User:Graeme Bartlett/Planetary hum

Planetary hum or continuous oscillation of the Earth is a mysterious infrasonic noise detected by geophones. The noise consists of Rayleigh waves propagating though the solid rock of the earth. The sound switches seasonally between hemispheres rising and falling over a period of 6 months. The frequency of the sound is around 0.01 Hertz. It is caused by coupling from the oceans.

These vibrations in the rock can be used to map the interior of the Earth via computed tomography, They have the advantage that they are continuously produced, at many places apart from earthquake zones, and thus can be used to help map the subsurface of seismically quite areas.

Source
The source of planetary hum was originally though to be due to volcanoes and earthquakes, but this has been ruled out. Speculation of the cause of the hum has been strong seasonal winds, or the waves caused by these winds. When the vibration of waves is carried downwards from the surface to ocean depths, this is called infragravity waves. The earthquake contribution of tiny continuous quakes to background noise is calculated as 0.06 ngal. However the major contribution to this background is bigger quakes of magnitude five of above, and larger earthquakes happen less frequently the more powerful they are.

For energy vibrations with frequencies below 3 mHz, the large peak is caused by atmospheric pressure variations. Vibration energy between 50 and 200 mHz peaking at 143 mHz is mainly due to waves on the ocean.

Noise with frequencies above 1 Hz are mainly made by humans.

Characteristics
The sounds making up planetary hum are a mixture of very low frequency tones that each play for several minutes. The spectral range covers about two octaves (factor of four). The spectrum is roughly flat above 3.5 milliHertz but rises steeply below 3 mHz. The spectrum between 2 and 7 mHz contains many peaks, and most of these correspond to fundamental mode vibration frequencies of the whole Earth. The vibration overtones are for this hum have terms designated oS13, oS14, and so on to around oS60. A broad peak of intensity occurs around 7 to 9 mHz.

The intensity is stronger in the afternoon and evening, and weaker in the morning. However the vibrations are nonstop through the day. The acceleration intensity is slightly under 1 nanogal. The whole planet vibrates continuously even when not triggered by earthquakes.

The six monthly variation in intensity of the 2 to 10 mHz vibrations has its peaks in December to February, and June to August. The intensity variation can be up to 6% above the average for the peaks and similarly 6% lower than average for the equinox troughs. The seasonal peaks in intensity are generated from large waves in the winter time in the North Pacific North from Japan, and the North Atlantic, and also in the winter though most of the Southern Ocean.

The modes of the low frequency waves below 0.3 Hz are Rayleigh waves travelling at around 3.5 km/s, but above this frequency faster P waves appear with velocities of 13.5 km/s.

History
The existence of continuous background seismic noise was first noticed by Bertelli in 1872 in Italy. He used a pendulum, and saw that it moved more in response to changing weather in the area. Indian Monsoons were associated with microseisms in 1924 by Banerji. in 1941 Bernard found that the main frequency was double that of swell.