Crustal Deformation of the Yellowstone Volcanic Field From Precise Measurements of Temporal Gravity Changes and Supplementary Leveling and GPS Data

Beschreibung

The Yellowstone caldera in north-west of Wyoming, U.S.A, shows high rates of height and gravity changes. The height changes were monitored by precise leveling, which was first conducted in 1923 and re-measured every few years since 1975. Gravity measurements were first carried out in 1977 and repeated almost annually since 1983. Both measuring types were concentrated on a line crossing the northern part of the caldera. A GPS network was installed additionally in 1987. It covers the whole national park region and was re-measured every second year. The measurements showed, that the Caldera experienced an uplift of up to 0.8 m from 1923 to 1984. Since 1984, the trend reversed and the heights decreased up to 14 cm. The gravity values along the same line changed up to -60 ± 12 u.gal ( 1 u.gal = 10 "8 m s"2 ) from 1977 to 1986. Until 1993, the gravity increased again up to +60 ± 12 jigal. The GPS results showed within the bounds of accuracy the same height changes as the precision leveling. In addition, horizontal displacements of up to 3 cm towards the middle of the caldera were observed. Furthermore, it showed that the area of downshift expands partly considerably over the borders of the Yellowstone caldera. The factor between the height and gravity changes is a value that can be used to detect mass changes at depth. This value varies strongly from year to year. Over longer time spans, it converges towards the free-air gravity gradient of -0.3 mgal/m, where no mass changes occur. Finite element modeling shows, that the earth's crust in the Yellowstone National Park does not behave like an ideal elastic material. The horizontal displacements are significantly smaller than expected from the height changes. This is explained by the geology heavily disturbed by a lot of fault zones.

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Geografische Informationssysteme Gravimetrische Vermessungen Gravitationsfeld Krater Vermessungstechnische Aufnahmen Yellowstone Nationalpark

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