Hydrogeochemical changes were detected by monitoring ice age meteoric waters before and after a magnitude (M) 5.8 earthquake on 16 September 2002 in the Tjörnes Fracture Zone, northern Iceland. Significant Cu, Zn, Mn, and Cr anomalies reached our sampling station 1, 2, 5, and ≥10 weeks before the earthquake, respectively. By comparison with published experimental, geophysical, and geochemical studies, we suggest stress-induced source mixing and leakage of fluid from an external (hotter) basalt-hosted source reservoir, where fluid-rock interaction was more rapid. Rapid 12%–19% increases in the concentrations of B, Ca, K, Li, Mo, Na, Rb, S, Si, Sr, Cl, and SO4, and decreases in Na/Ca, δ18O, and δD, occurred 2–9 days after the earthquake. The rapidity of these changes is consistent with time scales of fault sealing due to coupled deformation and fluid flow. We interpret fluid-source switching in response to fault sealing and unsealing, with the newly tapped aquifer containing chemically and isotopically distinct ice age meteoric water. Variation in Na/Ca ratio appears to be sensitive to the changing stress state associated with M > 4 earthquakes. This study highlights the potential of hydrogeochemical change in earthquake-prediction studies.