Earthquake Research in China Volume 30, Number 3, 2016 Upper Crustal Velocity Structure along the Yangtze River from Ma̓anshan to Anqing1 Tian Xiaofeng, Wang Fuyun, Liu Baofeng, Yang Zhuoxin, Zheng Chenglong, and Gao Zhanyong Geophysical Exploration Center, China Earthquake Administration, Zhengzhou 450003, China We applied the 3D first arrival travel time tomography method to the Anhui active seismic source experiment data, and obtained the imaging of the upper crust velocity structure beneath the Yangtze River from Ma̓anshan, Tongling to Anqing. Data fitting reveals the tomographic model fits the data with uncertainties, without overfitting, and with a minimum of complexity. The tomographic result shows an obvious heterogeneous upper crust which consists of a series of uplifts and depression basins. The velocity model and region imply that this region has experienced crustal uplift and extensional tectonism with concomitant magmatism since the Cenozoic. Key words: Seismic active source Airgun 3D seismic tomography Survey geometry INTRODUCTION With the unified platform of “ Yangtze River Geoscience Project” , the Anhui artificial source scientific experiment of underground structure exploration set the Anhui section of the Yangtze River as the early experimental zone to explore the subsurface structure of the Yangtze River and obtain the three⁃dimensional subsurface structure model by airgun seismic source generation in the Yangtze River waterway where permanent seismic stations and a three⁃dimensional mobile observation system were set up to receive artificial seismic wave signal within the scope of around 300km, which help to study the issues related to the regional special tectonic environment, formation mechanism of the metallogenic belt and the end effect of the giant strike slip fault2 . Anhui Province is located in the coupling zone between the North China block and the Yangtze block, which is the intersection of the Qinling⁃Dabie orogenic belt and Yangtze block. In Received on April 1, 2016; revised on June 16, 2016. This project was jointly sponsored by the National Natural Science Foundation of China (41574084) , and the Spark Program of Earthquake Sciences ( XH15059) . 2 Chen Tao, 2015. Introduction to the Anhui Artificial Seismic Source Scientific Experiment of Underground Structure Exploration Experiments for the Yangtze river, http: / / www. cea. gov. cn / publish / dizhenj / 121 / 379 / 20150917154613397737090 / index. html 1 Volume 30, Number 3 431 the foreland of the intersection between Qinling⁃Dabie orogenic belt and the Tan⁃Lu( Tancheng⁃ Lujiang) fault zone cutting the crust, there is a V⁃shaped metallogenic belt, narrow in the SW and broad in the NE, which is centered by the Yangtze River in the range of the 50km - 100km in the north and south respectively ( Chang Yinfo et al. , 1991 ) ( Fig. 1) . With the intra⁃ continental orogeny from the remote extrusion force caused by the NW low⁃angle subduction of the paleo⁃Pacific plate during the middle to late Jurassic⁃early Cretaceous and the subsequent concomitant massive volcanic⁃intrusive magmatism extension, an alternative tectonic framework of uplifts ( mineral gathering areas as Tongling, Ningzhen, Guichi ) and depressions ( mineral gathering areas as Ningwu, Luzong) was formed in the metallogenic belt of the Middle and Lower Reaches of the Yangtze River ( Chang Yinfo et al. , 1991; Lu Qingtian et al. , 2014 ) . The northwest part of the metallogenic belt is adjacent to Kongling⁃Dongling terrane in the Anqing area while its southeast is connected to the southern terrane of the Yangtze River. Therefore, the “ Poly⁃Basement with one cover” tectonic pattern of the Yangtze block, ( Chang Yinfo et al. , 1996) is embodied in the crust of this region. First⁃arrival travel time tomography is the main method to obtain high⁃resolution crustal velocity structures ( Zelt et al. , 1998; DuanYonghonget al. , 2002; Xu Zhaofan et al. , 2006; Pan Jishun et al. 2008; Jia Yupeng et al. , 2012 ) . The high⁃resolution profiling along the Yangtze River obtained from the Anhui experiment of the “ Yangtze River Geoscience Project” has realized dense observation on the upper crust of above⁃mentioned metallogenic belt and the Kongling⁃Dongling terrane in the Anqing area, which provides a good database for research on the regional crustal structure, tectonic evolution and mineralization mechanism. 