Geophysical characterization of the Krafla volcanic area from seismic tomography and attenuation

Exploration and exploitation of natural resources, such as geothermal energy, require a proper understanding of the physical properties of the upper crust, where they are mostly allocated. Indeed, the transition from brittle to ductile deformation (BDT), occurring at these depths, marks a progressive change in crustal rheology and a reduction in the rock’s permeability. Therefore, the characterization of underground conditions is crucial for planning explorative studies in geothermal systems. It has been recently demonstrated that the analysis of the propagation of seismic waves provides information on physical rocks’ behavior and an alternative assessment of the BDT depth [1]. In particular, the decay of the amplitude of the seismic waves (i.e. seismic attenuation), which is usually described by a “quality factor” Q, depends on the seismic frequency, temperature, water content, and grain size of the rocks. Depending on the seismic scale, it could be used as an indicator of subsurface heterogeneities.    

In this study, we investigate the seismic velocity and attenuation sensitivity to the crustal heterogeneities in areas affected by young tectonics and hot thermal conditions. To this aim, we implement a Q seismic tomography in the volcanic system of Krafla. The volcanos of age 0.5–1.8 Myr extend over an area of 21 km by 17 km and are characterized by faults and fissures, which allow water to penetrate and circulate at shallow depths [2] easily. In these geothermal fields, the temperatures, in a range of 400-600 °C at a depth < 5 km [3], make the BDT depth close to the surface.

We apply the method that solves Q_p perturbations, using a combination of a spectral decay technique to retrieve the attenuation operator (t*) and tomographic inversion [4]. The distribution of seismic wave velocities is obtained from a 3D tomographic inversion, using 1453 earthquakes detected from a local seismic network (2009-2012) [2]. Q_p inversion is performed with the simul2014 algorithm [5], while a linearized technique solves a nonlinear problem that uses a damped least-squares inversion for model perturbations.

We obtain a map of Q_p variations for the first 4 km, which we jointly interpret with the seismic wave velocities [2]. In this way, we can discriminate between anomalies related to temperatures and compositional heterogeneities. We also test the possibility to detect the BDT depth on the base of the reduction of the Q_p, related to hot temperatures/melt conditions. The obtained results will contribute to understanding the dynamics of the tectonic features and help plan explorative studies of high enthalpy geothermal systems, adding constraints to the correlation between viscous rocks’ deformation and their seismic attenuation.

References

[1] Natale Castillo et al., 2022. Gloplacha 219, 103978, ISSN 0921-8181, https://doi.org/10.1016/j.gloplacha.2022.103978.

[2] Schuler et al., 2015. J. Geophys. Res. Solid Earth 120, 7156–7173, doi:10.1002/2015JB012350.

[3] Scott et al., 2015. Nature communications 6, 7837. 10.1038/ncomms8837. https://doi.org/10.1038/ncomms8837.

[4] Lanza et al., 2020. J. Volc. Geoth. Res. 393, 106804, ISSN 0377-0273. https://doi.org/10.1016/j.jvolgeores.2020.106804

[5] Evans et al., 1994. US Geological Survey Open File Report OFR 94- 431, p. 101. https://doi.org/10.3133/ofr94431