Hydrogeochemical changes before and after a major earthquake

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 Tjornes Fracture Zone, northern Iceland. Significant Cu, Zn, Mn, and Cr anomalies reached our sampling station 1, 2, 5, and > or =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 SO (sub 4) , and decreases in Na/Ca, delta (super 18) O, and delta D, 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.