Upper Crustal Velocity Structure along the Yangtze River from

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
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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
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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.
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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.
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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
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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
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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.
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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.
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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