Changes in groundwater radon concentrations caused by the 2016 Kumamoto earthquake

•Radon-222 concentrations in spring water changed after the 2016 Kumamoto earthquake.•All springs with large Rn increases were in the earthquake’s compressive quadrant.•We explained the change of Rn concentrations as a direct consequence of porosity change.•Net porosity changes from pre- to post-seismic was 30‒70% by considering Rn release equation.