| The mainland of China and its adjacent areas are located in the southeast of Eurasian plate,and are surrounded by India,Pacific and Philippines Sea plates.The interaction between these plates makes continental China one of the most complex tectonic settings and the most active tectonics on the Earth.Thus,it is of great significance to determine high-resolution lithospheric structure and accurate earthquake locations for continental China to have a better knowledge of its internal structure,to understand the generation mechanisms of strong earthquakes,and to study continental geodynamics.Based on previous studies,this dissertation has conducted seismic imaging on Vp,Vs and Vp/Vs of lithosphere of continental China,and relocated earthquakes recorded by permanent seismic stations operated by China Earthquake Administration.The results obtained from this study provide a reference model for further understanding the seismogenic environment in continental China and the crust and upper mantle structure.The main research contents include three aspects:(1)Determining the 3-D P wave and S wave velocity structure of the lithosphere in the continental ChinaUsing the data of seismic arrival times recorded during the two years from 01/2013 to 01/2015 by the Digital Seismic Network of Chinese mainland,we have constructed a high-resolution lithospheric three-dimensional(3-D)Vp and Vs models(USTClithol.0)with a horizontal resolution up to 0.50 for continental China.Based on the multi-grid inversion strategy,the regional-scale double-difference(DD)seismic tomography method was adopted.Overall,compared with the existing lithospheric velocity models under continental China,the new models have a relatively high resolution and better characterize the finer 3-D velocity structure of the lithosphere.For the inverted velocity models,a variety of methods were used for evaluating the model resolution and uncertainty.Firstly,the checkerboard resolution test shows that the Vp and Vs models can achieve a higher resolution of 1° in the horizontal direction for most parts of continental China,and even achieve a resolution of 0.5° in most parts of eastern China.In addition,the distribution of seismic ray densities shows that the continental China has denser ray coverage except for the surrounding areas.Secondly,by comparing the observed travel times of the active sources not used for inversion,the Vp and Vs models are further verified.Furthermore,the inverted Vp and Vs models can also be used to fit the dispersion data of Rayleigh surface wave.Finally,our inverted Vp model is consistent with the results of deep seismic sounding survey and the inverted Vs model is also consistent with Vs model of Shen et al.(2016).(2)Determining the Vp/Vs structure of the lithosphere pf continental ChinaUsing the method of directly inverting Vp/Vs(Fang et al.,2019),and the same earthquake dataset used for constructing the USTCLitho1.0 model,a three-dimensional Vp/Vs model with a horizontal spacing of 20 of the continental China lithosphere has been determined.Based on the checkerboard resolution testing results,the model has relatively good resolution in the depth range of 5-100 km.The Vp/Vs model shows that there are obviously high Vp/Vs anomalies in the shallow crust of the Songliao Basin,north China Basin,Sichuan Basin;and the Junggar Basin,but the Tarim Basin and the Qaidam Basin are associated with low Vp/Vs anomalies in the shallow crust.The further analysis suggests that the above basins have different sedimentary ages and different lithologies.The shallow crust beneath the Tibetan Plateau is associated with low Vp/Vs values.There are clear high Vp/Vs anomalies in the middle-lower crust beneath both the middle-northern part of the Qiangtang block and the southeastern part of the Songpan-Garz block,indicating the existence of high temperature,partial melting.The new Vp/Vs model reveals high Vp/Vs anomalies in the middle and lower crust of eastern China,which are approximately consistent with those of low velocity and high conductivity layers in the same region.From the comprehensive analysis of previous results,it is suggested that the main cause of high Vp/Vs anomalies could be due to the fluids released by the dehydration of various water-bearing minerals in the middle and lower crust,but it is also possible that partial melting in some regions could also be the reason.The upper mantle beneath the central and eastern North China Craton and the South China block shows high Vp/Vs anomalies,indicating the existence of hot and soft asthenospheric materials.In addition,the results also show that there are local high Vp/Vs anomalies in the middle crust as well as upper mantle beneath Datong,Tengchong,and Changbaishan volcanoes,suggesting that there could be magma upwelling beneath these volcanoes related to the subduction of the surrounding plates.The average crustal Vp/Vs of continental China above 40 km is close to 1.73(Poisson's ratio ?=0.249),which is much lower than the global average of 1.78(?=0.27).This indicates that the mafic crust is generally deficient in the lowest crust of the continental China.(3)Systematically relocating earthquakes in continental ChinaBased on the 3-D Vp and Vs models of USTClithol.0,91,583 seismic events were relocated for the period of 01/2013 to 12/2016 that were recorded by the Digital Seismic Network of Mainland China using the double-difference seismic tomography method.Compared with the CEA catalog locations,the artifacts that many earthquakes are located along the same depth are greatly reduced.Overall,the earthquakes are deeper in the western part of continental China and shallower in the eastern part,The average depth of the earthquakes with Ml?2.0 is(11.2 ± 6.6)km,which is slightly greater than the initial average depth of(9.3±5.4)km.For different blocks,the average earthquake depth in the Western Block is(13.6±8.2)km and is the deepest,and the average depth in the South China Block is the shallowest and is(7.7±3.8)km.The Longmenshan seismic zone in the Sichuan province was selected for further analysis.It shows that the earthquakes are mainly distributed along the Longmenshan fault zone in the NE direction with a width of about 20-40km,and the earthquakes was mainly distributed in the upper crust of 0-20km.According to the distribution characteristics of the earthquakes,the related faults are delineated,which indicates that the Longmenshan tectonic belt has been pushed eastward by the deep crustal materials of the Tibetan Plateau since the Cenozoic,and is in the thrusting state.In order to verify the reliability of the location results,11 strong earthquakes with Ml?6 are selected for comparison with the existing results.The comparison shows that they are consistent with the previous location results,except for the Ms6.2 Qinghai Zaduo earthquake on October 17,2016.In addition,27 earthquakes in the lower crust of North China Basin are also consistent with the existing location results of Dong et al.(2018).Our relocation results indicate that there are earthquakes with a focal depth below 30km,mostly distributed to the west of the North-South gravity gradient zone.They are mostly concentrated in the three areas,the Tianshan seismic belt,the western margin of the Tarim block and the North South seismic belt.Some deeper earthquakes are also sporadically distributed in the Southern Himalaya block and the Lhasa block,and the Northeast Xingmeng orogenic belt.We select the South-North Seismic belt for further analysis.It is found that the deeper earthquakes are associated with Vp/Vs ratio of 1.68-1.82,of which 84%is greater than 1.73 and 6%is greater than 1.80.We have compared 54 "deep earthquakes" in the North-South seismic belt with the relocation catalogue released by China Seismic Experimental Site.These comparisons show that the relocated results from this study are relatively accurate.The causes of the deeper earthquakes in the lower crust are analyzed.It is speculated that there are two possible mechanisms.For the deeper earthquakes in the Tianshan and southern Tibetan Plateau areas in the western part of continental China,the main cause could be due to the presence of an anhydrous granulite-facies metamorphic assemblage in the lower crust,which can maintain metastable and mechanically strong.For the central and eastern parts of continental China,the deep earthquakes in the lower crust could be due to the existence of high temperature fluids in the lower crust. |
PDF Full Text Request