Gravity changes associated with recent great earthquakes from a decade-long observation of GRACE gravity fields: Inversion of the earthquake source parameters and constraint to the Earth's viscoelastic structure
Abstract
We quantify gravity changes after great earthquakes present within the 11-year-long time-series of monthly global GRACE gravity fields. Using the normal-mode formulation, we present our estimates of the source parameters of moment tensor and double-couple for the events of the 2004 Sumatra-Andaman, 2007 Bengkulu, 2010 Maule, 2011 Tohoku-Oki, 2012 Indian Ocean strike-slip earthquakes. For the 2012 Indian Ocean earthquake (the first strike-slip event detected by GRACE), the GRACE gravity data delineate a composite moment of 1.9×1022 N-m regardless of centroid depth, comparing favorably with the total seismic moment of the main ruptures and aftershocks. The smallest event we successfully analyzed with GRACE was the 2007 Bengkulu earthquake with M0 ~ 5.0×1021 N-m. We found that the gravity data constrain the focal mechanism with the centroid only within the upper and lower crustal layers for thrust events. Deeper sources (i.e., in the upper mantle) could not reproduce the gravity observation as the larger rigidity and bulk modulus at mantle depths inhibit the interior from changing its volume, thus reducing the negative gravity component. Focal mechanisms and seismic moments obtained in this study represent the behavior of the sources on temporal and spatial scales exceeding the seismic and geodetic spectrum. In addition, the large-scale postseismic gravity changes following the 2004 Sumatra-Andaman, 2010 Maule, and 2011 Tohoku-Oki earthquakes were evident in the GRACE time-series of the moment tensor components. Our preferred interpretation of the long-wavelength postseismic gravity change is biviscous viscoelastic flow. We present our estimates of the Earth viscoelastic structures by delineating a range of transient and steady-state viscosities. Finally, we discuss how these solutions could be used to correct the GRACE observations for the studies on climate change.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.G21C..05R
- Keywords:
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- 1217 GEODESY AND GRAVITY Time variable gravity;
- 7215 SEISMOLOGY Earthquake source observations;
- 1236 GEODESY AND GRAVITY Rheology of the lithosphere and mantle;
- 1240 GEODESY AND GRAVITY Satellite geodesy: results