Sustained crustal helium enrichment associated with postseismic viscoelastic relaxation following the 2025 Myanmar earthquake

Earthquakes can alter fluid flow far from a rupture, but the processes that sustain such changes for months remain uncertain. Here we report a sustained increase in gas-phase helium concentration at a thermal spring on the Red River Fault, about 600 kilometres from the 2025 magnitude 7.7 Myanmar earthquake. Minute-scale monitoring recorded intermittent fluctuations before the earthquake and a progressive concentration increase afterwards. Groundwater level, rainfall, atmospheric pressure and conservative estimates of instrumental drift do not reproduce this evolution. He–Ne isotope measurements from a nearby spring indicate that the added helium was predominantly radiogenic and crustal, with no resolvable increase in mantle contribution. Regional seismicity and stress calculations are consistent with time-dependent postseismic Coulomb stress loading of the Red River Fault, involving positive shear and extensional normal-stress changes. In the model, time-dependent lower-crustal relaxation produces postseismic stress changes comparable to the coseismic change within about a decade. Together, these observations suggest that prolonged crustal deformation, potentially reinforced by local earthquakes, contributed to the sustained far-field helium-concentration anomaly. Slip during the 2025 M7.7 Myanmar earthquake triggered high He-emissions that may persist for several decades along the Red River Fault, located around 600 km away from the epicentre, according to elevated gas-phase He concentrations in thermal-spring groundwater.