An Estimation of the Average Viscosity of the Asthenosphere in Japan from Seismic Deformations Using GPS and GRACE

Our Earth behaves as an almost viscoelastic layered body in time range over several months. The viscoelastic behaves occur in various geophysical processes such as the time-dependent deformations due to mass redistribution from huge earthquakes, glacial ice sheet evolutions, and seasonal hydrological circulations. These induced deformations provide us a way to investigate the rheology model of the Earth material through the surface global geodetic observations.

The viscoelastic deformations excited by different sources can reflect the depth-dependent viscoelastic parameters of the Earth. Seismic deformations whose sources are at a depth of tens to hundreds of kilometers, show us complex deformation patterns; and this mirrors the rheology structure of some viscoelastic layers in the asthenosphere and upper mantle.

Modern geodetic observations are expected to have great potential to estimate the rheology parameters in different depths. Some published papers have estimated the regional averaged viscoelastic parameters based on different rheology models like Maxwell solid, Kelvin-Voigt body, and Burgers body using GPS and GRACE observation for the huge earthquake. However, the estimated viscoelastic parameters have significant discrepancies when one considers different rheology models in different time spans.

In this study, We use GPS and GRACE to test whether these observations can constrain the viscoelastic parameters and distinguish different rheology models both in different temporal and spatial scales. Firstly, we will implement some forward calculation for displacement and gravity changes of Tohoku-Oki earthquake (Mw=9.0) in flat and spherical earth model. Then we use monthly GRACE and daily GPS data in Japan and the adjacent region to inverse the viscoelastic parameters assuming in different rheology models (Maxwell solid, Kelvin-Voigt body, and Burgers body).

Our Earth behaves as an almost viscoelastic layered body in time range over several months. The viscoelastic behaves occur in various geophysical processes such as the time-dependent deformations due to mass redistribution from huge earthquakes, glacial ice sheet evolutions, and seasonal hydrological circulations. These induced deformations provide us a way to investigate the rheology model of the Earth material through the surface global geodetic observations.

The viscoelastic deformations excited by different sources can reflect the depth-dependent viscoelastic parameters of the Earth. Seismic deformations whose sources are at a depth of tens to hundreds of kilometers, show us complex deformation patterns; and this mirrors the rheology structure of some viscoelastic layers in the asthenosphere and upper mantle.

Modern geodetic observations are expected to have great potential to estimate the rheology parameters in different depths. Some published papers have estimated the regional averaged viscoelastic parameters based on different rheology models like Maxwell solid, Kelvin-Voigt body, and Burgers body using GPS and GRACE observation for the huge earthquake. However, the estimated viscoelastic parameters have significant discrepancies when one considers different rheology models in different time spans.

In this study, We use GPS and GRACE to test whether these observations can constrain the viscoelastic parameters and distinguish different rheology models both in different temporal and spatial scales. Firstly, we will implement some forward calculation for displacement and gravity changes of Tohoku-Oki earthquake (Mw=9.0) in flat and spherical earth model. Then we use monthly GRACE and daily GPS data in Japan and the adjacent region to inverse the viscoelastic parameters assuming in different rheology models (Maxwell solid, Kelvin-Voigt body, and Burgers body).