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Hydrogeochemical changes before and after a major earthquake

¹ Department of Geology and Geochemistry, Stockholm University, 106 91 Stockholm, Sweden, and Nordic Volcanological Institute, ASKJA, Sturlugata 7, IS-101 Reykjavik, Iceland
² Department of Geology and Geochemistry, Stockholm University, 106 91 Stockholm, Sweden
³ School of Geosciences, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, Scotland
⁴ Geologisches Institut, Julius-Maximilians-Universität, Pleicherwall 1, 97070 Würzburg, Germany
⁶ Department of Geology and Geochemistry, Stockholm University, 106 91 Stockholm, Sweden
⁷ Department of Molecular Medicine, Karolinska Institutet, 171 76 Stockholm, Sweden

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, ¹⁸O, and 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.

Key Words: hydrogeochemistry • earthquake prediction • source mixing • source switching • ice age meteoric water • Iceland