Using the atmosphere as a mirror to detect earthquakes is the idea of Giovanni Occhipinti’s team and two PhD students of the IPGP, with Jean-Philippe Molinie (ONERA) and Thomas Farges (CEA). They thought to use the properties of the ionosphere, located beyond 80 km altitude, to transform radars into seismometers.
In concrete terms, when a seismic wave emitted in depth reaches the surface of our planet, it causes a deformation (a Rayleigh wave). This vibration of the Earth's crust creates an infrasonic wave that propagates in all directions in the atmosphere: reaching 80 km altitude, it creates strong variations in the speed and density of ionised particles (ions and electrons) of the ionosphere. However, these particles have the particularity of returning, reflecting the radio waves towards the ground: by emitting towards the ionosphere, the radars can thus follow the propagation of the sound wave, the signature of the Rayleigh wave, and deduce therefrom the intensity of the earthquake that produced it.
The Rayleigh wave (R) and its signature in the atmosphere (R atmo), detected on the ground by seismometers in red, and in the ionosphere by trans-horizon radars (OTH for "Over the horizon") and Doppler, in blue.
The authors studied 38 events and were able to establish a connection between the magnitude of earthquakes with ground seismometers and the intensity of the wave in the ionosphere. The reliability of these data even proved to be independent of the ionosphere weather conditions (time of year and sunshine) and the characteristics of the earthquake (depth and distance from the epicentre). This makes it a reliable source that opens up interesting prospects for seismology.
This new method would first increase the coverage of the least accessible areas from the ground, and improve the tsunami warning. With the support of CNES, the IPGP proposes satellite ideas applied to these uses, but also to space exploration: the use of this method on other bodies of the solar system would provide valuable information on their seismic activity, and therefore on their internal structure, remotely, from a probe in orbit, without even having seismographs on their surfaces. A considerable advantage in the case of Venus for example, where the average temperature is 460 ° C, and where the atmospheric pressure reaches 90 times that of the Earth. So far, the only probes to have reached the ground of Venus are those of the Soviet program Venera, between the ‘60s and ‘80s. And they did not survive these extreme conditions more than 2 hours, which made the seismic measurements impossible. At least until today.
Occhipinti, G., F. Aden-Antoniow, A. Bablet, J.-P. Molinie, T. Farges, Surface waves magnitude estimation from ionospheric signature of Rayleigh waves measured by Doppler sounder and OTH radar, Scientific Report 8, Article number: 1555 (2018)
- Giovanni Occhipinti , Institut de Physique du Globe in Paris : ninto at ipgp.fr
- Mioara Mandea,Solid Earth Programme Manager at CNES : mioara.mandea at cnes.fr
See also « connaître la magnitude des seismes en observant l’atmosphère » on the IPGP website