How is it possible to have a drop in gravity when a earth quake happens?
Not my field, but I can't see it happening on the whole. If the quake was big enough, maybe it caused a temporary shift in the centre of gravity. If the satellite was aimed at a small, specific area, maybe there was a local drop.
Aerial measurements of gravity are useful for various purposes, eg. identifying economically feasable ore deposits. Try googling "Gravity Recovery and Climate Experiment"?
A large mass was suddenly displaced, thus the earth's gravitational field was slightly altered, very suddenly. A planet is not a uniform sphere; there are inhomogenities in mass distribution, which can be mapped by measuring the gravitational field. Look at the picture - the scale is in parts per 10^-5, these are very tiny differnces.
Note carefully - "drop in gravity" is not a change of physics, it's simply the result of a change of topography.
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