Ground deformation, gravity and magnetics

Freymueller, Jeffrey T.; Murray, John B.; Rymer, Hazel and Locke, Corinne A. (2015). Ground deformation, gravity and magnetics. In: Sigurdsson, Haraldur; Houghton, Bruce; McNutt, Stephen; Rymer, Hazel and Stix, John eds. The Encyclopedia of Volcanoes. USA: Academic Press, pp. 1101–1123.

DOI: https://doi.org/10.1016/B978-0-12-385938-9.00064-X

Abstract

Volcanic eruptions are preceded by the upward migration of magma, and sometimes by the long-term accumulation of magma in the shallow subsurface. Upward migration of magma causes pressure changes within magma or hydrothermal fluid reservoirs, which cause ground deformation that can be measured as displacement at the surface or as tilt or strain. Deformation measurements are made using GPS and InSAR, and sometimes also using terrestrial surveying techniques. Displacements from volcanic sources can range from millimeters to a few meters. Tiltmeters and strainmeters are far more sensitive than GPS and InSAR to small deformations, but suffer from instrumental drift and require the removal of Earth tides and many smaller non-volcanic signals; however, they can measure rapid changes in deformation very well. Local changes in subsurface density may be produced by magma movements (intrusion/withdrawal) and by variations in the degree of magma vesiculation. These changes can be detected by measuring variations in the acceleration due to gravity. Variations on the order of 10s to 100s of µGal are typical. Magmatic processes may also induce changes in the total magnetic field strength. Magnetometers can measure field strength to 0.1 nanoTesla and fluctuations of a few nanoTesla to a few thousand nanoTesla are typical at volcanoes. Measurements of gravity, magnetic fields, and ground deformation can be made during and between eruptions, providing information on the processes going on inside a volcano when it is not erupting.

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