small annual variation with the most energy in June and July. We also use 4 years of simultaneous pressure/seismometer data from the TAM station to show how to improve the resolution of these oscillations by subtracting the pressure effect on the acceleration signal. Removing the pressure effect allows us to isolate some very low frequency spheroidal fundamental modes as low as angular order l=2 after large earthquakes and down to l=8 during quiet periods.

[1]  Duncan Carr Agnew,et al.  Vertical seismic noise at very low frequencies , 1978 .

[2]  J. Hildebrand,et al.  Estimating shear velocities in the oceanic crust from compliance measurements by two‐dimensional finite difference modeling , 1998 .

[3]  J. Hinderer,et al.  Effective barometric admittance and gravity residuals , 1995 .

[4]  R. Warburton,et al.  The influence of barometric‐pressure variations on gravity , 1977 .

[5]  Jean-Paul Montagner,et al.  Towards multiscalar and multiparameter networks for the next century: The French efforts , 1998 .

[6]  Toshiro Tanimoto,et al.  Cause of continuous oscillations of the Earth , 1999 .

[7]  Hiroo Kanamori,et al.  COMPUTATION OF SEISMOGRAMS AND ATMOSPHERIC OSCILLATIONS BY NORMAL-MODE SUMMATION FOR A SPHERICAL EARTH MODEL WITH REALISTIC ATMOSPHERE , 1998 .

[8]  T. Tanimoto.  Excitation of normal modes by atmospheric turbulence: source of long-period seismic noise , 1999 .

[9]  Eleonore Stutzmann,et al.  GEOSCOPE Station Noise Levels , 2000 .

[10]  W. Zürn,et al.  On noise reduction in vertical seismic records below 2 mHz using local barometric pressure , 1995 .

[11]  Naoki Kobayashi,et al.  Earth's continuous oscillations observed on seismically quiet days , 1998 .

[12]  Fukao,et al.  Resonant oscillations between the solid earth and the atmosphere , 2000, Science.

[13]  Kazunari Nawa,et al.  Incessant excitation of the Earth’s free oscillations , 1998 .

[14]  N. Kobayashi,et al.  Statistical features of Earth's continuous free oscillations , 1999 .

[15]  Gregory C. Beroza,et al.  Searching for slow and silent earthquakes using free oscillations , 1990 .

[16]  D. L. Anderson,et al.  Preliminary reference earth model , 1981 .

[17]  Thomas H. Jordan,et al.  Teleseismic Detection of a Slow Precursor to the Great 1989 Macquarie Ridge Earthquake , 1993, Science.

[18]  S. Webb,et al.  Broadband seismology and noise under the ocean , 1998 .

[19]  J. Montagner,et al.  The GEOSCOPE program: its data center , 1999 .

[20]  W. Zürn,et al.  Bichromatic excitation of long‐period Rayleigh and air waves by the Mount Pinatubo and El Chichon volcanic eruptions , 1992 .

[21]  D. Banka,et al.  Noise levels of superconducting gravimeters at seismic frequencies , 1999 .

[22]  P. Ihmlé.  Frequency-dependent relocation of the 1992 Nicaragua slow earthquake: an empirical Green's function approach , 1996 .

[23]  Walter Munk,et al.  Searching for the Earth's free oscillations , 1959 .

[24]  J. Orcutt,et al.  Relationships between ocean bottom noise and the environment , 1994, Bulletin of the Seismological Society of America.

[25]  Naoki Kobayashi,et al.  Continuous excitation of planetary free oscillations by atmospheric disturbances , 1998, Nature.

[26]  T. Jordan,et al.  Teleseismic Search for Slow Precursors to Large Earthquakes , 1994, Science.

[27]  W. Munk,et al.  Propagation of ocean swell across the Pacific , 1966, Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences.

[28]  Fukao,et al.  Earth's background free oscillations , 1998, Science.

[29]  J. Hildebrand,et al.  Seafloor Compliance Observed by Long-Period Pressure and Displacement Measurements , 1991 .

[30]  P. Lognonné,et al.  The effects of the atmospheric pressure changes on seismic signals or how to improve the quality of a station , 1996, Bulletin of the Seismological Society of America.

[31]  Jeffrey Park.  Radial mode observations from the 5/23/89 MacQuarie Ridge Earthquake , 1990 .

[32]  Raoul Beauduin.  Etude du bruit de fond sismique a l'aide des donnees geoscope et des donnees de l'experience ofm/sismobs , 1996 .