China Northeast West Pacific Subduction Zone Volcanic Region Crust And Upper Mantle Structure

The Western Pacific subduction zone in the northeast part of China is located at boundary of the Eurasia and the Western Pacific plate. The Western Pacific plate subducted beneath the northeast part of China with dip angle of 29° at Japan trench. The subduction action of the Western Pacific plate is main source of tectonics and Huichun deep earthquakes in the northeast part of China, There are many Quaternary volcanos in the studied area, such as Tianchi volcano, Wudalianchi volcano, Jinbohu volcano and Longgan volcano and so on. Research results revealed that Changbaishan Tianchi volcano is a most dangerous volcano that has potential possibility to erupt. The subducted zone is one of the oldest active subduction zone and good place for studying on the effections of subducted activity to Eurasia tectonics, relation between the subduction plate and mantal discontinuities, the subduction plate weather pierced the 660km discontinuity. So it is signification to study deep structure, volcano activity and focus of deep earthquake. The main work of this dissertation is as follow:Based on understanding teleseismic waveform utilization, history of receiver function development, extract receiver function, receiver function inversion and stack, debug the receiver function extract and inversion routine. Similar as Dueker(1997) common conversion point stack method, debug the receiver function stack routine.We first cut the waveform data from Changbaishan array (cooperative project between the State University of New York at Binghamton and the Research Center of Exploration Geophysics) that consist of 19 seismic stations recorded during 1998-1999 and Jinbohu array consist of 16 stations. Then pre-processed the waveform data with filtering and adding head. We obtained 53 teleseismic waveform records in Changbaishan array and 18 teleseismic waveform records in Jinbohu array. In all 243 receiver functions were obtained using these teleseismic waveform data. For each station, select the receiver functions with high signal-to-noise ratio, high quality waveform to be stacked. Finally, 34 radial components and tangential components of receiver functions that describe the response of medium beneath 34 stations were obtained. Inversing these radical receiver functions we obtained 34 1-D S-wave models beneath the stations. We think the depth of S-wave velocity 4.3km/s should be the Moho interface after analyzing the velocity structure beneath each station and comparing with results from wide-angle reflected/refraction sounding in Changbaishan volcanic area. To understand the character of velocity structure beneath the studied area intuitively, we got 2-D S-wave velocity structure and distribution of Moho depth by interpolation using 34 1-D velocity models that were gotten from receiver function inversion. From these results we can see that S-wave velocity contour is in the northeastern direction, this is agreement with geological structure trend on surface in studied area. Moho depth is between 33-36km beneath the most of studied area but beneath volcano crater Moho depth can reach to 39km. The S-wave velocity structures are different between volcanic area and non-volcanic area. We think that magma uplift is the main reason causing these differences. Receiverfunction inversion results are agreement with the results from wide-angle reflected/refraction sounding.We first filter 243 receiver functions using Butteworth filter, then calculated the conversion point positions using IASP91 velocity model at the stack depths. Stacking the conversion points within common the circle bin with 100km radius at the stack depth, we obtained structures of the AA' ,BB' ,CC' ,DD' four stack profiles respectively. From stack results we can see that Moho, 410km discontinuity and 660km discontinuity are obvious in the stack profiles. 520km discontinuity also can be seen in some profiles but it not always can be traced in all profiles. 410km discontinuity is obvious uplift beneath Changbaishan volcano area and its undulate is positive correlation with 660km discontinuity. The thickness of transition zone is about 250km approaching the average thickness of the globe's. From phases existed in the transition zone, we can deuce that the front part of the western Pacific plate may have been split into several blocks in transition zone. These blocks have different moving directions in the transition zone. Huichun deep earthquake zone is just located the most front part of the subduced plate and focus of the deep earthquakes may have relation with these blocks. 660km discontinuity is consisted of multi-interface and has characters of complicated discontinuity. From undulate of 660km discontinuity we can see that although the subduced plate may do not pierced through 660km discontinuity, but it has made great effects on 660km discontinuity.Seismic tomography is method for studying velocity structure. In recent twenty years it has great development. Tomography has been a powerful tool for studying inner structure of the Earth with digital seismogram widely used. When we have enough seismic data and reasonable ray distribution, for example some dense seismic station distribution area in Japan, tomographic resolution can be reach to kilometer scale. This dissertation reconstruct the velocity structure beneath studied area using DLSQR method developed by Prof. Dapeng Zhao. Epicenter and earthquake time are been corrected before next iteration using new velocity model and Geiger method. There are three data sets were used in the inversion. The first data set is 133 earthquake travel-times recorded by Jilin seismic network with magnitude >ML2.0 from 1982-1998. The second data set is 12 local earthquake first arrival travel-times recorded by Jingbohu seismic array. The third is Pg data from Changbaishan wide-angle reflected/refraction sounding. Checkboard was used to check resolution of different network using the three data sets. The checkboard results showed that the inversion resolution is about 0.4° using the three data sets. According checkboard results we divided model into 14×19×10. The space of network is 0.3° at Changbaishan and Jingbohu volcanic area and 0.3° ~0.5° at the other area. We also test the damp value used in inversion, test results showed that when damp is 5 model error and travel-time rms reach to balance. P-wave travel-time rms from 0.64s decreased to 0.55s after 42 iterative inversions. S-wave travel-time rms from 0.95 decreased to 0.81 after 52 iterative inversions. We obtained P-wave and S-wave velocity structure at depths of 2km, 5km, 8km, 12km, 17km, 22km and 34km respectively. From these results we can see that lower velocity anomaly at depth of 2-5km is distribution in northeast direction and agreement with geological structure...