1 OBSERVATION AND DATA The Anhui experiment of the “ Yangtze River Geoscience Project ” is located in Anhui Province. With the Yangtze River as the center, we laid out 9 survey lines parallel to the Yangtze River, 2 survey lines nearly perpendicular to the Yangtze River and more than 700 three⁃ component digital seismographs, including 350 PDS⁃2 digital seismographs, 350 DZS⁃1 deep digital seismograph and 50 test instruments with a sampling rate of 200 made by Chongqing Geological Instrument Factory. The observation system is shown in Fig. 1. The continuous observation is conducted in mobile observation to receive signals generated by the airgun in the Yangtze River. In order to ensure the observation quality, all seismographs are placed in pits with diameter of more than 30cm and the depth of more than 40cm, and the platforms are set up and waterproofed in part of the region. In this paper, the study object is the data produced by 20 fixed⁃point excitations observed in the survey line along the Yangtze River where there are a total of 100 instruments deployed with the average survey point distance of 2km. The weak signal is extracted by linear stacking ( Wang et al. , 2012 ) of the data generated by the 20 fixed⁃point excitations and finally 2000 stack records are obtained. A typical stack record is the 16 th fixed⁃point excitation ( Fig. 2 ) . All records are 3 - 8Hz bandpass filtered, then the first arrival Pg phase is picked up and the seismic phase picking error is obtained, which is 100ms. Pg travel time can be traced to the distance of 60km - 85km. The vertical axes are the reduced travel time with 6 km/ s and bandpass filtered between 2 -8Hz. 2 REGULARIZED TRAVEL⁃TIME IMAGING The regularization method is to understand the underdetermined part or to prevent the over fitting of data by the introduction of some constraints in solving ill⁃conditioned equations. These 432 Earthquake Research in China Fig. 1 3D survey geometry of the Anhui seismic experiment constraints are usually punitive measures on the complexity of the solution, such as the smoothness control of the imaging. Therefore, the regularization process, to a certain degree, can be regarded as the inversion of the travel time curve rather than the single⁃point travel time fitting ( Zhang et al. , 1998) . Thus, we can construct an objective function that includes the smoothness of the velocity model and the data fitting degree ( Zelt et al. , 1998) . Φ ( m ) = δ t T C d-1 δt + λ [ m T C h-1 m + s z m T C v-1 m ] (1) Where, m is model vector; δt is data residual; C d is data covariance matrix; C h and C v are horizontal and vertical smoothness matrix; λ is coordination factor of the data fitting and smoothing degree; s z is weighting factor of vertical smoothness which is coordination with the weight of horizontal smoothness. So, every linear iteration can boil down to one solution of δm, so that the objective function is the minimum ( Zelt et al. , 1998) . -1 / 2 -1 / 2 éê C d L ùú éê C d δt ùú (2) êê λ C h úúδm = êê - λ C h m0 úú ë sz λ Cv û ë - s z λ C v m0 û Where L is the partial derivative matrix of the objective function; m0 is the current model; δm is the model to be disturbed; the new model vector is m = m0 + δm. Formula (2) can be solved by the LSQR algorithm. Volume 30, Number 3 433 Fig. 2 Examples of in⁃line data for shot No. 1, No. 8, No. 16, and No. 20 in the profile along the Yangtze River Due to the introduction of the horizontal and vertical disturbance factors when applying the regularization algorithm, vertical resolution is greatly improved. With grids in different horizontal and vertical widths and the reasonable selection of a regularization factor, we could give a reasonable smooth solution on part of the no⁃ray coverage area along the profile, and then reasonably obtain the smoothest solution of the minimum grid point in the horizontal and vertical directions throughout the study area ( Zelt et al, 1998) 2 1 Selection of Initial Model For the seismic first arrival travel time imaging, a reasonable and objective initial model should be the one dimensional model where the longitudinal velocity gradient distribution is consistent with the crustal velocity gradient in the study area ( Zelt et al. , 1998) . In this paper, the best one dimensional average model is set as the initial model obtained from multiple imaging method ( Zelt et al. , 2003) , and the initial model is shown in Fig. 3( a) . Fig. 3( b) is the travel⁃ time contrast of the initial model in calculation and in observation. In Fig. 3( b) , the travel⁃time curve calculated from the initial model is in the mid observed travel times, so the model is relatively reasonable. 