A Unified Approach to Modeling the Effects of Earthquakes on the Three Pillars of Geodesy
Besides generating seismic waves that eventually dissipate an earthquake also generates a static displacement field everywhere within the Earth, causing the geometrical shape of both the Earth’s outer surface and of internal boundaries such as the core-ma
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A Unified Approach to Modeling the Effects of Earthquakes on the Three Pillars of Geodesy R.S. Gross and B.F. Chao
Abstract Besides generating seismic waves that eventually dissipate an earthquake also generates a static displacement field everywhere within the Earth, causing the geometrical shape of both the Earth’s outer surface and of internal boundaries such as the core-mantle boundary to change. By rearranging the Earth’s mass earthquakes also cause the Earth’s rotation and gravitational field to change. Earthquakes therefore affect all three pillars of geodesy, namely, the Earth’s geometrical shape, rotation, and gravity. These effects of earthquakes are usually modeled separately, with flat Earth models typically being used to compute changes in site positions and spherical Earth models being used to compute changes in the Earth’s rotation and global gravitational field. Here, a unified approach to computing changes in the three pillars of geodesy is described. As an example of this approach it is applied to the 2004 Sumatran earthquake. A preliminary comparison of predicted and SLR-observed degree-2 zonal gravitational field coefficients does not reveal the expected step-like change at the epoch of the earthquake.
85.1 Introduction The Earth is a dynamic system with a fluid, mobile atmosphere and oceans, a continually changing global distribution of ice, snow, and water, a fluid core that
R.S. Gross () Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109-8099, USA e-mail: [email protected]
is undergoing hydromagnetic motion, a mantle both thermally convecting and rebounding from the glacial loading of the last ice age, and mobile tectonic plates. In addition, external forces due to the gravitational attraction of the Sun, Moon, and planets also act upon the Earth. These internal dynamical processes and external gravitational forces exert torques on the Earth or displace its mass, thereby causing its shape, rotation, and gravity to change. These three fundamental properties of the Earth – its shape, rotation, and gravity – are the three pillars of geodesy. But these three pillars of geodesy do not stand alone. They are interconnected by both common observing systems and by common excitation mechanisms. For example, the space-geodetic measurement technique of satellite laser ranging (SLR) can be used to determine changes in the positions of the laser ranging stations and hence changes in the shape of the Earth, can be used to determine changes in the orientation of the network of laser ranging stations with respect to the orbiting satellites and hence changes in the Earth’s rotation, and can be used to determine changes in the orbits of the satellites and hence changes in the Earth’s gravitational field. Global navigation satellite systems can also be used to determine changes in all three pillars of geodesy, while very long baseline interferometry can be used to determine changes in shape and rotation but not in gravity. The three pillars of geodesy are also connect
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