Volcanoes and seismic activity

The various forms of seismo-volcanic activity

The observation of seismic vibrations generated by volcanoes is one of the main sources of information on the activity and physical properties of magmatic systems, on the phenomena that occur there and on their structure. It is therefore an essential tool for volcanic monitoring and eruption prediction. The seismic activity of volcanoes manifests itself in several forms that correspond to various processes.

**Volcano-tectonic earthquakes often the first sign of a volcano’s awakening

A volcanic building is intersected by many faults of all sizes. When magma is injected into the volcano, its structure is subjected to stress variations that trigger ruptures on faults in the same way as on tectonic faults. These events are called "volcano-tectonic earthquakes" (VT) to emphasize that the physical process involved is similar to that of tectonic earthquakes. This type of earthquake is also called "type-A" or "high frequency". P- and S-wave arrivals are generally distinguished in their seismograms and are characterized by dominant frequencies in their spectra in the range of 5 to 15 Hz. They can be located and studied by classical seismological methods. This type of seismic activity is often the first sign of a volcano’s awakening. However, it can last a few days, months or years before an eruption begins.

**Volcanic earthquakes and tremors resulting from fluid movement

A volcano contains pressurized fluids (magma, gas, water, steam) in magmatic pipes or chambers, cracks, groundwater. Different phenomena can occur: hydraulic fracturing phenomena, single or two-phaase fluid movements through irregularly shaped pipes, formation or disappearance of bubbles, vaporization of water in contact with magma. These phenomena are noisy, i.e. they are generally accompanied by the emission of seismic waves, which are therefore in a way the sounds of volcanic piping.

By propagating through the fluid-filled cavities and interacting with their solid walls, these waves can cause resonance phenomena in the same way as organ pipes. These vibrations can be brief, so they are called "volcanic earthquakes" or "long-period earthquakes (LP)" or "type B earthquakes". But they can also last for minutes, hours or be permanent; they are then called "volcanic tremors". These two types of signals are therefore probably produced by the same sources and share the following characteristics:
– Their onset is generally emergent, i.e. their amplitude gradually increases, unlike VT earthquakes, whose onset is impulsive.
– S-wave waves are not distinguished in the recordings and the dominant frequencies of their spectra are in the range 1 to 5 Hz, hence their name "LP earthquake".
– Spectra often have one or more dominant peaks that are usually the mark of resonance phenomena in fluid-filled cavities.

Volcanic explosions also generate seismic and acoustic waves. They can be likened to LP earthquakes because they result from fluid movement - gas reaching the surface - and the signal characteristics are similar.

This type of seismic activity often precedes volcanic eruptions and is therefore one of the most interesting precursors, since the mechanisms involved are directly related to the state of the magmatic or groundwater systems. Its characteristics mean that traditional methods of location and study are often ineffective. Specific methods are then used, such as the use of low aperture seismic networks, acting as seismic antennas, and appropriate signal processing.

**Listen to volcanoes!

Seismic signals emitted by volcanoes are generally inaudible because the human ear is sensitive to the frequency range 20 - 20,000 Hz. However, they can be transformed into sounds by multiplying their frequency by an appropriate factor. Below are some examples of volcanic signals whose frequency has been multiplied by 40 by passing the recording 40 faster than the original. You can compare the different volcano sounds by distinguishing between high frequency and short signals from VT earthquakes and lower frequency signals from LP earthquakes and tremors. Some signals contain almost pure frequencies that can vary rapidly over time. This gives an idea of the complexity of the mechanisms involved. We also see that volcanoes not only offer a spectacle for the eyes, but also generate sounds of musical quality for those who know how to hear them...