Long-term monitoring by absolute gravimetry: Tides to postglacial rebound

Abstract

Absolute gravity measurements over nearly a decade at 10 field sites in North America have provided insights on a range of phenomena from tides to postglacial rebound. In this overview we demonstrate the potential of long-term, terrestrial gravity measurements at selected sites to assist in the interpretation of temporal variations in the global gravity field from the Gravity Recovery and Climate Experiment (GRACE) mission. Absolute gravity measurements can be used to calibrate annual soil moisture effects observed by GRACE and to complement GRACE results over periods longer than the expected mission lifetime. Although the preferred terrestrial gravity monitoring system is a global network of superconducting gravimeters (Global Geodynamics Project) in combination with regular absolute gravity measurements, major advances can be made using absolute gravimetry alone. Operating the FG5 gravimeter in continuous mode at a near-shore site shows that special attention must be paid to possible sea-level related biases on gravity values as a result of near-shore hydrological effects. Where soil becomes saturated annually, simple soil moisture models can be used to remove the annual soil moisture effect from gravity, or to invert gravity observations for the soil moisture effect on space missions. Our measurements show that the phenomenon of “episodic tremor and slip” (ETS) in the Cascadia Subduction Zone is accompanied by gravity change that is most likely caused by mass redistribution and not height change. An inter-annual variation of unknown origin with a “period” of around 7 years is present at 10 North American field sites, as well as in data from Table Mountain Gravity Observatory, Boulder, Colorado. Correcting the long-term gravity trends for the inter-annual variation brings the trends into line with GPS vertical rates from four, co-located, continuous, GPS sites in the mid-continent and allows conclusions to be drawn on the thickness of the Laurentide ice-sheet.