Sumatra tsunami affects observations by GRACE satellites

Abstract

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.