Abstract
SUMMARY Satellite-derived gravity data offer a novel perspective for understanding the physics of megathrust earthquakes at subduction zones. Nonetheless, their temporal resolution and observational errors make it difficult to discern the different phases of the seismic cycle, as the elastostatic deformation (co-seismic) and the stress relaxation by viscous flow (post-seismic). To overcome these difficulties and to take advantage of the physical constraints on the temporal evolution and on the spatial pattern of the earthquake-induced gravity disturbances, we have jointly estimated the fault slip of the 2011 Tohoku earthquake and the rheological stratification by means of a Bayesian inversion of GRACE data time series and within the framework of spherically symmetric self-gravitating compressible viscoelastic Earth models. This approach, in addition to improve the exploitation of satellite-derived gravity data, allows us (i) to constrain the fault slip taking advantage of information from both the co- and post-seismic signatures and (ii) to investigate the trade-off between the fault slip and the shallow rheological stratification. In this respect, it can be used to improve the modelling of crustal displacements from GPS data, even if their higher accuracy and temporal resolution allow to discriminate well the co-seismic signature from the others.
Highlights
These signatures result from earthquake-induced elastostatic deformation and stress relaxation by viscous flow, and are superimposed on the background gravity field, which varies through time due to hydrology, continental ice variations, residual ocean circulation and solid Earth processes other than the earthquake (De Linage et al 2009; Matsuo & Heki 2011)
Satellite-derived gravity data can recover only the long wavelength gravity field and, so, they miss a significant part of the gravity disturbance caused by earthquakes, being characterized by shorter wavelengths
The spatial filtering of the Gravity Recovery And Climate Experiment (GRACE) data damps the earthquake signatures, limiting further the information from the observations that we can use for constraining the earthquake source and the rheological stratification
Summary
Solid Earth mass rearrangement and ocean water redistribution caused by great earthquakes are made visible by their co- and postseismic signatures on the Earth’s gravity field, nowadays detectable by the Gravity Recovery And Climate Experiment (GRACE) and Gravity and Ocean Circulation Explorer (GOCE) satellites (De Linage et al 2009; Matsuo & Heki 2011; Cambiotti & Sabadini 2013; Han et al 2014; Broerse et al 2015; Fuchs et al 2016). The monthly temporal resolution and the observational errors of GRACE data make it difficult to distinguish the co- and post-seismic signatures only on the basis of their temporal evolutions, especially when we have to estimate at the same time the characteristic times controlling stress relaxation These relaxation times, vary with the wavelength and so depend on the geographical location (De Linage et al 2009; Broerse et al 2015). Differently from annual and semiannual periodic signals which can be removed from the 15 yr of GRACE data, additional trends in the TBG (due to non-periodic hydrological, oceanic and ice mass re-arrangements; Didova et al 2016) can affect the estimate of the post-seismic signature (De Linage et al 2009)
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