The Earth's Hum Observed at the Indian Ocean Seafloor
Abstract
The Earth's « hum » is defined as continuous oscillations of the earth, generally observed at periods greater than 30 seconds. This very weak signal has not previously been observed using ocean bottom seismometers (OBS), which have higher noise in this frequency band, mostly due to seafloor currents and ocean waves. We present observations of the hum on broadband INSU ocean bottom seismometers deployed in the Indian Ocean in 2012/13 at depths up to 4.8 km, as part of the RHUM-RUM experiment. The hum, invisible in the raw data, becomes visible after removing the effect of ocean infragravity waves (compliance) and seafloor currents (tilt) using transfer functions between the pressure, horizontal and vertical components. The noise levels vary according to site and season, depending on the water depth, presence and thickness of sediments, and ocean infragravity wave amplitudes. We were able to reduce the compliance signal by up to 15 dB and the tilt contribution by up to 25 dB. The resulting spectra reach a base level of -174 dB in acceleration for the frequency band of 3 to 9 mHz, which is remarkably low compared to results from other Ocean Bottom Station experiments. The best results are obtained using transfer functions calculated using data from the same day as the noise removal. This low level enables to observe normal modes spectral peaks in the presence of an earthquake and for seismically quiet periods, with signal to noise ratios up to 0.1. Finally, we present the hum's temporal variation and we model it using a numerical model that is based on the interaction of ocean infragravity waves at the continental shelf. We compare the real and modeled hum to validate the source generation theory of the hum.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFM.S43A2794D
- Keywords:
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- 3384 Acoustic-gravity waves;
- ATMOSPHERIC PROCESSESDE: 0728 Ice shelves;
- CRYOSPHEREDE: 4560 Surface waves and tides;
- OCEANOGRAPHY: PHYSICALDE: 7255 Surface waves and free oscillations;
- SEISMOLOGY