Study For1-D Velocity Structure And3-D Finite-frequency Full-wave Tomography

The basin and range province is two primary tectonic unit in the of plate tectonics, and play important role in the lithospheric structure and its evolution. In this paper, two different basin and range system at northwest and north China, one is collision area between east part of the north Tianshan Mountains and Junggar Basin (TSM-JGB), the other is the North China metropolitan area, where to improve1-D crustal velocity model and conducte full-wave tomograply on full3-D finite-frequency theory, respectively, will help us further understand the interaction between adjacent plates in the basin and range system, and the relationship between tectonic structure and seismic activity.The widely used three-dimensional seismic tomography is a direct and effective means to study the crustal velocity structure in the world. The precise inversion data base, advanced inversion theory and technology are necessary guarantee to improve the reliability of the inversion results. The data base as the initial reference velocity model and source parameters has an strong impact for seismic tomography. Inappropriate initial velocity structure could affect determining of the source parameters inaccurately, and also bring unreal results for tomography. Therefore, to improve1-D crustal velocity model for TSM-JGB region, considering coupling of the initial velocity structure and source parameters, we selected1370P-wave and1396S-wave travel time data of172high quality local earthquake recorded Urumqi Telemetry Network and10temprary broadband seismic stations, apply the’minimum1-D model’approach to carry out jointly inversion of the initial velocity structure and hypocenter. Particularly, we prepared several reference models and seismic events datebase to avoid’local minima’problem during inversion, and eventually achieved optimized the regional1-D crustal velocity model The results show that the one-dimensional P-wave and S-wave velocity model we build as well as the stations correction well represent the general geological characteristics for regional structure of the area. Compared with previous model, our result indicated relative low velocity at shallow depth range (0-10km), middle crust (about30km) and lower crust (about45km), respectively. Additional, we revealed several transition layers with thickness about12km below lower crust (about48km) which may imply complexity of uppermost mantle, which is consistent with deep seismic sounding measurements by the converted wave detection and seismic imaging profiles across the Tianshan orogenic belt. Then we relocated2370earthquakes occurred from1988to2005on this improve1-D crustal model. The most relocated output781epicenters are distinctly aligned south of the surface trace of the Manas-Tugulu anticline and foldbelts of north Bogda piedmont, and distributed at depth from5to25km with the average depth16.8km. The average horizontal and vertical error outputted by method were reduced greatly, which suggests that we improve the accuracy of hypocenter determination by this new model.Properly reflecting the geological structure and the improvement of precise location suggest that we have established the regional one-dimensional crustal velocity model is reliable, which lay the foundation for future3D crustal seismic tomography with sufficient data and other related seismology research, provide evidence for evolution of basin and range tectonics and exploration of oil and gas resources.Additionally,3-D seismic tomography in traditional ray theory has the following limitations:extremely high frequency approximation, the ray path migration caused by low velocity zone, wave front healing problem, imaging resolution cannot beyond λL (width of the first Fresnel zone), waveform information invalidity, etc. Recent developments in the full-wave finite-frequency theory reveal more information for real physical characteristics of the seismic wave and its propagation in the earth, which could extract more structure information from seismic waves, thus improve resolution of the earth interior medium. However, we could not further conduct3-D finite-frequency inversion in the TSM-JGB area with limitations of inadequate date. Comparing with the TSM-JGB region, there is relatively richer database and previous structural study in the North China metropolitan area.We calculated50Strain Green Tensors (SGTs) on3-D velocity model for broadband seismic station with the full-wave finite-frequency theory on cluster computing sever; then achieved calculated seismograms on previous3-D velocity structure through3-D staggered-grid finite-difference method; and measured the delay travel time by cross-correlation between the observed and calculated seismic waveforms; and then we computed the3-D Frechet sensitive kernel for delay travel time by SGTs with lateral velocity variation, which avoid the error caused by FFT transform in time-frequency domain, and improve the efficiency of the numerical simulation by the hypocenter-station reciprocity principle; Due to the high quality requirements for the seismic waveform data recorded for nearly a decade in the metropolitan area, however, after careful selection we could not afford sufficient measurements to gain higher resolution than before. As a result, finally, we conducted full3-D waveform tomography with1°×1°horizontal resolution, and produced good resolution for northeast study area above15km depth. After LSQR iteration, the total RMS travel time residual of measurement dropped to0.710sec from0.756sec after inversion.The results show that the P-wave velocity structure presents significant lateral heterogeneity in the mid-upper crust, the image in upper crust show good coherence with surface geology and lithology, like low-velocity anomalies beneath depression or mountain basin area, high-velocity anomalies beneath uplift mountains or exposed bedrock area. And distinct low-velocity anomalies exist beneath some board area of regional block and Bohai region, especial under Zhangjiakou-Bohai depression belt zone, which is similar to preview research. Under the combined effects of the extrusion stress produced by the Pacific Plate subduction from west to east and the maximum principal stress in the North China region, the low-velocity zone below Bohai Basin at5km depth spread to north by the east side of Yanshan Mountains, and to west by the south side of the Yanshan Mountains, respectively, when colliding with the hard uplift Yanshan Mountains tectonic block. And the shallow transition zone between Yanshan Mountains from and the North China basin indicate low velocity, not as previous studies with high-velocity distribution, where distribute a large number of fault zones,and gradually appear distinct high-speed anomaly from south to the north until near the range of the Beijing Miyun region (latitude40.5°).Although inadequate data limit image resolution, the advantages of3-D full-wave finite-frequency theory remains on the resolution improvements and the details of velocity structure. This is the first application in China of3-D finite-frequency tomography method for crustal velocity structure, which build the finite-frequency sensitive kernel based on the3-D initial velocity model to conduct full wave tomography. This pilot study for North China metropolitan area in this paper provide a meaningful practice, which is befit to utilize and develope of the promising theory.