No doubt that the land in the central valley is sinking, quadrillions of tons of water being sucked out of the ground over the last hundred and fifty years. Think of the continental crust under that as a boat on the Earth's mantle. When the boat gets lighter, the draft is reduced and the top of the boat sits higher from the water line. Static GPS is accurate enough to measure that rise of the top in millimeters per year. Just my perspective and two cents as a geodetic surveyor.
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