Seismic tomography and attenuation models for the geophysical characterization of the Krafla volcanic area.

The characterization of the underground structures is crucial for planning explorative studies in geothermal systems. As recently demonstrated, analysis of the propagation of seismic waves provides information on physical rocks’ behavior and alternative assessments of the brittle-ductile transition (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, depending on the seismic scale, could be used as indicator of the subsurface heterogeneities. In this study, we investigate the sensitivity of the seismic velocity and attenuation to the crustal heterogeneities, by implementing a Q seismic tomography of the Krafla volcanic system, an area affected by young tectonics and hot thermal conditions. The applied method solves for Qp perturbations, using a combination of a spectral decay technique to retrieve the attenuation operator (t*) and tomographic inversion [2]. The distribution of seismic wave velocities is obtained from a 3D tomographic inversion, using 1453 earthquakes detected from a local seismic network (2009-2012) [2,3]. The obtained Qp variations up to a depth of 4 km is interpreted together with the seismic wave velocity values [3]. The joint interpretation helps discriminate between anomalies related to temperatures and compositional heterogeneities, defining the main structures of the area. We also try to define the BDT depth, on the base of the strong reduction of the Qp, related to hot temperatures/melt conditions.