Why Earth Hums


C. Coimbra photo

C. Coimbra photo

We may not be able to hear it, but the Earth is constantly oscillating and creating a hum that has been likened to the ring of a bell.

It’s long been known that earthquakes and their seismic activity contribute to this ringing, but researchers have now proved ocean waves also play a role.

As so-called short waves collide near the surface they create weak microseismic waves, which combine with long, more powerful waves dragging across the ocean’s floor to create the constant hum.

These vibrations aren’t powerful enough to be detected by humans, but are strong enough to register on seismometers and similar.

WHAT CAUSES THE EARTH’S HUM? 
Earthquakes shake the planet in such a way that Earth can oscillate for months after the seismic event.

This oscillation is caused by vibrations and researchers in the mid-1980s found that the hum can drop as low as an E flat, 20 octaves below middle C on the musical scale.

In the late 1990s, seismologists found that the planet doesn’t just oscillate following quakes, and instead is vibrating at extremely low frequencies all of the time.

A team of Japanese and Californian seismologists first suggested ocean waves may cause this hum in 2004.

Using data from 2008, a French team found that ocean waves colliding generate seismic activity, but these waves only take 13 seconds or less to complete a single oscillation.

They didn’t explain why slow seismic waves – which provide the most powerful bell-like ringing – can last for longer than 50 seconds.

So the team studied the movement of ocean waves at the bottom of the seabed.

As these long waves travel over the ocean floor they meet resistance, especially as they pass over the steep continental shelves, and this friction helps create long seismic waves, that oscillate much slower.

They proposed that as ocean waves move in opposite directions and collide they create microseismic activity.

But Fabrice Ardhuin, a senior research scientist at Centre National de la Recherche Scientifique in France, and his team wanted to put this theory to the test.

They began by studying and measuring seismic activity across the globe, including earthquakes, storms and the movement of ocean waves, throughout 2008.

During this time, two seismic events with magnitudes of 6.9 and 7.1, took place on 3 March and 20 March.

But the largest hum coincided with the strongest seismic waves during the Johanna storm on 10 and 11 March.

That storm generated waves with a peak period of 16 seconds that reached heights of up to 42ft (12.8 metres).

During this storm, a spectrogram found energy levels reached 10 mHz, and lasted 36 hours.

By looking at how much energy was generated by the storm, the researchers were able to conclude that the hum was caused by the storm, and wasn’t the remnants of seismic activity from the events.

They could then look at the reasons why.

‘Ocean waves provide most of the energy that feeds the continuous vertical oscillations of the solid Earth,’ said the researchers.

‘The hum contains periods longer than 30 seconds, and the primary and secondary peaks are centered around 15 and five seconds respectively.’

Motions in all three bands are recorded everywhere on the planet and the team said they can provide information on both the solid Earth structure, the ocean wave climate over the past century, and the properties of short period ocean waves.

‘Yet, the use of seismic data is limited because where and how ocean waves rock the Earth is not known, in particular for the hum,’ continued the team. ‘The hum is the least understood part of Earth’s oscillations.’

They didn’t explain why slow seismic waves – which provide the most powerful bell-like ringing – can last for longer than 50 seconds.

So the team studied the movement of ocean waves at the bottom of the seabed. These waves are known as long because they travel from the floor to the coast and back again.

As these long waves travel over the ocean floor they meet resistance, especially as they pass over the steep continental shelves, and this helps create long seismic waves that oscillate much slower.

This means that all three theories – earthquakes, colliding oceans and long waves – combine to create the Earth’s hum. Although the majority comes from the latter.

The findings are published in Geophysical Research Letters, a journal of the American Geophysical Union (AGU).

Mr Ardhuin said that by measuring and understanding long, microseismic waves, researchers can learn more about how the Earth is structured.

‘Now we know where this ringing comes from and the next question is: what can we do with it,’ he told the AGU.

–From The Daily Mail

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Categories: Discovery, Oceanography

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