Structure Of The Mantle Transition Zone And Neighbouring Regions Beneath East China Using Triplicated Phases

East China is consisted of Northeast China,North China Craton and the South China block.Influenced by the subduction of the west Pacific and the Philippine Sea plates,the structure of the Mantle Transition Zone(MTZ)and its adjacent areas had a complex change.The 410 km discontinuity is the upper boundary of the mantle transition zone and the 660 km discontinuity lies underneath the mantle transition zone,which lateral changes are frequently studied by many scientists these years.The character of the travel time curves and the amplitude of the triplicated waves are sensitive to the depth of discontinuity and its adjacent structure.Based on the waves received from China Digital Seismograph Network,seismic triplication method can be an efficient instrument to study the structure of the Mantle Transition Zone and its adjacent areas.This paper uses two mid-deep earthquakes of 4 June 2008 at Hokkaido of Japan and 7 August 2010 at the northeastern Ryukyu islands recorded by the China Digital Seismograph Network.Using P wave seismic triplication,we learn the structure of the Mantle Transition Zone and its adjacent areas of lower eastern Yangtze Craton,the southern Huabei Basin,northeastern North China Craton and the Korean Peninsula.By the way,we also analysis the anomaly of the temperature in the Mantle Transition Zone.To better illustrate how velocity structure above and below the discontinuity affects the theoretical traveltime curves and waveforms,we first show synthetic seismograms from a series of models modified by ourselves based on iasp91 model.We regard them as constraints on final P wave velocity models.In order to study the influence caused by the lateral inhomogeneity of the velocity structure more vividly,we divided our study areas into 5 subregion according to different azimuth.Next we take mesures of preprocess towards original waveforms.Using waveforms received by stations which distance range from 10°-30°,we take the method of trial and error to find the structure of the Mantle Transition Zone(MTZ)and its adjacent areas underneath East China.Our results indicate that around a 130 km-thick low velocity zone exists in the 80-210 km upper mantle of the lower eastern Yangtze Craton and the southern Huabei Basin.The decrease of the P-wave velocity is about-2%.There also exists a 140-150 km-thick low velocity zone in this areas.A high-velocity layer is revealed above the 660 km discontinuity and it show a increasing trend from the South to the North.The velocity anomaly is extremely high at the depth of 560-610 km,which reachs 1.67%-3.4%.We also find that a 180 km-thick low velocity zone exists in the 80-260 km upper mantle of the northeastern North China Craton and the Korean Peninsula.The decrease of the P-wave velocity is about-1%.The high velocity anomaly reaches at 3.67%-4.1% in these areas.So it is obviously higher than that of the lower eastern Yangtze Craton and the southern Huabei Basin.A low velocity anomaly nearly at 1% is found below the 660 km discontinuity under the the northeastern North China Craton.And a depression of the 660 km discontinuity is found under the Korean Peninsula,where we also can see a 2% high velocity anomaly below the 660 km discontinuity.To sum up,the low velocity layer is generally discovered atop the 410 km discontinuity in our study areas.High velocity layer is found generally in the Mantle Transition Zone.The 660 km discontinuity of the Huabei basin is a gradient zone.