Estimation of PGR Induced Absolute Gravity Changes at Greenland GNET Stations

ABSTRACT In 2006 a gravimetric pavilion was installed inside the Geodynamic Laboratory (LG) in Książ. The pavilion was equipped with two pillars intended to serve relative and absolute gravimetric measurements. Installation of measurement platform for absolute gravity measurements inside gravimetric pavilion of LG made it possible to perform four sessions of absolute gravity measurements: two of them in 2007 (June 10-12 and Nov. 21-22), one in 2008 (Apr. 21-22) and one in 2011 (June 19-21). In 2007 the absolute measurements were performed using two FG5 ballistic gravimeters. In April 2007 the measurements were performed by Dr Makinen from Geodetic Institute of Finnish Academy of Science with application of FG5 No. 221 absolute gravimeter. In June 2007 and in the years 2008 and 2011 such gravimetric measurements were performed by the team from Department of Geodesy and Astronomical Geodesy of Warsaw University of Technology using FG5 No. 230 absolute gravimeter. Elaboration of observation sessions from both gravimeters was performed in the Department of Higher Geodesy following the procedures used in constituting of uniform gravimetric system of geodynamic polygons reference. This constituting of gravimetric system comprised inter alia application of identical models of lithospheric tides (global model by Wenzel, 1997) and ocean tides (Schwiderski, 1980) (reduction of absolute measurements with tidal signals). Observations performed during summer of 2007, autumn of 2007, and spring of 2008 and 2011 indicated existence of small changes of absolute gravity of the order of 1 Gal. Maxima of accelerations appear in the spring period, and minima in the autumn period. This effect is connected with the influence of global hydrological factors the annual amplitude of which is ca 1,5 Gal and achieve extreme values in the spring-autumn interval. Very small value of observed amplitude of gravity changes in the period of extreme variability suggests that the observed gravity changes in LG are caused only by global phenomenon. This proves high degree of „independence” of gravimetric measurement base in LG from the local environmental factors such as ground water level variations, ground humidity, impact of snow cover, etc. At this moment the instrumental environment of absolute measurements obtains particular value, especially in the case of the tiltmeters and relative the gravimeter Lacoste& Romberg (LR-648). The relative gravity measurements as performed simultaneously with absolute gravity measurements enable us to determine the local tidal ephemeredes which makes it possible to replace the global tidal modal with ocean tidal model with the more realistic, locally determined tidal parameters (the local tidal ephemeredes).

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Measured and modelled absolute gravity changes in Greenland

Journal of Geodynamics ◽

10.1016/j.jog.2013.09.003 ◽

2014 ◽

Vol 73 ◽

pp. 53-59 ◽

Cited By ~ 4

Author(s):

J. Emil Nielsen ◽

Rene Forsberg ◽

Gabriel Strykowski

Keyword(s):

Absolute Gravity ◽

Gravity Changes

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Comparison between a six-year (2015-2020) continuous time series from an iGrav superconducting gravimeter and absolute gravity data at Mt. Etna volcano (Italy).

10.5194/egusphere-egu21-9119 ◽

2021 ◽

Author(s):

Filippo Greco ◽

Daniele Carbone ◽

Alfio Alex Messina ◽

Danilo Contrafatto

Keyword(s):

Time Series ◽

Continuous Time ◽

Gravity Data ◽

Total Error ◽

Superconducting Gravimeter ◽

Repeated Measurements ◽

Absolute Gravity ◽

Data Sets ◽

Gravity Changes ◽

Mt Etna

Since September 2014, iGrav#016 superconducting gravimeter (SG; by GWR) has recorded continuously at the Serra La Nave Astrophysical Observatory (SLN; 1730 m elevation; ~6.5 km from the Etna&#8217;s summit craters; Italy).Here we present results of a comparison between a six-year (2015-2020) time series from iGrav#16 and absolute gravity data collected through the Microg LaCoste FG5#238 absolute gravimeter (AG), in the framework of repeated measurements that were performed at the same installation site of the SG. Both AG and SG records have been corrected for the local tides, local atmospheric pressure and for the polar motion effect.The comparison allows to estimate the long-term drift of the SG, defined as the total SG trend minus the observed trend in AG measurements, which is of the order of 9 microGal/year. Once the drift effect is removed,&#160; there is a remarkably good fit between the two data sets. The differences between absolute gravity changes and corresponding relative data in the continuous time series from the SG are within 1-2 microGal (the total error on AG measurements at this station is typically +/- 3 microGal).After being corrected for the effect of instrumental drift, the time series from the SG reveals gravity changes that are due to hydrological and volcanological effects.Our study shows how the combination of repeated AG measurements and continuous gravity observations through SGs can be used to obtain a fuller and more accurate picture of the temporal characteristics of the studied processes.

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Secular gravity changes: measurements and predictions for selected Canadian sites

Canadian Journal of Earth Sciences ◽

10.1139/e91-049 ◽

1991 ◽

Vol 28 (4) ◽

pp. 557-560 ◽

Cited By ~ 9

Author(s):

A. M. Tushingham ◽

A. Lambert ◽

J. O. Liard ◽

W. R. Peltier

Keyword(s):

Geological Survey ◽

Surface Gravity ◽

Theoretical Modelling ◽

Absolute Gravity ◽

Adjustment Process ◽

Hudson Bay ◽

Gravity Changes ◽

Secular Variations

Secular variations in surface gravity (ġ) over Canada are dominated by the Late Wisconsin deglaciation signal. Theoretical modelling of this deglaciation shows that the maximum values for ġ are to be expected around Hudson Bay. Using the JILA-2 absolute gravity apparatus of the Geological Survey of Canada, it should be possible to determine ġ at three sites near Hudson Bay by about 1993 and at more sites soon afterward. Secular rates of the ongoing postglacial-adjustment process will soon be determined at inland sites where no contemporary rates have been available.

