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  • 1.
    Andrén, Margareta
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Stockmann, Gabrielle
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Sturkell, Erik
    Mörth, Carl-Magnus
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Guðrúnardóttir, Helga Rakel
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Keller, Nicole Simone
    Odling, Nic
    Dahrén, Börje
    Broman, Curt
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Balic-Zunic, Tonci
    Hjartarson, Hreinn
    Siegmund, Heike
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Freund, Friedemann
    Kockum, Ingrid
    Coupling between mineral reactions, chemical changes in groundwater, and earthquakes in Iceland2016In: Journal of Geophysical Research - Solid Earth, ISSN 2169-9313, E-ISSN 2169-9356, Vol. 121, no 4, p. 2315-2337Article in journal (Refereed)
    Abstract [en]

    Chemical analysis of groundwater samples collected from a borehole at Hafralaekur, northern Iceland, from October 2008 to June 2015 revealed (1) a long-term decrease in concentration of Si and Na and (2) an abrupt increase in concentration of Na before each of two consecutive M 5 earthquakes which occurred in 2012 and 2013, both 76km from Hafralaekur. Based on a geochemical (major elements and stable isotopes), petrological, and mineralogical study of drill cuttings taken from an adjacent borehole, we are able to show that (1) the long-term decrease in concentration of Si and Na was caused by constant volume replacement of labradorite by analcime coupled with precipitation of zeolites in vesicles and along fractures and (2) the abrupt increase of Na concentration before the first earthquake records a switchover to nonstoichiometric dissolution of analcime with preferential release of Na into groundwater. We attribute decay of the Na peaks, which followed and coincided with each earthquake to uptake of Na along fractured or porous boundaries between labradorite and analcime crystals. Possible causes of these Na peaks are an increase of reactive surface area caused by fracturing or a shift from chemical equilibrium caused by mixing between groundwater components. Both could have been triggered by preseismic dilation, which was also inferred in a previous study by Skelton et al. (2014). The mechanism behind preseismic dilation so far from the focus of an earthquake remains unknown.

  • 2. Chatterjee, Sayantan
    et al.
    Bhatnagar, Gaurav
    Dugan, Brandon
    Dickens, Gerald R.
    Stockholm University, Faculty of Science, Department of Geological Sciences. Rice University, USA.
    Chapman, Walter G.
    Hirasaki, George J.
    The impact of lithologic heterogeneity and focused fluid flow upon gas hydrate distribution in marine sediments2014In: Journal of Geophysical Research - Solid Earth, ISSN 2169-9313, E-ISSN 2169-9356, Vol. 119, no 9, p. 6705-6732Article in journal (Refereed)
    Abstract [en]

    Gas hydrate and free gas accumulation in heterogeneous marine sediment is simulated using a two-dimensional (2-D) numerical model that accounts for mass transfer over geological timescales. The model extends a previously documented one-dimensional (1-D) model such that lateral variations in permeability (k) become important. Various simulations quantitatively demonstrate how focused fluid flow through high-permeability zones affects local hydrate accumulation and saturation. Simulations that approximate a vertical fracture network isolated in a lower permeability shale (k(fracture) >>k(shale)) show that focused fluid flow through the gas hydrate stability zone (GHSZ) produces higher saturations of gas hydrate (25-70%) and free gas (30-60%) within the fracture network compared to surrounding shale. Simulations with a dipping, high-permeability sand layer also result in elevated saturations of gas hydrate (60%) and free gas (40%) within the sand because of focused fluid flow through the GHSZ. Increased fluid flux, a deep methane source, or both together increase the effect of flow focusing upon hydrate and free gas distribution and enhance hydrate and free gas concentrations along the high-permeability zones. Permeability anisotropy, with a vertical to horizontal permeability ratio on the order of 10(-2), enhances transport of methane-charged fluid to high-permeability conduits. As a result, gas hydrate concentrations are enhanced within these high-permeability zones. The dip angle of these high-permeability structures affects hydrate distribution because the vertical component of fluid flux dominates focusing effects. Hydrate and free gas saturations can be characterized by a local Peclet number (localized, vertical, focused, and advective flux relative to diffusion) relative to the methane solubility gradient, somewhat analogous to such characterization in 1-D systems. Even in lithologically complex systems, local hydrate and free gas saturations might be characterized by basic parameters (local flux and diffusivity).

  • 3.
    Skelton, Alasdair
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Liljedahl-Claesson, L.
    Wästeby, Niklas
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Andrén, Margareta
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Stockmann, G.
    Sturkell, E.
    Mörth, Carl-Magnus
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Stefansson, A.
    Tollefsen, Elin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Siegmund, Heike
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Keller, N.
    Kjartansdóttir, R.
    Hjartarson, H.
    Kockum, I.
    Hydrochemical Changes Before and After Earthquakes Based on Long-Term Measurements of Multiple Parameters at Two Sites in Northern IcelandA Review2019In: Journal of Geophysical Research - Solid Earth, ISSN 2169-9313, E-ISSN 2169-9356, Vol. 124, no 3, p. 2702-2720Article, review/survey (Refereed)
    Abstract [en]

