believe they have pinpointed the origin of a low frequency "hum"
that emanates from the Earth.
The scientists say they have ruled out
earthquakes as the source
The noise - which can be picked up in the two and seven mHz
(millihertz) range - occurs far below the threshold of human
hearing, US researchers said.
A University of California team propose the hum is a product of
storm energy being converted into other forms in the Pacific and
Details of the study appear this week in the academic journal
The daily release of energy required to generate the hum is
equivalent to a magnitude 5.75 to six earthquake, say Junkee Rhie
and Barbara Romanowicz of the University of California, Berkeley.
They show that the hum is not distributed over the entire surface
of the globe as previously thought. Instead, they have pinpointed
the source of the "noise" to the seas not the land.
But the seismologists say that this noise cannot be explained by
summing the contributions of small earthquakes. So they analysed
seismic waves emitted from the Earth.
These so-called Rayleigh waves were measured on two networks of
seismic instruments in Japan and California. The Rayleigh waves are
usually generated from medium to large magnitude earthquakes. But
not in this case.
They filtered their data to detect and locate non-earthquake
sources for these waves.
Both sets of measurements agree that the "hum" originates from
the northern Pacific Ocean during the northern hemisphere winter and
the Southern ocean during southern hemisphere winter.
These locations also correspond to the regions of maximum storm
activity in the northern and southern winters.
Rhie and Romanowicz think some of the energy contained in
powerful waves generated by ocean storms at mid-latitudes is
transferred via infragravity waves to the seafloor.
Infragravity waves are probably generated in shallow water from
more common types of ocean wave and are indirectly driven by winds
over ocean basins.
On the seafloor, these are converted to seismic waves and it is
these that generated the hum.
"The Earth's hum is generated by the interaction between
atmosphere, ocean and seafloor," the researchers write in Nature.
The efficiency with which the seismic waves are generated depends
on the shape and depth of the ocean floor and on the strength and
persistence of the storms.