Where does the sound during an earthquake come from?
The rumbling during an earthquake is caused by rocks cracking. It can release so much energy that stone melts.
Many people across Eastern Norway heard Sunday's earthquake as a prolonged rumble.(Image: menur / Shutterstock / NTB, Photo of Hans Jørgen Kjøll: University of Oslo)
A 3.6 magnitude earthquake had its epicenter near Oslo on April 26. It is very rare for people to notice earthquakes in this area.
Because the quake occurred close to Norway’s most densely populated region, large numbers of people both felt it and heard the sound. In a Facebook neighbourhood group of eastern Oslo, residents speculated whether nearby blasting or underground tunnel work could be responsible.
So what are people actually hearing during an earthquake?
Can create molten rock in an instant
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"We hear the rock deep underground cracking due to stresses in the Earth's crust," Hans Jørgen Kjøll tells Science Norway. He is a researcher at the University of Oslo's Centre for Studies of the Physics of the Earth (Njord).
These stresses are what trigger earthquakes. A fault shifts, and the surrounding rock fractures.
A fault can also form when the Earth's crust cracks open and moves along the fracture, Kjøll explains.
The researcher adds that the sudden movement of such massive rock formations releases enormous amounts of energy.
"Most of this energy goes into heating the surrounding rock," he says.
Researcher Hans Jørgen Kjøll at the University of Oslo.(Photo: University of Oslo)
It can become extremely hot in an instant. The intense friction during powerful earthquakes can melt the rock into glass-like materials. It remains unclear whether Sunday’s earthquake was strong enough for that to occur, according to Kjøll.
Even so, the release of energy creates enormous heat underground.
Seismic waves
A large portion of the energy is also used to generate seismic waves: waves that travel through rock and the Earth's crust. These waves cause the ground to shake, but they also produce sound.
"If you blast rock or strike it with a sledgehammer, it creates sound," he says.
Sound is movement in the air that we can detect. When the air moves, the waves in the air can also move our eardrums.
The seismic waves move the ground – which in turn moves the air, Kjøll explains.
"When the waves reach the surface, the ground literally vibrates, and that energy is also transferred through the air," he says.
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The graphic shows when and where the earthquake was detected across Norway on 26 April.(Image: NORSAR)
Low and high frequency
If you've watched a speaker while it's playing music, you can see it vibrating rapidly. This is how energy is transferred from the speaker into the surrounding air.
When an earthquake occurs relatively close to the Earth's surface, we may hear more high-frequency sounds, Kjøll explains – sounds that fall within the range that humans are capable of hearing.
These sounds may be perceived as rumbling or booming underground. If the quake is very shallow, it can produce louder bangs or rumbling.
However, human hearing is limited to a specific frequency range.
Some wavelengths are too short, such as dog whistles, while others are too long. These are sounds far below what we perceive as bass or rumbling.
If an earthquake originates deeper underground, some of the high-frequency sounds may lose their energy.
"The low-frequency sounds can be felt as shaking, but we cannot hear them," says Kjøll.
The same applies the farther away you are from the epicentre, which is the centre of the earthquake. The seismic waves gradually become lower in frequency.
This is why earthquakes may still be measured clearly at great distances, even when people far from the epicenter hear nothing at all. This is shown in NORSAR’s graphic, where the quake was detected as far away as Karasjok, far north in Norway, a few minutes after the initial event.