434 Earthquake Research in China Fig. 3 ( a) 1⁃D initial models used in the minimum⁃structure approach, ( b) Complete set of first arrival picks reduced at 6 0km / s versus source⁃receiver offset without regard to position along the profile 2 2 Model Parameterization and Selection of Forward and Inversion Parameters The 2 nd order finite difference algorithm is used to solve the wave⁃front algorithm of the eikonal equation ( Hole et al. , 1995; Zelt et al. , 1998) . The 0 25km × 0 25km grid is adopted in the forward model of the study area, and finally the number of grids for the forward model is 1081 in the horizontal direction and 41in the vertical direction, with a total of 44,321 nodes. 1 00km × 0 25km grid is used in the inversion model, namely, grid point spacing is 1000m in horizontal direction and 250m in the vertical direction, forming 270 horizontal grids and 40 Volume 30, Number 3 435 vertical grids, with a total of 10800 inversion grids. Horizontal and vertical smoothing weight factor s z takes the value of 0 15. 2 3 Data Fitting Error and Stability Analysis The root mean square error of initial model ray tracing is 198ms, and the χ2 value is 6 13. The reduction rate of damping factor is 100 for every 10 iterations, and the final root mean square error is 80ms and χ2 value decreases to 1 after 58 iterations. The travel⁃time residual distributions of initial and final models are shown in Fig. 4, from which the travel time fitting error range of the initial model is ± 0 6 and that of the final model constraints is within ± 0 1s, in accordance with Fig. 4 Traveltime residuals as a function of frequency for ( a) preferred initial model and ( b) preferred final model from regularized inversion Earthquake Research in China 436 seismic phase picking error χ2 value of 1. This inverse fitting data is neither overfitting nor lacking. The final model ray cover is shown in Fig. 5, from which we can see a total of 628 rays that form a better coverage of the upper crust in the study area. It indicates that the imaging results are robust and reliable. 3 IMAGING RESULTS AND DISCUSSION Fig. 6 shows the upper crustal velocity structure model of Tongling⁃Anqing⁃Ma̓anshan, where the crystalline upper crustal structure has an obvious horizontal difference, which fits the zoned characteristics. In the Anqing area, west of the Lujiang⁃Zongyang basin, the depth of basement is deep, about 4km - 6km; the upper crust presents a typical alternating tectonic characteristic of two uplifts and two depressions. The measuring point is located on the northern shore of the Yangtze River, with thicker sedimentary cover and a surface velocity of 3 5km / s where the surface is manifested as a large⁃area alluvial plain. Fig. 5 Ray coverage through the final model The buried depth of the crystalline basement in the Lujiang⁃Zongyang basin is about 4km - 5km and its sedimentary cover shows an obvious morphology of depression basin, which indicates the extension⁃depression process since the Mesozoic. The surface velocity is about 3 5 - 4 0km / s, while there are obvious low⁃velocity anomalies at about 3km depth beneath the edge of this basin which may be related to volcanic⁃magmatic activities since the Mesozoic, suggesting that the basement depression is characterized by volcanic basin features. Combined with the deep seismic reflection and MT imaging results carried out in Guichi area, the low⁃speed medium ( at the depth of 3km) in the basin may correspond to the channels of magmatic activity in the region ( Dong Shuwen et al. , 2010) . The measurement points to the east of Tongling ( across the Yangtze River) are located on the southern bank of the Yangtze River. The near⁃surface velocity varies significantly in the horizontal direction. After crossing the Yangtze River, the surface velocity soars to about 4 5km / s and the sedimentary cover significantly uplifts with the basement depth being about 2 - 4km. From the east of Tongling to Wuhu, the uplifting characteristics of the sedimentary cover correspond to the complex reflection in the upper crust showed by the deep seismic reflection in this region ( Lu Qingtian et al. , 2002 ) , which could be inferred as fold, thrust and intrusive structures, indicating that strong compressive deformation has occurred in the upper crust. Volume 30, Number 3 437 Fig. 6 Upper crustal velocity model for profile along the Yangtze River obtained by first⁃arrival traveltime tomography ACKNOWLEDGEMENTS We are grateful to Professor Zhang Xiankang for the instructive suggestions on dealing with the experimental data and to Professor Wang Baoshan and Professor Yao Huajian for their support and help in airgun data processing. REFERENCES Chang Yinfo, Dong Shuwen, Huang Dezhi. On tectonics of “ Poly⁃Basement with One Cover” in Middle⁃Lower Yangtze Craton China [ J] . Volcanology & Mineral Resources, 1996, 17( I) :1 - 15( in Chinese with English abstract) . Chang Yinfo, Liu Xiangpei, Wu Yanchang. Copper and Iron Metallogenic Belt in the Middle and Lower Reaches of the Yangtze River [ M] Beijing: Geological Publishing House, 1997 1 - 239( in Chinese) Dong Shuwen, Xiang Huaishun, Gao Rui et al. Deep structure and ore formation within Lujiang⁃Zongyang volcanic ore concentrated area in Middle to Lower Reaches of Yangtze River [ J] . Acta Petrologica Sinica, 2010, 26(9) :2529 - 2542( in Chinese with English abstract) . Dong Shuwen, Xiang Huaishun, Gao Rui et al. Deep structure and ore formation within Lujiang⁃Zongyang volcanic ore concentrated area in middle to lower reaches of Yangtze River [ J] . Acta Petrologica Sinica, 2010, 26(9) :2529 - 2542( in Chinese with English abstract) . Duan Yonghong, Zhang Xiankang, Fang Shengming. Three⁃dimensional finite⁃difference tomography of velocity structure of the upper crustal in North China [ J] Chinese Journal of Geophysics, 2002, 45(3) :362 - 369( in Chinese with English abstract) . Hole J. A. , Zelt B. C. 3⁃D finite⁃difference reflection travel times [ J] . Geophys J Int, 1995, 121: 427 - 434. Jia Yupeng, Wang Fuyun, Tian Xiaofeng et al. Tomography of high resolution seismic refraction traveltimes in Tianjin⁃Beijing Profile [ J] . Northwestern Seismological Journal, 2012, 34(4) :375 - 382 34(4) :375 - 382 ( in Chinese with English abstract) . 438 Earthquake Research in China Lv Qingtian, Dong Shuwen, Shi Danian et al. Lithosphere architecture and geodynamic model of middle and lower reaches of Yangtze Metallogenic Belt: A review from SinoProbe [ J] . Acta Petrologica Sinica, 2014, 30(4) : 889 - 906( in Chinese with English abstract) . Lv Qingtian, Huang Dongding, Hou Zengqian et al. Deep seismic reflection image of crustal structure in Tongling Ore District [ J] . Mineral Deposits, 2002, ( Suppl. ) :1173 - 1176( in Chinese with English abstract) . Pan Jishun, Zhang Xiankang, Zhao Ping et al. 2D multiscale non⁃linear seismic velocity imaging [ J] . Chinese Journal of Geophysics, 2008, 51(1) :197 - 205( in Chinese with English abstract) . Wang B. , Ge H. , Wei Y. , et al. Transmitting seismic station monitors fault zone at depth [ J] . Eos Transactions American Geophysical Union, 2012, 93(5) :49 - 50. Xu Chaofan, Zhang Xiankang, Zhang Jianshi et al. Ray hit analysis method and its application to complex upper crustal structure survey [ J] . Acta Seismologica Sinica, 2006, 19(2) :173 - 182 ( in Chinese with English abstract) . Zelt C. A. , Barton P. J. 3D seismic refraction tomography: A comparison of two methods applied to data from the Faeroe Basin [ J] . J Geophys Res, 1998, 103: 7187 - 7210. Zelt C. A. , Sain K. , Naumenko J. V. , et al. Assessment of crustal velocity models using seismic refraction and reflection tomography [ J] . Geophys J Int, 2003, 153:609 - 626. Zhang J. , Toksoz M. N. Nonlinear refraction traveltime tomography [ J] . Geophysics, 1998, 63 ( 5 ) : 1726 - 1737. About the Author Tian Xiaofeng, born in 1979, is a senior engineer at the Geophysical Exploration Center, China Earthquake Administrator. His major is the study of seismology, travel time imaging, and crustal lithosphere evolution. E⁃mail:tiandler@ hotmail. com
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