On the Earth's Hydrostatic State
Abstract
A hydrostatic state for a planetary body is that the planetary body behaves like an inviscid fluid in response to rotational and tidal force, such that the planetary shape coincides with an equipotential surface. While some planetary bodies like the Moon clearly deviate from their hydrostatic state, the Earth is generally assumed to be in a hydrostatic state. This assumption has implications for a range of geophysical questions, including the origin of Earth's non-hydrostatic gravitational anomalies and the rotational dynamics. Given that the Moon's non-hydrostatic state is likely caused by dynamic interplay between thickening lithosphere and de-spinning of the Moon (Qin et al., 2018), we proceed to investigate the possible effects of Earth's lithosphere on its hydrostatic state. In this study, we model the Earth's response to its centrifugal potential using the newly upgraded 3-D spherical loading software package CitcomSVE (e.g., Zhong et al., 2003) that solves dynamic loading equations for a viscoelastic Earth with 3-D mantle structure. First, we show that CitcomSVE reproduces fluid Love numbers computed from analytical solutions to an accuracy better than 99.99% for a homogeneous mantle (i.e., the hydrostatic case) and also a mantle with a lithosphere of uniform thickness. Second, we show that when a global elastic lithosphere thickness model is employed, the gravitational potential at the loading harmonic (l=2 and m=0) is reduced by ~0.5% (or ~100 meter of geoid anomalies) compared with the hydrostatic value, and significant gravitational anomalies are also generated at other harmonics due to mode coupling. Additionally, stress close to 1 GPa is generated in the lithosphere. Third, when weak/faulted plate boundaries are included in the lithosphere model, we find that the response is nearly the same as that for the hydrostatic state, and that lithospheric stress is greatly reduced to below tens of MPa, indicating that the weak plate boundaries tend to release lithospheric stress and restore the hydrostatic state. Finally, we consider a hypothetical but arguably more realistic scenario in which the Earth starts from a hydrostatic state with the lithosphere thickening with time to the present-day values. This model allows us to examine the effects of more realistic initial state on the Earth's hydrostatic state.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMDI001..07Z
- Keywords:
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- 1213 Earth's interior: dynamics;
- GEODESY AND GRAVITY;
- 1217 Time variable gravity;
- GEODESY AND GRAVITY;
- 1239 Earth rotation variations;
- GEODESY AND GRAVITY;
- 1507 Core processes;
- GEOMAGNETISM AND PALEOMAGNETISM