Multi-scale Analysis Of The Gravity Field And Its Variations In The Eastern Qinghai-Tibet Plateau

Since the Cenozoic,the Qinghai-Tibet plateau is uplifting sharply during the India-Eurasian violent collision.Its crust has in north-south shorting and east-west extrusion.The eastwardly moving mass is stopped by hard blocks such as Alax,Ordos and South China block beneath the eastern Qinghai-Tibet plateau.Under the interaction,a complex structural belt with a north-south direction is formed,called the North-South structural belt.And it is also known as the “North-South Seismic Belt”because of dense distribution and high intensity of earthquakes.Therefore,studies of the crustal structure and mass movement characteristics have scientific significance to reveal the mechanism of tectonic activity and earthquake incubation beneath the eastern Qinghai-Tibet plateau.It will help improve capabilities of regional earthquake preparedness and disaster mitigation.The paper uses multi-scale wavelet analysis to decompose the gravity fields and its changes in the eastern Qinghai-Tibet Plateau,and obtains subsets of Bouguer gravity anomalies and flowing gravity changes at different scales and depths.The crustal structure,mass movement and its seismogenic environment are studied in the eastern margin by both dynamic and static perspectives.Based on the global gravity field model WGM2012 data,the Bouguer gravity data in the eastern Qinghai-Tibetan Plateau is decomposed in the 5th order by using multi-scale wavelet analysis.The results show that 1)the small-scale gravity anomaly of the 2nd and 3rd order mainly reflects the deformation information of the middle and upper crust,indicating that the strong earthquakes mainly occur in the high gravity gradient zone and the boundary of the active blockin this area.A comparative analysis of gravity anomalies at various scales reveals that seismogenic environment is not only controlled by the structure of the upper and middlecrustal fractured blocks but also related to the changes in the density of the lower crust.The lower crust at its epicenter appears as a low anomaly zone.This dynamic process of interaction between shallow and deep crust may be an important condition for earthquakes in the study area;2)the mesoscale gravity anomaly of the 4th order shows there is a low Bouguer gravity anomaly trap in the Longzhong Basin sandwiched by the Tarim Basin and the Qilian block,which is just on the Longshoushan fault and the northern edge of the Qilianshan fault.It is speculated that this local is in mass loss status.And the existence of a low Bouguer gravity anomaly trap in the Songpan-Garze block.It is consistent with the observation results of that there is a thicker,low-velocity and low-resistance layer in the crust of Bayan Har block.It may be related to the large thickness of the lithosphere,the higher temperature of the lower crustand the melting of parts of the middle and lower crust at high temperatures.In the Panzhihua area,there is a high gravity anomaly trap,which may be caused by the mass residues in the middle and lower crust by the deep high-density mass ascent during the Panxi ancient rift period;3)the large-scale gravity anomaly of the 5th order shows a regional negative anomaly in the Chuan-Dian block,which provides indirect evidence about the existence of “lower crustal flow” in the eastern Qinghai-Tibet Plateau.A four order multi-scale decomposition of the gravity field changes data was carried out in the eastern Qinghai-Tibet Plateau in 2015?2020 years.The results show that 1)The first-order wavelet detail at each interannual scale mainly reflect the surface and sediments of the eastern Qinghai-Tibet Plateau.The mass accumulation of the layer basically does not contain the geological structure information of the area;2)The second-order wavelet detail at each interannual scale mainly reflect the mass migration and density structure disturbance of the upper crust in the eastern Qinghai-Tibet Plateau,the change transition zone of the its positive and negative gravity is basically consistent with the block boundary and the strike of the active fault in the eastern Qinghai-Tibet Plateau;3)The third-order wavelet detail at various interannual scales better reflect the eastward movement of mass in the middle crust of the eastern Qinghai-Tibet Plateau and the uplift of the Longmenshan area.It can also reflect the coseismic effect of the 2017 Ms7.0 Jiuzhaigou earthquake and the mass adjustment effect after the earthquake;4)The details of the fourth-order wavelet at each interannual scale better reflect the large-scale tectonic activity information of the lower crustal in the eastern Qinghai-Tibet Plateau,and the detailed information at some interannual scales is in good agreement with the "lower crustal flow" model.It also reflects the large-scale tectonic activity information of the uplift of the Qinghai-Tibet Plateau caused by the collision of the Indian plate and the Eurasian plate.