At frequencies too low for human hearing, the solid earth hums a tune that has a tropical flair, according to a pair of physicists at Scripps Institution of Oceanography at UC San Diego.
The earth’s “hum,” a wash of background noise at frequencies between 3 and 50 mHz, had been detected more than a decade ago but its source was unclear. Scripps physicists Peter Bromirski and Peter Gerstoft used data from the USArray Earthscope’s transportable seismic network to identify the Pacific coast of Central America as a strong hum source region, and secondarily over continental shelves along the western coasts of North America and Europe.
“These locations have big waves and the nearshore bathymetry and coastline configuration have the right characteristics,” Bromirski said.
The pair described the association of hum with waves traveling along the West Coast by analyzing data collected by US array, which monitors earth movements across the United States. Though the hum had been thought to be the result of atmospheric disturbances, Bromirski and Gerstoft describe an ocean-wave path that starts with storms over the deep ocean that generate swell.
When swell energy reaches coastlines, it is transformed into long-wavelength waves known as infragravity waves. The energy from these coastal waves generates the solid earth hum.
“This gives us an indication of the amount of wave energy that is impacting coasts,” said Bromirski. “There are potential uses for investigating crustal and upper mantle structure once these signals are better understood.”
This is not Bromirski’s first look into the interactions of the solid earth with the planet’s ocean waves. He is using seismograms archived over decades to produce a history of microseisms generated by waves along the West Coast. The record of wave energy serves as a retrospective indicator of climate change and violent storm occurrence dating back to 1930.