This news is associated with the following publication: Insights from Mount Etna recent activity, Geophysical Research Letters, DOI:10.1029/2019GL085525, 2019). DOI:10.1029/2019GL085525, 2019

The Earth’s surface is subjected to forces that constantly distort it. The best known is the lunisolar tide, generated by the attraction of the moon and the sun, which causes the surface to rise and fall by tens of centimeters each day. Another "tide" (in the sense of deformation of the Earth’s surface) is that resulting from centrifugal forces from the Earth’s variable rotation around its axis, a rotation whose precise measurement is one of the strengths of SYRTE. In this tide, one distinguishes a part that comes from the change in rotational velocity (let’s call it the "rotational tide"; it is a few millimeters with periodicities ranging from day to century), from a part ten times greater associated with the motion of the rotation pole about the mean pole over periods from year to decade (this is the "pole tide").

Figure 1. A comparison between the displacements of the surface at the latitude of Mount Etna generated by the lunisolar (daily sampling), rotational (Length-of-Day, or LOD) and pole tides.

Figure 1 compares the three "tides": lunisolar, rotational, and polar. If the range of the pole tide is small compared to the lunisolar tide, it is nevertheless slow and therefore its effect is accumulated over several years and becomes dominant at interannual time scales. If the influence of the lunisolar tide on volcanism, as at Stromboli, seems established, and if recent studies seem to show a link between variations in the Earth’s rotational rate and volcanism at subduction zones, especially along the large Pacific trenches, could it be that the pole tide, stronger than the rotational tide, and by altering the stress in the volcanic edifice, has an effect on its seismic or eruptive activity?

Let’s head on Sicily and Mount Etna, a volcano particularly well monitored during recent decades. Located at the plunge of the African plate under the Eurasian plate, its seismicity is that of a tectonically active region with frequent earthquakes of moderate magnitudes and depths. In Figure 2, at the top, the small yellow dots are the earthquakes recorded by the INGV, the main Italian geophysics organization, while the triangles are monthly averages. (The values are not magnitude but seismic energy.) At the bottom of the figure, we see the radius of the trajectory described by the rotation pole about the mean pole, a radius that varies slowly, typically in 6-7 years. Visually, the same variation appears in seismicity with the same phase. Statistical tests show a largely significant correlation above 99.5%, excluding a fact of chance. Further examination of the data shows that some regions of Mount Etna are more susceptible to the pole tide: closer to the crater (within a radius of 30 km), shallower (less than 20 km) and that weak earthquakes (magnitude between 1 and 2.5) are more concerned.

Figure 2. At the top, the seismic energy, in Joules, accumulated in monthly windows around Etna. At the bottom, the distance, in meters between the rotation pole and the mean pole averaged over monthly windows.

This work opens over some perspectives. First of all, it will be necessary to rapidly extend the search for such effects to other volcanic systems (but which have sufficient observational data ...) around the world and to gradually build a synthetic vision of the effects of tides on seismicity and volcanism (the strong eruptions of Stromboli in the summer of 2019 are an example: separated by two months, they are concomitant with the maxima of tides) or even weight the possibility of forecasting eruptions. Secondly, if the rotation of the Earth has been the subject of the study of the SYRTE for several decades, and if it has long been considered as the result of various phenomena (atmospheric and oceanic circulation, rotation of the nucleus, lunisolar tides and even large earthquakes!), it should now be seen as the possible origin of other phenomena, some of them having a large societal impact.

Finally, it is interesting to note that Greek mythology places the forges of Hephaestus under Mount Etna. Cyclops make weapons of the gods of Olympus such as the trident of Poseidon or the lightnings of Zeus. The dull noises escaping from the volcano are nothing but the pounding of tools on the anvils. The rhythm of this work is ultimately given by the relative regularity of the oscillations of the pole, which are themselves under the control of the oceans and waves, the atmosphere and its lightning: Poseidon and Zeus themselves... There’s always a bit of truth in legends…

Mount Etna and the city of Catania, Sicily.