D3 Geodesy and Deformation

Topic

Modern geodesy measures the position, shape, and deformation of the Earth's surface with millimeter-level precision or better. A continuous satellite positioning system allows for the determination of tectonic plate movement in near real-time, the monitoring of deformation in an active fault—both during an earthquake and in the intervening years—and the detection of volcanic inflation or deflation prior to an eruption.

A continuous satellite positioning network, comprising dozens or even hundreds of stations distributed around an active fault, yields a velocity field for the entire region; this velocity field is then mathematically inverted to determine how fault slip is distributed at depth.

An interferometric radar technique combines the phase data from multiple satellite radar images of the same area acquired at different times; the phase difference between two such acquisitions encodes the ground surface displacement along the satellite's line of sight, achieving millimeter-level precision over areas spanning thousands of square kilometers simultaneously. This interferometric radar technique is the most powerful method available for mapping subsidence, volcanic inflation, slow-moving landslides, and coseismic deformation on a regional scale.

Differential gravimetry, in conjunction with satellite gravimetry missions, measures variations in the Earth's gravitational field—variations that directly reflect changes in subsurface mass distribution, such as aquifer recharge, glacier melting, land subsidence, or crustal isostatic rebound. The principle underlying this technique is that the gravitational acceleration measured at a specific point depends on the change in gravitational potential between that point and a nearby location—a potential derived from the sum of contributions from all mass distributed within the volume surrounding that point. The expectation is that the integrated use of continuous satellite positioning, interferometric radar techniques, and gravimetry will provide a complete, three-dimensional picture of Earth's surface deformation—whereas each method individually measures only a single component or partial projection of that same deformation—thereby bridging the gap between the millimeter-level precision mentioned at the start of this description and the comprehensive image that modern geodesy can reconstruct by combining its various methods.

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