Geology Reference
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Box 5.2 Climate-tectonic linkages at seasonal time scales.
Although the largest load in a sedimentary basin derives from the mass of sediments within it,
water that is either stored or passing through the basin creates an additional load. For basins
where rainfall is highly seasonable, cyclic variations in that water load might be expected. Over
the past decade, studies that combined remotely sensed altimetry with continuous GPS records
have documented not only the variations in the height of water within a basin, but also coeval
basement subsidence as the crust responds elastically to a seasonal water load (Bevis et al .,
2005). Remotely sensed gravity data now allow the changing mass of water in a basin to be
quantified at daily to weekly time scales. As a consequence, we can now estimate the crustal
flexure that would be expected to occur in response to changing seasonal water loads.
Every summer the Indian monsoon dumps 1-4 m of rain on the Himalaya and the adjacent
foreland basin. Rivers flood and the ground becomes saturated. In most of the foreland, about
70-80% of the annual rainfall falls during those summer months and causes the crust to subside
under that temporary load. Surprisingly, this rainwater loading has a spectacular impact on
crustal stresses, crustal motion, and seismicity within the nearby Himalaya (Bettinelli et al .,
2008). A comparison (see figure A) of the water load (or height) within the foreland, changes in
the rate of north-south contraction in the Himalaya, and the frequency of earthquakes greater
than magnitude 3 shows that they vary with similar periods, but that times of high water load-
ing are anticorrelated with times of high seismicity and more rapid north-south shortening.
100
80
60
40
20
0
30
20
10
0
-10
-20
A
Seismicity
(Mag . 3 or Higher)
monsoonal water loading
modeled impact of water load on displacement
Continuous GPS data
Horizontal Component of Himalayan Contraction
-30
3
2
1
Satellite gravity
0
-1
-2
Water Level in Foreland
Satellite altimetry
-3
1998
1999
2000
2001
2002
2003
2004
Ye a r
A. Continuous time series of seismicity and geodesy in the Himalaya and water levels in the adjacent foreland.
Modified after Bettinelli et al . (2008).
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