    Hydrochemical changes before and after earthquakes have been reported for over 50years. However, few reports provide sufficient data for an association to be verified statistically. Also, no mechanism has been proposed to explain why hydrochemical changes are observed far from earthquake foci where associated strains are small (<10(-8)). Here we address these challenges based on time series of multiple hydrochemical parameters from two sites in northern Iceland. We report hydrochemical changes before and after M >5 earthquakes in 2002, 2012, and 2013. The longevity of the time series (10 and 16years) permits statistical verification of coupling between hydrochemical changes and earthquakes. We used a Student t test to find significant hydrochemical changes and a binomial test to confirm association with earthquakes. Probable association was confirmed for preseismic changes based on five parameters (Na, Si, K, O-18, and H-2) and postseismic changes based on eight parameters (Ca, Na, Si, Cl, F, SO4, O-18, and H-2). Using concentration ratios and stable isotope values, we showed that (1) gradual preseismic changes were caused by source mixing, which resulted in a shift from equilibrium and triggered water-rock interaction; (2) postseismic changes were caused by rapid source mixing; and (3) longer-term hydrochemical changes were caused by source mixing and mineral growth. Because hydrochemical changes occur at small earthquake-related strains, we attribute source mixing and water-rock interaction to microscale fracturing. Because fracture density and size scale inversely, we infer that mixing of nearby sources and water-rock interaction are feasible responses to small earthquake-related strains. Plain Language Summary Changes in groundwater chemistry before and after earthquakes have been reported for over 50years. However, few studies have been able to prove that the earthquakes caused these changes. Also, no study has explained why these changes are often reported far from where the earthquake occurred. Here we address these challenges based on measurements of groundwater chemistry made at two sites in northern Iceland over time periods of 10 and 16years. We used statistical methods to prove that the earthquakes caused changes of ground water chemistry both before and after the earthquakes. We showed that changes of groundwater chemistry before earthquakes were caused by slow mixing between different groundwaters, which triggered reactions with the wall rock that changed groundwater chemistry, and that changes of groundwater chemistry after earthquakes were causes by rapid mixing between different groundwaters. That these changes were detected far from where the earthquakes occurred suggests that cracking of the wall rock at a very small scale was all that was needed for mixing of different groundwaters and reactions with the wall rock to occur.

  • 4.
    Wästeby, Niklas
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Tollefsen, Elin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Andrén, Margareta
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Stockmann, Gabrielle
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Liljedahl, Lillemor Claesson
    Sturkell, Erik
    Mörth, Magnus
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Hydrochemical monitoring, petrological observation, and geochemical modeling of fault healing after an earthquake2014In: Journal of Geophysical Research - Solid Earth, ISSN 2169-9313, E-ISSN 2169-9356, Vol. 119, no 7, p. 5727-5740Article in journal (Refereed)
    Abstract [en]

    Based on hydrochemical monitoring, petrological observations, and geochemical modeling, we identify a mechanism and estimate a time scale for fault healing after an earthquake. Hydrochemical monitoring of groundwater samples from an aquifer, which is at an approximate depth of 1200 m, was conducted over a period of 10 years. Groundwater samples have been taken from a borehole (HU-01) that crosses the Husavik-Flatey Fault (HFF) near Husavik town, northern Iceland. After 10 weeks of sampling, on 16 September 2002, an M 5.8 earthquake occurred on the Grimsey Lineament, which is approximately parallel to the HFF. This earthquake caused rupturing of a hydrological barrier resulting in an influx of groundwater from a second aquifer, which was recorded by 15-20% concentration increases for some cations and anions. This was followed by hydrochemical recovery. Based on petrological observations of tectonically exhumed fault rocks, we conclude that hydrochemical recovery recorded fault healing by precipitation of secondary minerals along fractures. Because hydrochemical recovery accelerated with time, we conclude that the growth rate of these minerals was controlled by reaction rates at mineral-water interfaces. Geochemical modeling confirmed that the secondary minerals which formed along fractures were saturated in the sampled groundwater. Fault healing and therefore hydrochemical recovery was periodically interrupted by refracturing events. Supported by field and petrographic evidence, we conclude that these events were caused by changes of fluid pressure probably coupled with earthquakes. These events became successively smaller as groundwater flux decreased with time. Despite refracturing, hydrochemical recovery reached completion 8-10 years after the earthquake.

  • 5.
    Zhao, Zhihong
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    An assessment of the role of nonlinear reaction kinetics in parameterization of metamorphic fluid flow2014In: Journal of Geophysical Research - Solid Earth, ISSN 2169-9313, E-ISSN 2169-9356, Vol. 119, no 8, p. 6249-6262Article in journal (Refereed)
    Abstract [en]

    Based on inverse modeling of reaction progress data using a numerical framework that considers coupled advection and diffusion, linear and nonlinear reaction kinetics, and with effective diffusivity given by Archie's law, we show that (1) choice of reaction order has little effect (<0.3 orders of magnitude) on estimates of time-integrated and time-averaged metamorphic fluid fluxes and metamorphic fluid flow durations based on reaction progress data, (2) reaction order must be known for robust determination of time-averaged net reaction rates based on reaction progress data and that underestimation of this term by more than 3 orders of magnitude can arise from assuming linear reaction kinetics, (3) differing reaction orders between laboratory experiments and natural metamorphic systems and/or a nonlinear dependence of effective diffusivity on porosity can explain order-of-magnitude discrepancies between field-based and laboratory-based estimates of time-averaged net reaction rates, and (4) parameterization of metamorphic fluid flow is limited to time-averaged values which fail to account for the possibility that metamorphism occurs in short-lived pulses during longer time periods of metamorphic quiescence.

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