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