Evidence of both M2 and O1Earth tide waves in radon‐222 air concentration measured in a subglacial laboratory

Abstract

Many earthquake‐related radon‐222 temporal changes have been recorded since the 1960s and are frequently discussed, sometimes initiating a controversial debate on the relevance of radon‐222 as an earthquake precursory signal. The diurnal S1–O1 and semidiurnal S2–M2 Earth tide signatures in radon signals are acquired in a natural context. This can be used to calibrate the radon changes under strain accumulation close to epicentral areas, which are often discussed but are rarely evidenced by experimental data. The analysis of a 10 month time series acquired in the subglacial laboratory of the Argentière glacier, Mont Blanc Massif, French Alps, demonstrates here the unambiguous episodic appearance of the M2–O1 waves in the radon signal with significant amplitudes of 36 and 50 Bq m−3, respectively. We thus prove that radon variations induced by gravitational M2 and O1 waves are detectable in a natural environment. In this particular place, the radon response is probably amplified by cyclic stress variations applied on the upstream side of the natural rock dam into which the laboratory is drilled. The amplification of the radon signal is induced by poroelastic deformation under this particular mechanical forcing. This can elucidate why most precedent studies failed to detect M2–O1 signatures in radon signals recorded in other underground laboratories.