Amplification of seismic stress-induced radon fluxes near surface seals: Application to earthquake precursor detection

Reliable identification of earthquake precursors remains challenging due to weak signal amplitudes and environmental noise. Radon flux from soil is a promising indicator, but its effectiveness is often limited by low signal-to-noise ratios. In this study, we propose the use of surface seals for strengthening the signal to noise ratios of Rn-222 fluxes, based on a theory of amplification of flux signals induced by seismic stresses in their vicinity. The surface seals could either be those installed specifically for this purpose or could be the pre-existing concrete pavements, building foundations or asphalt roads. A two-dimensional advection-diffusion model is formulated and solved analytically using dual integral equation techniques to describe radon transport in partially sealed soils. The analysis demonstrates that sealed surfaces act as transport barriers, leading to subsurface radon accumulation and elevated fluxes near seal edges. Under conditions of seismic stress, the onset of Darcian flow further amplifies these fluxes relative to unsealed environments. Field measurements conducted near a concrete structure corroborate the model predictions, showing that radon flux at seal edges can increase by up to a factor of four compared to open soil. These findings establish seal-edge regions as effective passive amplifiers of radon signals, significantly improving the detectability of radon-based seismic precursors. By strategically placing radon monitors near sealed surfaces, observatories can detect seismic signals more clearly and earlier. The approach may also apply to other underground gases, opening new possibilities for earthquake research and environmental monitoring.