J. Sauber et al., Crustal deformation associated with glacial fluctuations in the eastern Chugach Mountains, Alaska, J GEO R-SOL, 105(B4), 2000, pp. 8055-8077
The changes of the solid Earth in south central Alaska in response to two m
ajor glacial fluctuations on different temporal and spatial scales have bee
n estimated and we evaluated their influence on the stress state and ongoin
g tectonic deformation of the region. During the recent (1993-1995) Bering
Glacier surge, a large transfer of ice from the Bagley Ice Field to the Ber
ing Glacier terminus region occurred. We estimated the elastic displacement
of the solid Earth due to ice mass redistribution from Global Positioning
System (GPS) measurements at sites near the surging glacier. We can account
for these displacements by transfer of an ice volume of about 14 km(3) fro
m the surge reservoir area to the terminus region. We examined the backgrou
nd seismicity (M-L greater than or equal to 2.5) before, during, and after
the surge. We found that the occurrence of small earthquakes (M-L less than
or equal to 4.0) in the surge reservoir region increased during the surge
time interval possibly in response to a decrease in ice mass. This suggests
that a Small decrease in the vertical stress, sigma(3), could be enough to
modulate the occurrence of small, shallow earthquakes in this dominantly t
hrust fault setting. During this century the southern Alaska coastal glacie
rs have been undergoing an overall decrease in volume. Based on our compila
tion of changes in the extent and thickness of the coastal glaciers between
the Malaspina and Bering, we calculated surface displacements due to the E
arth's viscoelastic response to annual thinning and to the cumulative retre
at over the last 100 years. The uplift of the region due to an average annu
al thinning rate of 1-6 m/yr in the ablation region is 1-12 mm/yr. For our
reference model with a viscosity of 5 x 10(19) Pa s for depths between appr
oximate to 40 and 200 km the total viscoelastic response due to the retreat
over the last century may be as much as a couple of meters within the coas
tal ablation zone near Icy Bay. The maximum decrease in sigma(V) between 0
and 10 km was approximate to 1.0 MPa, which is significant in relation to t
he stress drops in recent earthquakes (approximate to 2 to 10 MPa) but smal
l in relation to the estimated tectonic stress magnitude. Therefore the occ
urrence of an earthquake such as the St. Elias (1979, M-S = 7.2) may have b
een advanced in time; however, most of the ongoing stress accumulation woul
d be primarily due to tectonic forces.