Changes in Groundwater Chemistry before and after Earthquakes in Northern Iceland

Wednesday, 17 December 2014: 12:05 PM
Alasdair Skelton1, Lillemor Claesson Liljedahl2, Niklas Wästeby1, Margareta Andrén1, Elin Tollefsen1, Helga Rakel Gudrunardottir1, Gabrielle Jarvik Stockmann1, Colin Graham3, Sigurjon Jonsson4, Ingrid Kockum5, Hrefna Kristmannsdóttir6, Magnus Morth1, Erik Sturkell7, Arny Erla Sveinbjörnsdottir8, Hreinn Hjartarson9 and Heike Siegmund1, (1)Stockholm University, Stockholm, Sweden, (2)SKB Swedish Nuclear Fuel and Waste Management, Stockholm, Sweden, (3)Edinburgh University, Edinburgh, United Kingdom, (4)King Abdullah University of Science and Technology, Thuwal, Saudi Arabia, (5)Karolinska Institutet, Stockholm, Sweden, (6)University of Akureyri, Akureyri, Iceland, (7)University of Gothenburg, Gothenburg, Sweden, (8)University of Iceland, Institute of Earth Sciences, Reykjavik, Iceland, (9)Landsvirkjun, Reykjavik, Iceland
We report changes in stable isotope values (δ2H and δ18O) and concentrations of dissolved elements in groundwater before and after earthquakes in northern Iceland. Groundwater samples have been collected from 2 boreholes that penetrate basalt and basalt-derived sediments for >5 years at Hafralækur and for >10 years at Húsavík, northern Iceland. Comparison of δ2H and δ18O with the global meteoric water line reveals mixing between pre-Holocene and modern day groundwater and groundwater affected by water-rock interaction.  M >5 earthquakes occurred in 2002, 2012 and 2013.  There were also several M >4 earthquakes during our study.

At Hafralækur, we report δ2H maxima that are not part of a normal distribution (confirmed using the Shapiro-Wilk test for normality) and which coincide with M >5 earthquakes in 2012 and 2013. These began before each earthquake with a gradual increase of δ2H towards heavier values and ended with a gradual return towards lighter values. At both sites, we report concentration maxima of dissolved elements which coincide with the M >5 and some M >4 earthquakes. Using a binomial test, we confirmed that the δ2H and concentration maxima were associated with the earthquakes. Possible causes of this association are mixing of groundwater components or exposure of fresh rock surfaces to groundwater. Both changes can be explained by expansion of the rock volume (dilation) enhancing permeability.

The M >5 and some M >4 earthquakes were followed by abrupt changes of δ18O and of dissolved element concentrations. These changes might be explained by different mixing ratios between groundwater components caused by switching between fracture pathways due to seismically induced changes of water pressure.

Our results show that changes in groundwater chemistry before earthquakes can be statistically verified provided that measurement campaigns are long term. Although these changes are specific for northern Iceland, we infer that chemical and isotopic signals of dilation might be detected elsewhere before earthquakes.