I am working on an earthquake of Magnitude Mw 7.9 occurred in Southern China in 2008. Found significant variations in groundwater radon data in some stations situated almost 1500 km away from epicenter. Is it acceptable to correlated the variation recorded before and after with this event????????????????????
From my point of view, radon variations can be caused by an earthquake, the epicenter of which is located at a distance of more than 1500 km. I will try to explain why! Of course, it is important at what depth the epicenter is located and what geological structures are located on the line between the epicenter and the observation wells. But even rough estimates give acceptable results. Suppose that our medium is elastic and isotropic. The earthquake source can be represented as a spherical inhomogeneity of elastic properties, the size (radius) of which is determined by the magnitude of the event as 10^(0.414 M-1.696) km.
Then we will assume that such inhomogeneity causes a change of volumetric strain of the geomedia (variations of volumetric strain can lead to an increase in permeability and an additional release of radon). Suppose that a perturbation of a volume strain can lead to an instrumentally detectable change in the radon emanation if it is larger than the tidal strain. The distance over which variations in volumetric strain are 10 ^ -7 / -8 (the level of tidal deformations) can be estimated as 10^(0.43 M) [Dobrovolsky, I.P., Zubkov, S.I. & Miachkin, V.I. PAGEOPH (1979) 117: 1025. https://doi.org/10.1007/BF00876083]. For M=7.9 this distance is 2494.59 km.
There are other estimates according to which the size of the precursors manifestation zone is no more than 50 radii of heterogeneity. Both in the first and in the second case this size is greater than 1500 km.
And if we add here that it is only in an elastic medium, the perturbations decay with distance in proportion to the square of the distance, and in a non-elastic medium, this dependence is inversely proportional. As well as the fact that large geological disturbances in a certain way oriented can “intensify” the disturbance of the volume deformation, we find that the initial assumption may have the right to life.
It depends on the quake magnitude, time span between the quake and the reported elevated Rn, if there were any other quake with lower magnitude in the wells region and on the lithology and structures along those 1500 Km. If you have such relation, so it is more likely that you also have elevated Rn readings in the wells located closer to the epicenter.
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