GRACE gravity data help constraining seismic models of the 2004 Sumatran earthquake

Abstract

The analysis of Gravity Recovery and Climate Experiment (GRACE) Level 2 data time series from the Center for Space Research (CSR) and GeoForschungsZentrum (GFZ) allows us to extract a new estimate of the co‐seismic gravity signal due to the 2004 Sumatran earthquake. Owing to compressible self‐gravitating Earth models, including sea level feedback in a new self‐consistent way and designed to compute gravitational perturbations due to volume changes separately, we are able to prove that the asymmetry in the co‐seismic gravity pattern, in which the north–eastern negative anomaly is twice as large as the south–western positive anomaly, is not due to the previously overestimated dilatation in the crust. The overestimate was due to a large dilatation localized at the fault discontinuity, the gravitational effect of which is compensated by an opposite contribution from topography due to the uplifted crust. After this localized dilatation is removed, we instead predict compression in the footwall and dilatation in the hanging wall. The overall anomaly is then mainly due to the additional gravitational effects of the ocean after water is displaced away from the uplifted crust, as first indicated by de Linage et al. (2009). We also detail the differences between compressible and incompressible material properties. By focusing on the most robust estimates from GRACE data, consisting of the peak‐to‐peak gravity anomaly and an asymmetry coefficient, that is given by the ratio of the negative gravity anomaly over the positive anomaly, we show that they are quite sensitive to seismic source depths and dip angles. This allows us to exploit space gravity data for the first time to help constraining centroid‐momentum‐tensor (CMT) source analyses of the 2004 Sumatran earthquake and to conclude that the seismic moment has been released mainly in the lower crust rather than the lithospheric mantle. Thus, GRACE data and CMT source analyses, as well as geodetic slip distributions aided by GPS, complement each other for a robust inference of the seismic source of large earthquakes. Particular care is devoted to the spatial filtering of the gravity anomalies estimated both from observations and models to make their comparison significant.

Effects of Earth's layered structure, gravity and curvature on coseismic deformation

“…They claimed a substantiation of the three important effects; they also show that in order to explain the GRACE observed gravity anomalies following the 2004 SE Asia earthquake it is essential to gravitationally self-consistently solve the sea level equation, and the important role played by compressibility of the solid Earth layers is detailed. Cambiotti et al (2011) also enlighten the gravitational effect of the ocean water washed out from the epicentral area. Sun & Zhou (2012) applied an approximated approach to make seawater corrections.…”

“…That is, the ocean response to coseismic ocean bottom deformation should be carefully considered in computing coseismic deformations or fault inversions (Melini & Piersanti 2006;Linage et al 2009;Heki & Matsuo 2010;Melini et al 2010;Broerse et al 2011;Cambiotti et al 2011;Sun & Zhou 2012). The dislocation theories above are valid for a solid elastic Earth, and the corresponding computing program assumed dry Earth, so that the surface subsidence on the Earth surface is replaced with air.…”

Ocean contribution to seismic gravity changes: the sea level equation for seismic perturbations revisited

“…When a uniform ocean model is used for interpretation of gravity observations, one of the effects is that the slip will appear to be deeper than its actual depth, as deeper slip increases the ratio positive peak to negative peak (Cambiotti et al 2011;Han et al 2013).…”

“…In the case of suboceanic earthquakes, changes in bathymetry have a first-order effect on coseismic gravity changes (de Linage et al 2009;Broerse et al 2011;Cambiotti et al 2011). Namely, when the bathymetry changes during an earthquake-up to a few metres of uplift and subsidence have been measured for recent megathrust earthquakes (Meltzner et al 2006;Farías et al 2010)-the height of the water column is adjusted, resulting in a redistribution of water.…”

On the response of the Earth to a fault system: its evaluation beyond the epicentral reference frame

“…This formalism thus aims at improving our understanding of the relation between the static perturbation and the whole fault system, and to make easier the implementation of spherically symmetric Earth models in geodetic problems, like the modelling of long-wavelength gravity or geoid anomalies, nowadays detectable by Gravity Recovery and Climate Experiment (GRACE) and Gravity field and steady-state Ocean Circulation Explorer (GOCE) space missions (Han et al, 2006;Panet et al, 2007;de Linage et al, 2009, Cambiotti et al, 2011Matsuo and Heki, 2011;Han et al, 2011;Wang et al, 2012;Sabad ini, 2012, 2013;Fuchs et al, 2013).…”