Sumatra tsunami affects observations by GRACE satellites

Changes in the Earth’s gravity fi eld are
caused by the redistribution of mass within
the Earth and on or above its surface.While
previous studies [Tapley, 2004; Wahr, 2004]
showed that the Gravity Recovery and Climate
Experiment (GRACE) satellite mission, executed
by NASA,had successfully determined
oceanic mass redistribution, the relative motions
between two GRACE satellites caused by
the 2004 Sumatra tsunami is still uncertain.
This present study combines a numerical
model of the tsunami and GRACE orbit data
to estimate the realistic effect of oceanic mass
redistribution on the inter-satellite range-rate
change between two GRACE satellites.The
GRACE mission is designed to map out the
Earth’s gravity field to high accuracy. Instead of
measuring the Earth gravity field or mass variations
directly, GRACE estimates a set of spherical
harmonic coefficients denoting the Earth
gravity fi eld each month by measuring range
changes between two spacecrafts [Chambers
et al., 2004].
The aftermath of this study has shown that
the sea surface height anomaly due to the 2004
Sumatra tsunami can cause GRACE inter-satellite
range-rate change (i.e., the small relative motions
between twin GRACE satellites).When the
GRACE trajectories are near the tsunami regions,
the range-rate variations are large enough to be
observed by GRACE.
On 26 December 2004, the fourth strongest
earthquake over the past century occurred
in the Indian Ocean off the western coast of
northern Sumatra, Indonesia. Measuring 9.0 in
magnitude, the earthquake generated a massive
tsunami that struck the Indian Ocean countries
and Somalia.The sudden and violent
vertical displacement of the seafl oor caused
a disturbance to the overlying water column
that propagated rapidly across the whole Indian
Ocean.As the tsunami reached shallow water,
the height of the wave drastically increased
and produced huge inundations and runup
heights of up to a few tens of meters.

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