Spatiospectral localization of global geopotential fields
from the Gravity Recovery and Climate Experiment (GRACE)
reveals the coseismic gravity change
owing to the 2004 Sumatra-Andaman earthquake

Shin-Chan Han¹ and Frederik J Simons²

1 Planetary Geodynamics Laboratory
NASA Goddard Space Flight Center
Greenbelt MD 20771, USA

2 Geosciences Department
Princeton University
Princeton NJ 08544, USA

Abstract

Regional mass fluxes due to transport and adjustment within the Earth system that are implicitly contained in the monthly GRACE global geopotential coefficients are revealed by localizing global spectra using spatiospectrally concentrated window functions. We have analyzed 45 monthly global GRACE harmonic coefficient series in order to find the coseismic signature associated with the 2004 great Sumatra-Andaman earthquake. A significant gravity change after the earthquake is found in the time-series of the GRACE coefficients after localization with a single band-limited window centered near the north of the island of Sumatra. This change is undetectable from the original global coefficients or from coefficients localized elsewhere on the globe. A step function with its discontinuity at 26 December 2004 usefully models the coseismic gravity change. The localized GRACE coefficients contain the jumps (associated with the earthquake) up to degree and order 55, although not all of them within this band produce changes that are statistically significant. The gravity change calculated from the localized GRACE coefficients displays 30 µGal peak-to-peak variations which are very well correlated with an independently derived seismic model based on elastic dislocation theory.

Figures

1. Figure 02 Bandlimited (un)localized rms variations of the gravity field determined by GRACE
   
2. Figure 03 Geopotential time series localized around Sumatra with an L=15 Slepian window
   
3. Figure 04 Geopotential time series as part of the global, unlocalized L2 data set
   
4. Figure 05 Geopotential time series localized increasingly away from Sumatra with an L=15 Slepian window
   
5. Figure 07 Spatial rendition of the gravity change due to the earthquake from the localized GRACE time series
   
6. Figure 08 Spatial rendition of the modelled gravity change (displacement and density effects) and correlation with GRACE observations
   
7. Figure 09 Spatial rendition of the in-phase and out-of-phase annual gravity change: GRACE observations and LDAS predictions