(mm/yr) (mm/yr) (mm/yr)

Application of modern geodetic
measurements to the geophysics and
Earth’s s environmental problem
Tadahiro
SATO
National Astronomical Observatory of Japan

For the geosciences based on the
data obtained from the modern geodetic
measurements, GIA (Glacial
Isostatic Adjustment)
is important to study the Earth’s viscoelastic
structure and dynamics, and the
environmental problem of the Erath.

Following two topicst
will be introduced;
/ Experience at Ny-Ålesund
lesund, , Svalbard in the
Arctic,
/ Japan–US joint observation project in
Southern Alaska started in 2005**.
**
Abstract #053 of P41 by Freymueller et al.
last day talk closely relates to the 2 nd topics.

Experience at Ny-Ålesund
lesund,
Svalbard in the Arctic
(Sato et al., 2006, GJI, in press)

Geodetic observations at Ny-Ålesund
AG: Absolute Gravimeter
SG: Superconducting
Gravimeter
NALL & NAL1: GPS sites
VLBI: VLBI site
TG: Tide Gauge
L&R: LaCoste & Romberg
Gravimeter

Secular uplift rates at Ny-Ålesund
(mm/yr)
VLBI 4.8 (1.1)
GPS (NYA1) 4.8 (1.5)*
GPS (NALL) 5.3 (0.9)*
Tide Gauge 5.2 (0.3)**
----------------------------------
Mean 5.2 (0.6)
The GPS solutions derived by
Heflin (2004).
* The GPS rates shown here are corrected
for a GPS scale rate error of +1.6 mm/yr to
the VLBI rate, which was found from a
comparison between VLBI and GPS at the 30
collocation sites in the world.
** Assumed 1.7 mm/yr as a rate of global
secular sea level change

Absolute gravity observations
• Rate: -2.5+/ 2.5+/-0.9
μGal/yr
AG measurement in 2002 carried
out by the Luxembourg group.

Predicted PGR (Post Glacial Rebound) :
no more than 2-32
3 mm/yr
Observed uplift rates:
5.2+/-0.6 mm/yr
**Observation is two times larger than the
proposed model results for the PGR effect.
References: Lambeck 1995; Kaufmann & Wolf 1996;
Kaufmann & Wu 1998; Scherneck et al. . 2002.

Possible origins that we studied;
(1) Effect of the secular sea level (SSL(
SSL)
of a few 10 years in the oceans of the
northern part of Europe,
(2) Effect of the present-day ice melting
(PDIM),
(3) Effect of the ambiguity in the modeling
of PGR.

Computation of the PDIM effect
Method:
Convolution of the changing
mass with the loading Green’s
function (for the PREM earth
model) over the main 16 glaciers
approximated their shape with
elliptical cylinder.
Results:
Uplift: 2.04 mm/yr
Gravity: -0.53
μGal/yr
SVAL model by
Hagedoorn & Wolf
(2003)
For a mean melting rate of -45 cm/yr
over Svalbard that was inferred from
glaciology data.

Sensitivity test of the PGR rate
Large ice model
● : ARC3+ANT4
▼ : ICE-3G (large ice model)
Small ice model:
○ : ANU+ANT4
Computed by Okuno

Comparison between the observation and the prediction
• Displacement
NS EW Vertical
Gravity
(mm/yr) (mm/yr) (mm/yr) (μGal(
Gal/yr)
Observation Mean 0.1 (0.4) -1.8 (0.4) 5.2 (0.6) -2.5 (0.9)
Corrections
1. RSL 0.00 -0.01 0.12 -0.04
2. PDIM SVAL model 0.05 -0.83 2.04 -0.53
3. PGR ANU+ANT4 0.53 -0.71 1.18 -0.33
ARC3+ANT4 1.30 -2.98 1.46 -0.29
ICE-3G
1.16 -2.43 1.88 -0.31
Sum of the corrections
ANU+ANT4 0.58 -1.54 3.33 -0.88
ARC3+ANT4 1.35 -3.82
3.61 -0.84
ICE-3G
1.21 -3.26 4.03 -0.86

A geophysical interpretation
Assumed melting rate vs Estimated PDIM effect

Summary
Both the observed uplift and gravity rates at Ny-Ålesund
are larger than that expected from the PGR effects.
Our study indicates;
(1) Relative sea level (RSL) changes:
not major origin
(2) Present-day ice melting (PDIM):
If the recent ice melting rate increases, it can be
a strong candidate to explain the difference between the t
observed ed rates and the predicted ones.
(3) Ambiguity of the computation of PGR effect:
Computation is sensitive to both the ice mode and Earth’s
structural parameters.
At Ny-Ålesund
lesund, the horizontal rates are useful to
constrain the discussion of the vertical rate of the PGR.

Japan–US joint observation project
Period: 2005-2008
2008
in Southern Alaska
Leaders:
S. Miura (Tohoku Univ.)
J. T. Freymueller (Univ. Alaska, Fairbanks)
Members:
T. Sato (NAOJ),
H. Fujimoto (Tohoku Univ.),
Y. Miyagi, M. Kasahara, , H. Takahashi (Hokkaido Univ.),
T. Sugano, W. Sun, J. Okuno (ERI, Univ. Tokyo),
C. F. Larsen, R. Motyka (Univ. Alaska, Fairbanks) .

