The gentle rumble of the Earth

The forces of Earth are massive. They become evident in situations like tsunamis, earthquakes, giant floods, tornadoes and lightning. Generally, many of these happenings of nature are accompanied by very loud sounds.

However, people all over the world have heard and experienced a rumbling sensation from the Earth even when no such phenomena as those stated above were taking place anywhere nearby.

This ‘Hum of the Earth’ has caught the attention of geologists (those who study the Earth’s crust) and seismologists (those who study earthquakes) and there has been a lot of research to find out what really is behind this mysterious rumble.

A couple of reasons have already been found out regarding this queer phenomenon. Let’s see what they are.

Source of the rumble

This hum of the Earth has been there for a long time. Seismologists have found out that this is caused by some sort of vibration within the Earth’s surface. These vibrations have been caught on devices which are used to measure earthquake-related vibrations and they have appeared as a ‘noise signal’ (unwanted signal) in the collected data.

However, it is quite recently that scientists thought of following up to find out the source of this queer rumble.

It has been less noticeable to humans all this time as its frequency is lower than the lowest frequency audible to human beings. The audible range of humans is said to be between 20 Hertz and 20,000 Hertz and the frequency of these vibrations is believed to be in the range of millihertz, which is over a thousand times smaller than the lowest audible frequency of humans.

Scientists say that if the frequency of these hums were a few octaves higher, this rumble could have made enough sound to drown out the noise from a hundred TV talkshows.

Furthermore, when the vibrations of a full day are taken into account, their total level has been found out to be large enough to equal that of about six earthquakes.

Japanese scientists have been coming up with explanations for this since 1997 and further research has been carried out by the Earth-listening research station at the Black Forest Observatory in Germany, with supporting data from Japan and China. With all this data, researchers have come to believe that the atmosphere and the oceans might be the cause of these vibrations.

The effect from the atmosphere is believed to be due to the varying pressure applied on to the Earth’s surface by turbulences (disturbances) in the atmosphere. Similarly, the effect from the oceans was thought to arise when the pressure on the seabed varied with waves sloshing about.

Carrying out research has been quite challenging since the signal is very weak to capture by the measuring devices, although the total energy seems large. Researchers have always believed that this might sometimes be a result of a complex interaction between the land, sea and air.

Explanation

After much research, a decent explanation has now been given for this strange phenomenon. It is said that the Earth’s hum is generated by the interaction between the atmosphere, ocean and sea floor, probably through the conversion of storm energy to oceanic infra-gravity waves that interact with seafloor geography.

Gravity waves are vertical waves generated in a fluid medium or at the boundary between two media like the atmosphere or ocean, which have the force to restore the position of the fluid parcel that is moved.

This can be in the form of gravity or buoyancy (ability to float). Infra-gravity waves are waves with very slow periods spanning from 30 seconds to five minutes and carry a lot of energy.

Data has been collected from networks of seismometers in California, USA, and Japan, and researchers have worked out the direction that the hum signal was travelling on each of the 60 earthquake-free days the Earth experienced in one year. Using the directions measured at the two distant networks, they could trace the signal back to its source.

Finally, results have demonstrated that the Earth’s hum originates mainly in the northern Pacific Ocean during the Northern Hemispheric winter, and in the southern oceans during the Southern Hemispheric winter.

Humming on other planets

Since Venus and Mars have atmospheres, researchers think that the pressure variations on their surfaces might be inducing humming on those planets, just like on the Earth.

The proposed NetLander mission by the French Space Agency in collaboration with the European Space Agency had the objectives of measuring the seismic waves of the surface of Mars, and trying to identify a connection between its atmospheric behaviour and planetary vibrations. However, this mission was called off since it was getting too costly.

Researchers predict that even though there are hums on these planets, the amplitudes (breadth) of the signals would be different since the atmospheres are different, and the frequencies would still be in the millihertz range.

- Aravinda Dassanayake