Development of a three-dimensional velocity model for the crust and upper mantle in the greater Barents Sea region
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27th Seismic Research Review: Ground-based nuclear explosion monitoring technologies, page 13--22. Rancho Mirage, California, (September 2005)

We have compiled a 3D seismic velocity model for the crust and upper mantle in the greater Barents Sea region including northern Scandinavia, Svalbard, Novaya Zemlya, the Kara Sea, and the Kola-Karelia regions. While the general motivation for developing this model is basic geophysical research, a more specific goal is to create a model for research on the identification and location of small seismic events in the study region, and for operational use in locating and characterizing seismic events in the study region. The observational basis for the velocity model are previous, crustal-scale 2D seismic reflection and refraction profiles, and passive seismological recordings, supplemented by potential field data to provide additional constraints on the crustal structure. The model is defined at grid tiles spaced every 50 km, and each tile is represented by up to two sedimentary and three crystalline crustal layers (plus water and ice). For crustal regions not constrained by primary velocity data, we developed an interpolation scheme based on several defined geological provinces that are characterized by individual tectono-sedimentary histories. The interpolation utilizes the observed strong correlation between sediment and crystalline crustal thickness within continental provinces. For comparison, an alternative interpolation approach applies a continuous curvature gridding algorithm within each of the provinces. To provide a complete lithospheric model, we complemented the crustal model with an upper mantle velocity model based on surface wave inversion, thereby covering depths essential for Pn and Sn travel time modeling. As an extension to the previously existing data set, we recently retrieved a large amount of surface wave data recorded or excited in the European Arctic during the last three decades. The merged surface wave data set will enable us to refine the upper mantle velocity structure in the study region significantly. Preliminary group velocity maps for Rayleigh and Love waves reflect large-scale geological structures and demonstrate lateral velocity variations in the mantle. Validation of our velocity model includes travel time modeling and relocation of seismic events. For this purpose we compiled a set of Ground Truth (GT) events comprising chemical and nuclear explosions, and natural earthquakes. Phase arrival times of multiple events at some sites provide timing error estimates at some stations. With the GT events we obtain a rather good Pn and Sn ray coverage in the main target region. Besides the comparison of observed and modeled travel times along selected transects, we have computed source-specific station corrections (SSSCs) from our 3D model. The crustal velocity models are also evaluated by comparison of predicted gravity fields with the observed free-air gravity. To model the gravity field, we used standard velocity-density relationships for crustal rock types and the density structure of the upper mantle from previous studies. The inferred gravity fields both reflect and exaggerate the basic geological features. Accomplishments so far have been concerned with implementation of a forward modeling procedure and software development needed to support the complex 3D model structure. The forward modelling is done in order to reduce the misfit between observed and modelled gravity and finally to supplement our crustal velocity model with a density distribution.
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