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Background to >10 years of BIFROST activities
Jan M. Johansson1, Hans-Georg Scherneck1, Rüdiger Haas1, Sten Bergstrand1
Martin Lidberg1,2, Lotti Jivall2, and Bo Jonsson2
1. Onsala Space Observatory, SE-439 92 Onsala, Sweden, Email: [email protected]
2. National Land Survey, Gävle, Sweden
The BIFROST project monitors
movements of the earth crust in
three dimensions
In the BIFROST (Baseline Inferences
for Fennoscandian Rebound
Observations Sea-level and Tectonics)
project continuous GPS observations
are used to determine three-dimensional
motion at about 100 stations (see figure
to the right). The project has been active
since 1993 and is based on data
obtained from the GPS networks in the
northern Europe and the IGS global
network. In addition occasional GPS
field campaigns have been carried out
in order to include measurements at tide
gauge sites.
Ice and Ocean Load
The maximum glaciation stage of
the Pleistocene occurred 18 kyr
ago. Shown to the right are three
later stages of deglaciation at 10,9
and 8 kyr before present. The
right column shows the change of
the accompanying ocean load. The
receding ice edge and the
increasing sea level cause the
ocean load to increase while the
land uplift displaces sea water
outward from the rebound area.
The load model is fully consistent
with the visco-elastic selfgraviting planet.
For the GPS data analysis we use the
GIPSY, Bernese, and GAMIT software
packages.
GPS and Tide Gauges
GPS Time Series, daily processing since August 1993
We produce time series of daily determinations of geodetic
positions. In the figures below the bottom diagrams show the
vertical, the middle the east and the top the north position,
respectively. The GPS results are shown in black with green bars
representing the standard error. The motion has been reduced
with respect to the rigid plate that moves together with the most
important European stations. Predictions based on a model are
shown in red. The model comprises one constant rate of motion
in each component and occasional offsets at times when antenna
configuration was changed. In the predictions we also include a
regional common mode of motion that is derived from the
measurements at neighboring stations using an Empirical
Orthogonal Functions method. The fitted rate is shown in each
diagram. The left figure shows the case of a southern station
(Onsala) far away from the postglacial rebound center, and the
right one located more centrally (Sundsvall).
Deformation Rates
The map below shows observed
vertical motion in colors and
horizontal motion by red arrows.
The blue arrows denote
predictions from a model for
visco-elastic isostatic adjustment.
Vertical motion of the crust
relative to the gravity center of
the Earth is inferred from GPS.
This motion is compared to tide
gauge observations in the figure
below. Tide gauges observe the
change of land level relative to
the ocean. The change of the
gravity potential has been
modeled out. Plotting one data
set against the other reveals: 1)
residual unmodeled gravity
field effects as a deviation from
a uniform slope and 2) a
regional change of the sea level
as a negative intercept on the
abscissa. From the plot the sea
level appears to rise with about
1 mm/year.