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Gravity changes before and after the 2015 Mw7.8 Nepal earthquake

10.5194/egusphere-egu2020-4304 ◽

2020 ◽

Author(s):

Weimin Xu ◽

Shi Chen ◽

Hongyan Lu

Keyword(s):

Source Region ◽

Absolute Gravity ◽

Nepal Earthquake ◽

Source Regions ◽

Gravity Changes ◽

Seismogenic Source ◽

Before And After ◽

Gravity Measurements ◽

The Absolute ◽

Regional Gravity

Based on the absolute gravity measurements of 4 gravimetric stations (Shigatse, Zhongba, Lhasa and Naqu) in southern Tibet surveyed from 2010 to 2013, we modeled the source region as a disk of 580 km in diameter by Hypocentroid model, shown that the gravity increase at these stations may be related to mass changes in the source region of the 2015 Mw7.8 Nepal earthquake. We analyzed the characteristics of gravity variations from the repeated regional gravity network, which including the 4 absolute gravimetric stations and 13 relative gravimetric stations from 2010 to 2019, to study the characteristics of gravity changes before and after the earthquake.We firstly estimated the reliability of the absolute gravity measurements by the errors of each station, and considered the effect of vertical displacement, denudation of surface mass, GIA correction and the secular and background gravity changes. Secondly we employed the Bayesian adjustment method for the relative gravimetric network data analysis, which was more robust and adaptive for solving problems caused by irregular nonlinear drift of different gravimeters, and then carried out error analysis for the repeated relative gravity measurements. Furthermore, we took the Shigatse station as example, which covered absolute and relative measurements and was most close to the Hypocenter of the inversion Hypocentroid model, the hydrologic effects of the Shigatse station was modeled exactly, and the results shown that the secular and background gravity changes were much smaller than the observed gravity changes. Lastly we studied the characteristics of gravity changes before and after the earthquake through the Hypocentroid model, we found the coincident gravity increase both in absolute and repeated regional gravity results before the earthquake, and gravity decreased after the earthquake, which suggested that the pre-earthquake gravity increase may be caused by strain and mass (fluid) transfer in broad seismogenic source regions of the earthquake. Moreover, the study indicated that high-precision ground gravity measurements (absolute and relative) may provide a useful method for monitoring mass changes in the source regions of potential large earthquakes.&#160;Acknowledgment: This research is supported by National Key R&D Program of China (Grant No.2018YFC1503806 and No.2017YFC1500503) and National Natural Science Foundation of China (Grant No.U1939205 and No.41774090).

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Gravity Changes Before and After the 2008 Mw 7.9 Wenchuan Earthquake at Pixian Absolute Gravity Station in More Than a Decade

Pure and Applied Geophysics ◽

10.1007/s00024-019-02356-4 ◽

2019 ◽

Vol 177 (1) ◽

pp. 121-133 ◽

Cited By ~ 1

Author(s):

Yong Zhang ◽

Shi Chen ◽

Lelin Xing ◽

Mian Liu ◽

Zhitang He

Keyword(s):

Wenchuan Earthquake ◽

Absolute Gravity ◽

Gravity Station ◽

Gravity Changes ◽

Before And After

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Thirty years of precise gravity measurements at Mt. Vesuvius: an approach to detect underground mass movements

Annals of Geophysics ◽

10.4401/ag-6442 ◽

2013 ◽

Vol 56 (4) ◽

Author(s):

Giovanna Berrino ◽

Vincenzo d’Errico ◽

Giuseppe Ricciardi

Keyword(s):

Ground Deformation ◽

Reference Station ◽

Absolute Gravity ◽

Volcanic Area ◽

Gravity Station ◽

Gravity Changes ◽

Gravity Measurements ◽

Elevation Changes ◽

Gravity Network

Since 1982, high precision gravity measurements have been routinely carried out on Mt. Vesuvius. The gravity network consists of selected sites most of them coinciding with, or very close to, leveling benchmarks to remove the effect of the elevation changes from gravity variations. The reference station is located in Napoli, outside the volcanic area. Since 1986, absolute gravity measurements have been periodically made on a station on Mt. Vesuvius, close to a permanent gravity station established in 1987, and at the reference in Napoli. The results of the gravity measurements since 1982 are presented and discussed. Moderate gravity changes on short-time were generally observed. On long-term significant gravity changes occurred and the overall fields displayed well defined patterns. Several periods of evolution may be recognized. Gravity changes revealed by the relative surveys have been confirmed by repeated absolute measurements, which also confirmed the long-term stability of the reference site. The gravity changes over the recognized periods appear correlated with the seismic crises and with changes of the tidal parameters obtained by continuous measurements. The absence of significant ground deformation implies masses redistribution, essentially density changes without significant volume changes, such as fluids migration at the depth of the seismic foci, i.e. at a few kilometers. The fluid migration may occur through pre-existing geological structures, as also suggested by hydrological studies, and/or through new fractures generated by seismic activity. This interpretation is supported by the analyses of the spatial gravity changes overlapping the most significant and recent seismic crises.

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