Categorized | Sci-Tech

Volcano Watch: Good vibrations are the Earth’s music

(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

When an earthquake occurs, why do we feel the ground shake? The shaking we feel is from seismic waves radiating away from the earthquake. These waves are similar to waves radiating away from a stone thrown into water. They are the Earth’s way of dissipating energy released during an earthquake. 

Dissipation of earthquake energy is an example of the continuous process of converting energy from one form to another. In this case, elastic potential energy (in the form of underground stresses built up within the earth) is suddenly released during an earthquake and converted to kinetic energy (the energy an object has due to its motion). 

The same energy conversion happens when the string of a guitar or ukulele is plucked. As the player pulls on a string, its elastic potential energy increases. When the string is released, that energy radiates as kinetic energy in the form of a vibrating string. 

The string vibrates at a frequency (number of vibrations per second) that we can hear— and call music.  Frequency, known as “pitch” in music, is measured in cycles per second, or Hertz (Hz). Humans can hear a wide range of frequencies, varying from about 50 Hz (low bass) up to 14,000-20,000 Hz (high pitch).   

Around the globe, earthquakes happen continually, so seismic waves are constantly travelling throughout the earth. In essence, the earth is continually making music, but for the most part, this music is at a frequency below our hearing range.  

Most small, local earthquakes release most of their energy between 1-15 Hz. Larger earthquakes can produce significant energy down to much lower frequencies. 

While small earthquakes tend to dissipate their energy like ripples radiating away from a stone thrown into water, large earthquakes can actually vibrate the whole earth at once. These normal modes of vibration are often described as the earth ringing like a bell. 

Because most seismic energy is below our hearing range, and because it is difficult for seismic waves to travel from the earth to the atmosphere, we do not often hear the energy radiated from earthquakes.  

This is not always the case with volcanoes.  Volcanoes can release energy directly to the atmosphere, sometimes explosively, and radiate energy at a range of frequencies.   

The new eruptive vent at Kilauea’s summit has produced strong energy between 0.5-2.0 Hz.  This energy includes seismic energy recorded by local seismic instruments, as well as energy radiated directly to the atmosphere, which is recorded with nearby infrasound microphones.   

While this energy is below our hearing range, the vent has also produced audible sounds, including hissing and jet-engine-like sounds. These noises are also a form of energy dissipating from the volcano, but they are more closely related to the sound produced by an instrument, such as a trumpet, rather than the sound produced by the vibrating strings of an ukulele.   

When pressurized air is forced through a constricted opening (the player’s lips), the vibration produces music.  The pitch of the music is controlled both by the frequency of the vibration and by the shape and length of the brass tubing on the trumpet.   

On the volcano, pressurized gases are being forced through tight constrictions within the vent.  The frequency of the resulting vibrations is controlled by both the amount of pressure and by the shape and size of the vent. 

At a place like Kilauea volcano, the continual process of energy dissipation allows for a unique multi-sensory experience in which we can see, hear, and feel the Earth’s music.

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