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We reiterate the reasons for considering the force balance equations at the Earth’s surface separate from the full 3-dimensional problem, with the former involving the much simpler physics of purely-elastic behavior. This leads us to inverting for VDoHS rates, which are surface terms containing all information that can be deduced from surface observations about subsurface deformation sources. VDoHS rates have inherently high spatial resolution, and are much better than velocities or strain rates for identifying individual source signal. When source signals are small (<1 mm/year in horizontal velocity at the surface), low signal-to-noise-ratio can be counterbalanced by having very dense GPS networks (station spacing <0.5 times the source depth). We briefly discuss that our methodology can be extended to include InSAR and strainmeter data, and we conclude by outlining how the surface-based methodology and full 3-dimensional physical modelling are complementary tools. That is, the new methodology provides the most detailed surface images possible of the subsurface sources irrespective of their nature, whereas physical modeling provides understanding of what is actually happening below the surface.
KeywordsForce balance equations Earth’s surface Purely elastic behavior High spatial resolution Individual source signal InSAR and strainmeter 3-dimensional physical modelling Detailed surface images Subsurface sources
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