Long-term Change And Its Interpretation Of Absolute Gravity In Taiyuan Station

The temporal and spatial changes of the earth's gravitational field reflect the distribution and movement of matter in the surface and interior of the earth.Many research results show that the earth's gravitational field changes with time mainly including tidal changes and non-tidal changes,while the former presents periodicity,which is the main component of the earth's gravitational field change,and can reach 300 ?Gal,which can be accurately calculated based on the model.Since the non-tidal changes of the earth's gravitational field can be divided into periodic or quasi-periodic,long-term,and short-term,as well as sudden changes,there have been different opinions on how to accurately and carefully study non-tidal changes.Since the 1970 s,the measurement accuracy of gravity measuring instruments has been gradually improved.In particular,the accuracy of the FG5 absolute gravimeter can reach 1 to 2 ?Gal at a stable station,making earthquake disasters,volcanic incubation and outbursts,and sea level fluctuations.Non-tidal changes in the earth's gravitational field,such as changes,vertical deformation of the crust,groundwater level changes,and glacial equilibrium adjustment,have been fully observed,and non-tidal changes in the earth's gravitational field have been confirmed.The dynamic mechanism can be attributed to vertical crust Deformation and structural factors caused by the underground material migration caused by.The Taiyuan basin is located in the middle of the Shanxi rift system of China.The results of tectonic studies indicate that the Moho surface below the Taiyuan Basin is uplifted by 2-3 km.However,there is no quantitative evidence showing whether the Moho in the lower part of the fault zone is continuously increasing.In this paper,the retest data from the high-precision absolute gravimeter for the period from 2009 to 2016 in Taiyuan,Shanxi,were obtained.The preliminary calculation results show that the gravity change rate shows a very significant positive trend change,exceeding 10 ?Gal/yr.Therefore,we estimated the mantle upwelling rate of Taiyuan basin by the absolute gravimetric observation data,combined with GPS,groundwater level changes,GLDAS hydrological model and GRACE satellite gravity data,then the gravity effect caused by tectonic factors caused by the underground material migration is obtained,in order to establish a crust-and-mantle boundary deformation model.1)Use g9 software to process the absolute gravity observation data,including correction of solid tides,correction of polar shifts,correction of ocean tide loads,correction of atmospheric pressure,and correction of baseline height.2)The global terrestrial water storage load effect is calculated based on the GLDAS hydrological model divided into two parts: local and non-local.We select four layers of soil water of GDLAS from 2009 to 2016,combined with SRTM elevation data,and select appropriate boundary thresholds to correct local and non-local hydrological load effect.The global land load change rate is-0.37±0.03?Gal/ yr,which is same as the results with GRACE satellite gravity results.3)Analyze the groundwater level of the BM3 well at the same site as the absolute gravity station.The change rate in groundwater level is 1.82 ± 0.03m/yr.The porosity obtained from the properties of the borehole rock is 10% of the classic porosity,and the gravity change rate caused by the change in the groundwater level is 7.64 ± 0.33 ?Gal/yr.Taking into account the vertical deformation effect of the crust,the estimated porosity is compared with the classical porosity.4)Based on GPS observation data from 2011 to 2015,using gipsy software to perform precise point positioning calculations,we can conclude that the vertical displacement rate of the crust is-8.5±0.59 mm/yr,which indicates the land subsidence,and then the gravity generated by vertical deformation of the crust is 1.62 ± 0.11 ?Gal/yr.Under the assumption that there is no underground material transport,the absolute gravity change rate after the elimination of global terrestrial water and groundwater effects is estimated to be-14.52±1.2 mm/yr,which is a large difference from the GPS solution result,authenticating the existence of underground material migration produced by tectonic movements.After a series of corrections to the absolute gravity observation data,the rate of change of residual gravity is 0.79±0.63 ?Gal/yr.According to the CRUST1 model,the crust-and-mantle boundary deformation model of the Taiyuan Basin can be established,and the mantle uplift rate is 2.3±2.7 ?Gal/yr.Then,the uplift mechanism of the mantle in the Taiyuan Basin and the boundary deformation model of the crust and mantle are established.Combined with geology,seismology,geodesy and other results,it shows that the current Taiyuan Basin is characterized by land subsidence,thinning of the crust and expansion on both sides,and the continuous increase of the Moho surface.