Glacier Bay in Southern Alaska
60N
160W
140W
after C. Larsen (2002)

Rapid Uplift Rate Observed in Southern Alaska
GPS and tide gauges give consistent uplift rates each other.
after Larsen, C. F. et al., 2004 and 2005

Intensive GIA study was done in this area
by University of Alaska, Fairbanks
C.. F. Larsen, R. J. Motyka, , J. T. Freymueller,
K. A. Echelmeyer and E. R. Ivins,
Rapid uplift of southern 1118, Alaska caused by recent ice
loss,
Geophys. . J. Int., (2004) Vol. 158, P.
C. F. Larsen, R. J. Motyka, , J. T. Freymueller,
K. A. Echelmeyer, , E. R. Ivins
Rapid viscoelastic uplift in southeast Alaska caused
by post-Little Ice Age glacial retreat,
EPSL, (2005) 237, p. 548

Summary of the study by Alaskan group
1. Large uplift rates: 30 mm/yr at the maximum.
2. The observed rapid rates may not explain from the
tectonic rate.
3. The uplift pattern estimated from the GPS and TG data is
consistent each other in general.
4. A two-layer
viscoelastic model (i.e. Elastic crust and Low
viscosity half-space mantle) is not adequate to explain all
three data sets of the GPS, TG and raised shore line.
Three-layer
viscoelastic earth model (i.e.
Elastic
lithosphere,
low-viscosity
athenosphere and
high-
viscosity lower mantle) is favorable to explain
consistently all three data sets.
5. For the observed viscous motion to the long-term forcing,
there is a possibility that the effect of post-Little Ice Age
contributes.

Subjects in the project
1. Which global past-ice model among those now
proposed is the best to explain the data obtained in
Southern Alaska
2. What is the best combination of the viscoelastic
structural parameters for the study area
3. How we can precisely separate two contributions
from the past-ice and from the present-day ice
These questions are closely linked each other.

GPS measurements
100 km
after C. F. Larsen (2003)
45 Sites
Red: good data
Blue: need to be visited again

Collocation observation of GPS and gravity
• By collocating the position and gravity measurements.
we have an opportunity to discuss the elastic and
viscous problems separately (Wahr(
et al. . ,1995)
Key numbers;
+6.5 mm/μGal
Gal: : a ratio of the viscous displacement
to the viscous gravity change due to
the mass redistribution in the mantle.
-0.3
μGal/mm: Free-air gravity change
-0.15
μGal/mm
(i.e. -0.3+1/6.5):
viscous gravity rate

Observation and study in this project
1. Observation
/ GPS:
Survey at some sites which are suggested to visit again
from the previous observations
Continuous observations
/ Absolute
and relative gravity measurements
Gravity measurements with FG5 absolute gravimeter
Continuous observation of the gravity tide
2. Modeling
/ Modeling of PGR & PDIM
/ Improving the ocean tide model around the field
/ Application of GRACE data to this area by increasing
the spatial resolution of the analysis

Observation plan
● AG, ○ Tidal Gravity, ●, Continuous GPS, ● GPS survey

Absolute gravityg
measurement in Southern Alaska

Results of the AG measurement in 1987
9.81832420 (±2.8 Gal)
Sasagawa et al. (1989)
Expected gravity change:
δh = 18*(2006-1987)
= 342 mm
δg = -0.2*342
= 68 μGal at least

Tide Gauge Stations
100 km
after C. F. Larsen (2003)
20 sites
Black: Permanent site

Effect of the Ocean tide in Southern Alaska (SA)
The ocean tide in SA is very large. Moreover, SA is a place at
where the accuracy of the modern ocean tide models are low,
i.e. difference among the models are large.
This is a reason why we do the observation of gravity tide in
SA.
Ny-Ålesund
1yr
Ocean (cm) 6.1
Vertical (mm) 3.1
Gravity (μGal) 1.2
8 m
Alaska
Ocean (cm) 39
Vertical (mm) 18
Gravity (μGal) 6

Summary
/ Experiences in Svalbard and Southern Alaska
introduced here also show that the modern
geodetic measurements are worth very much
not only the GIA problem but also a tool to
consider the Earth’s s environmental problem via
the present-day ice melting.
/ Experiences in Ny-Ålesund
suggest that the
horizontal displacements are important to
constrain the ice problem, especially for an area
which locates at an edge of past- or present-
day ice sheets.
/ GRACE data may help to determine the low-
order gravity changes and its results should be
used by combing with the data from the ground
observations.

Thanks for your attention.

Observed displacement rates
• NS EW Vertical
• (mm/yr) (mm/yr) (mm/yr)
• VLBI 14.4 (0.5) 10.3 (0.5) 5.1 (1.1)
• 13.8 11.3 4.8
• GPS (NYA1) 14.6 (0.6) 10.0 (0.6) 6.6 (1.5)
• 13.8 10.7 6.4
• GPS (NALL) 14.2 (0.5) 10.7 (0.5) 7.0 (0.9)
• 13.5 11.4 6.9
• NUVEL1A-NNR NNR 13.6 13.0 -------
Note:
The first line for the VLBI and GPS (Heflin, 2004) indicates rates given in an
ITRF2000 frame. The second line indicates rates in a NNR-NUVEL-1A frame.