Some Applications of Satellite Gravity Data to Seismological Problems in the Future

Abstract:

Satellite gravity measurements by GRACE and GOCE have successfully revealed mass transport and the resultant surface loading of the solid Earth due to continental water, atmosphere, oceans and ice sheets. Mass redistributions caused by megathrust earthquakes have also been detected and physical models to explain the gravity variations have been constructed. In this presentation, we show that, in the following two aspects, satellite gravity data can be used for seismological studies, when its spatial resolution becomes higher. A first is the use of observed postseismic gravity change as a sensor to determine the viscosity distribution in a shallower portion of the asthenosphere. For this purpose, a new method for estimating postseismic viscoelastic relaxation of a spherical Earth model is developed. With a spectral finite-element approach, self-gravitation, compressibility and 3D viscosity distributions can be treated in a more natural manner than with the existing methods. As an example, it will be shown that mantle flow caused by a lateral heterogeneity in viscosity due to a slab can be detected. A second is the use of observed non-tidal ocean bottom pressure (OBP) to investigate long-term variations in seismicity. Recent studies revealed that solid and ocean tides modulate the occurrence rate of deep non-volcanic tremors on the plate interface through a non-linear fault friction law. Tremors slightly accelerate the plate subduction speed below the seismogenic zone, by which shallow earthquakes are more likely to be triggered. It will be shown that non-tidal slow variations in OBP as low as 100 Pa, originating from the PDO and the Kuroshio Current, can also perturb seismicity in the Tokai area in Japan when combined with the tidal effects. In this calculation, an ocean model of the Japan Meteorological Agency is used. It is expected that higher-resolution satellite gravity data reduce leakage effects from land hydrology to more accurately evaluate OBPs in plate boundary zones in the world. This implies that satellite gravity missions potentially play an important role for investigating climatic triggering of earthquakes.