First Observation of the Earth's Permanent Free Oscillations on Ocean Bottom Seismometers

The Earth's hum is the permanent free oscillations of the Earth recorded in the absence of earthquakes, at periods above 30 s. We present the first observations of its fundamental spheroidal eigenmodes on broadband ocean bottom seismometers (OBSs) in the Indian Ocean. At the ocean bottom, the e...

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Bibliographic Details
Published in:Geophysical research letters 2017-11, Vol.44 (21), p.10,988-10,996
Main Authors: Deen, M., Wielandt, E., Stutzmann, E., Crawford, W., Barruol, G., Sigloch, K.
Format: Article
Language:English
Subjects:
Autocorrelation
Broadband
broadband ocean bottom seismology
continuous oscillations
Data processing
Earth
Earth orbits
Earth Sciences
Earthquakes
Fourier transforms
Free vibration
Frequency dependence
Geophysics
glitch
hum
noise
Noise reduction
Ocean bottom
Ocean bottom seismometers
Ocean currents
Ocean floor
Oceans
Oscillations
Regression analysis
Response functions
Sciences of the Universe
Seismic activity
Seismographs
Seismometers
surface waves
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Summary:The Earth's hum is the permanent free oscillations of the Earth recorded in the absence of earthquakes, at periods above 30 s. We present the first observations of its fundamental spheroidal eigenmodes on broadband ocean bottom seismometers (OBSs) in the Indian Ocean. At the ocean bottom, the effects of ocean infragravity waves (compliance) and seafloor currents (tilt) overshadow the hum. In our experiment, data are also affected by electronic glitches. We remove these signals from the seismic trace by subtracting average glitch signals; performing a linear regression; and using frequency‐dependent response functions between pressure, horizontal, and vertical seismic components. This reduces the long period noise on the OBS to the level of a good land station. Finally, by windowing the autocorrelation to include only the direct arrival, the first and second orbits around the Earth, and by calculating its Fourier transform, we clearly observe the eigenmodes at the ocean bottom. Key Points Earth's eigenmodes observed at ocean bottom in the absence of earthquakes Linear regression and frequency transfer functions are effective methods to remove tilt and compliance Autocorrelation of continuous data and windowing to show eigenmodes at PREM frequencies
ISSN:0094-8276
1944-8007
DOI:10.1002/2017GL074892