How Tonga’s volcano eruption ‘was heard’ in the Pyrenees

Volcanic eruptions are common on planet Earth, as the Cumbre Vieja volcano on the island of La Palma has recently reminded us.

However, the eruption of the submarine volcano Hunga-Tonga-Hunga-Ha’apai on January 15 must be considered an exceptional event. The event generated a volcanic column more than 30 kilometers high, a tsunami detected on all Pacific coasts and a strong explosion in the atmosphere.

To be more exact, the energy of the eruption has been estimated by NASA at about ten megatons, 1,000 times greater than the nuclear bomb exploded in Hiroshima during World War II.

The sound of the explosion has been heard not only in nearby areas, but there are testimonies of having reached the Alaskan peninsula, located more than 9,700 kilometers from Tonga.

Shock waves spread across the planet

One of the most interesting effects of the explosion has been that the variation in atmospheric pressure generated has spread throughout the planet, moving like a shock wave with a speed of close to 1,100 kilometers per hour and producing variations in its path. between one and eight millibars.

Although the frequency of this signal is below the audible range, its intensity has allowed it to be detected by various types of sensors, from barometers built into weather stations to geodetic stations.

The sudden variation in atmospheric pressure produced at the arrival of this wave has caused various physical effects, among which the variations in sea level observed in many places on the planet stand out.

The effects of the Tonga volcano on the Iberian Peninsula

These effects have also been observed in the Iberian Peninsula. Towns on the coasts of Valencia and the Balearic Islands have reached variations of 50 centimeters in height.

It is even more curious to know that the ‘push’ produced by the passage of the shock wave not only displaces the masses of water, but also generates a small deformation of the ground, which has been detected by seismometers distributed throughout the planet.

Seismometers are basically highly sensitive pendulums that allow the movement of the ground to be recorded. Although the mission of these instruments is to detect the waves generated by earthquakes, in recent years seismographs have made it possible to study very diverse phenomena. For example, the degree of human activity during the confinements due to covid-19, the variations in the flow of mountain rivers or the activity of elephants in the African savannah.

Well, the records in some of the seismometers provide magnificent evidence of the deformation of the ground generated by the passage of the pressure wave caused by the volcanic eruption. Two examples are the one installed in the Canfranc Underground Laboratory (Huesca) or the seismic station of the Cartographic Institute of Catalonia in Ós de Balaguer (Lleida).

These seismic stations, both located in the Pyrenees, are located about 17,000 kilometers from the volcano, relatively close to its antipodal point, located in southern Algeria.

Analyzing the data in detail, the arrival of two different pressure waves can be identified, both generated by the Tonga volcanic explosion.

First, seismometers record the arrival of seismic waves produced by the eruption. These waves reach the Pyrenees about 20 minutes after the explosion, after passing through the Earth’s core. About 16 hours later, the arrival of the pressure wave occurs, which causes a minor deformation, but clearly detectable by the seismometer.

This first pressure wave, traveling along the shortest path between Tonga and the Pyrenees, arrives at the seismic station around 20:00 UTC. About four hours later, the arrival of the acoustic wave that had traveled the balloon in the opposite direction, following a longer path, is recorded.

Although the signals can be observed in all three components of the seismic records, their amplitude is greater in the component oriented north-south, since this is the relative orientation between Tonga and the Pyrenees area.

An exceptional observation

The seismic observation of the passage of these pressure waves after traveling more than 17,000 kilometers is an interesting fact, since it allows the interaction between the atmosphere and the solid earth to be revealed.

However, the exceptional character of this event is highlighted by noting that the seismic data allow the detection of the second and third steps of the pressure waves. These waves take about 36 hours to circumambulate the planet, first being detected around 10:00 on the 17th and later around 22:00 on the 18th of January, after traveling a distance of nearly 100,000 kilometres.

When large earthquakes occur, the generated surface waves circumnavigate the planet, using about three and a half hours on each turn. In these cases, it is relatively common to detect two or three of these steps in seismic sensors located in areas with little ambient noise.

However, the seismic detection of the ground deformation produced by the successive passages of pressure waves is undoubtedly an exceptional event, which highlights the great energy released by the Tonga volcanic explosion.