Radon tides on an active volcanic island: Terceira, Azores

Abstract

Twelve synchronous, continuously recording underground radon monitoring stations on the geothermally and tectonically active volcano Pico Alto, Terceira Island, Azores, are used to identify and quantify underground radon movements. Findings include:
(1) Meteorological variations have negligible effects on underground radon emanations.
(2) As a function of lunar month, radon emanations generate repetitive patterns in time and magnitude for several consecutive months (at least one year). Deviations from the established pattern indicate that additional, non-systematic events influenced radon emanations.
(3) Underground radon behaves as a continuous body, much like an aquifer. It exhibits "tides", distinct from marine and earth tides. Radon tidal "ages" have a negative magnitude, with emanation maxima occurring some days before the luni-solar conjunction and/or opposition. Absolute radon maxima are associated with the Full Moon in contrast to marine tidal maxima, which occur at New Moon.
(4) Radon emanations give rise to two maxima per day, one at dawn, the other at sunset. Peak times and separations vary with the season, depending on variations in the length of day.
(5) Marine tides influence the radon body along faults directly connected to the sea. These tidal effects are in the form of p hysical pumping action on buried aquifers and radon "masses".
(6) On Terceira Island, stations respond either to the stress system of the regional Azores/Gibraltar tectonic regime (NW-SE), or to the local NE-SW Pico Alto Volcano regime. Emanation maxima were seen to shift from one system to another as different tectonic regimes came into play.