The long arm of volcanos

16 January 2022

Yesterday erupted a volcano near the Tonga archipelago. The eruption was captured by satellite. The underwater eruption caused beside Tsunamis also pressure shockwaves that were recorded by meteorological stations worldwide. On the footage below, the pressure waves heading away from the eruption are well visible.

At SMEAR Estonia, we measure atmospheric pressure with 10Hz frequency and therefore the signal of the remains of the "atmospheric tsunamis" from the Tonga eruption were captured in Järvselja, Estonia.

Running a time series analysis of the data captured on 15th January revealed the effect of the volcano generated pressure anomaly at the SMEAR Estonia measurement station. We saw a multi-wave anomaly that took place from 8 pm to 9 pm.

The pressure anomaly had all together several waves that were recorded. Three mark the incoming pressure wave and another three or four transient oscillations after the pressure drop. The whole phenomenon is known as the "Doppler-effect", the sound that travels towards us will get higher frequecies, or pressure, and once it has passed the sound will drop to deeper frequencies. On the figure, we can see at least two well distinctive frequencies with approximately 10 minutes wavelengths (the darker purple colored markers) and about 15 minute wavelength (green markers). On the figure, the 10Hz original data are in blue and the smoothed data using a 2 minute running mean are in red color.

We were curious what would be the sound of the eruption if we could hear such low frequencies. So, we resampled the time series and when we got it to just about a second's (0.88 s) length, a short bump between the background noise is heard. Its saved to a wave form audio file and you can hear the roar of the volcano in Estonia.

Reflecting pressure waves last long.

The pressure waves travel around the globe and meet, in the ideal case at the volcano's antipode location. This place is somewhere in the Saharan desert, see the location here in a blog entry from Martin Heimann. From there, they should come back. Indeed, we could also measure at SMEAR Estonia the reflection of the eruption pressure wave at about 4:30 on 16th January.

But, that's not all, the pressure waves bounce back and forth between the eruption site in Tonga and the antipode, south of the city of Tamanrasset in southern Algeria. Diego Aliaga tweeted a short footage that these waves move around the globe several days.

Steffen M. Noe