Study Of Moho Changes Beneath The Northeastern Tibetan Plateau Based On Satellite Gravity Data

The Northeastern Tibetan Plateau(NETP)is the front part of inland expansion of the Tibetan Plateau from the India-Asia collision and accompanied with rapid deformation(both earth surface and interior).Uplift mechanism of the NETP has attracted significant attention from numerous scholars.The depth and variation of the Moho interface can provide an important reference for study the growth and expansion of the NETP.This study constrains the Moho variations beneath the Northeast Tibetan Plateau(NETP)by using multiple geodetic data and climate models.The hydrological and tectonic gravitational signals are indetified from multisource data,and these signals are removed from the satellite gravity observations.At last,the uplift mechanism of the NETP is discussed based on the results obtained and the seismological results,and the main conslusions are as follows:Taking the 2013 Mw8.3 deep-source earthquake as an example,this study tests the capablity of GRACE satellites to monitor gravity signals generated by deep crustmantle materials.This earthquake is the first deep-source earthquake with the source depth approximately 600 km,since the GRACE satellites were launched.The coseismic gravity changes caused by the 2013 Okhotsk Mw8.3 are extracted by difference method and time series least square fitting based on th GRACE Level-02 dataset.This earthquake is the first deep source earthquake with Mw>8.0 since the launch of GRACE satellites,and the source depth is more than 600 km.Both the gravity changes extracted by the two methods show obvious “positive-nagetive”patterns on both sides of the fault.The gravity signal extracted by the difference method is-1.1～0.4 μGal,and that extracted by the least square method is approximately-0.8～0.3 μGal.The spherical dislocation theory is applied to simulate the coseismic gravity changes caused by this earthquake to verify the reliability of the signal extraction,and the simulated gravity changes is approximately-1.1～0.6 μGal.The simulated results are generally consistent with the stacking results both in magnitude and spatial distribution.The GRACE’s detection ability to deep source earthquakes is verified,and the stacking method is recommended when extracteing the coseismic gravity signals.The coseismic gravity signals caued by vertical displacement and density changes at source are separated based on the GRACE observations and the spherical dislocation theory.The the mechanism of coseismic gravity changes are discussed,and the results show that gravity changes caused by vertical displacement are-0.8～0.3 μGal,and that caused by density changes at source are-0.2～0.2 μGal,respectively.The gravity signal induced by vertical displacement is approximately four times as much as that caused by density changes at source,while the predecessors suggested that the gravity signal generated by the vertical displacement of the surface is about 10 times of that generated by the density changes at source.That is because the half-space erath model was used and the influence of the earth’s layered structure and curvature were not considered.This study verifyed the necessity of layered spherical earth model to simulated the seismic deformation by a deep earthquake.The gravity signals in the NETP are analyzed based on the multi-geodetic data/models.The gravity effects caused by tectonic activities of the earth surface and hydrological factors,and the Moho’s vertical variation is simulated.The GRACE observation shows that material is accumulating in the NETP with the rate 9.0±0.1mm/yr(Equivalent Water Height,EWH).The EWH changing rate caused by terrestrial water storage is 0.9±0.2 mm/yr,and those induced by mass changes of glaciers and lakes are-1.3±0.1 mm/yr and 2.5±0.02 mm/yr,respectively.The EWH variation generated by the uplift of the earth surface is also simulated,with the rate at 5.1±0.5mm/yr.At last,the EWH changing rate is obrained by removing other sginals from the GRACE observations,and the result shows that the EWH changing rate caused by the variation of Moho interface is approximately 1.6 mm/yr,which is equal to the Moho interface is upliftting at a rate 2.46±0.92 mm/yr.The crustal uplift rate is evaluated based on the GPS and Leveling data,and the uplift rate is 1.92±0.2 mm/yr.The uplift rate of crust is generally consistent with that of Moho interface,which reveals that both the earth surface and moho interface is uplift,and the thickness of crust beneath the NETP is not thickening.This result suggests that the crustal uplift recently in NETP is maily controled by isostatic uplift rather than crustal shortened and thickened.In addition,the rate of moho uplift is slightly larger than that of crustal uplift.Combined with the seismological results,it is considered that there is partial melting in mid-lower crust beneath the NETP.