Analysis of active faults based on natural earthquakes in Central north China

J. Vis. Commun. Image R. 65 (2019) 102612

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Analysis of active faults based on natural earthquakes in Central north China q Junjie Zhou a,b,⇑,1, Guowei Zhu a, Qingchao Zhang a, Zhenqiang Yang a, Pengfei Sun b, Jiahao Liu a a b

China University of Mining & Technology, Beijing, Beijing 100083, China Hebei University of Engineering, Handan, Hebei 056038, China

a r t i c l e

i n f o

Article history: Received 3 June 2019 Revised 19 August 2019 Accepted 20 August 2019 Available online 21 August 2019 Keywords: Active fault zone Earthquake distribution Taihang mountain front fault Velocity inversion

q

a b s t r a c t As an important part of the integration of Beijing-Tianjin-Hebei, it is very important to analyze the seismic activity of active structures in Central north China. There are two sets of active faults belt in the lot, and there have been devastating earthquakes, which need to grasp the level of seismic activity. Located at the boundary of the third-order tectonic unit, there are a series of faults in the area, such as the north to the east Taihang mountain front fault, the north to the east Xinhe fault and the north to the west Cixiandaming fault, which intersect and cut each other to form fault depression basin. There are different scales of NE, NNE, and NW faults, which are considered to be the birthplace of the earthquake. At the same time, more than 6 magnitude earthquake magnitude have happened in the Cixian and Xingtai. The seismogenic structure of the research shows that these earthquakes associated with deep fault activities, the source location in the deep crust velocity structure mutation. In order to determine and analyze the P-wave velocity structure characteristics and the hypocenter distribution, and the activity characteristics of the deep space of active fault belt, the natural seismic data monitored by seismic network are collected and organized, which are used to analyze the relationship between seismic wave velocity and hypocenter position. Due to the deep migration of the crustal material and the horizontal principal compressive the NEE direction stress in North China, the crustal thickness on the west side of the Taihang mountain front fault is greater than that of the east side, from 1 km to 7 km. Along the trend, the epicenter of the small earthquake is mainly distributed in the crustal thickening area on the west side of this active fault, and the epicenter of the eastern plain is less distributed. The depth of the small earthquake is concentrated in the range of 8–20 Km. Comprehensive analysis shows that the seismic p-wave velocity structure characteristics can be divided into the sedimentary cover, upper crust, the earth’s crust and the lower crust structure, thickness of different location have change, the thickness of the sedimentary cover Taihang uplift zone thickness 0.1–3 km, to 5–7 km in Handan fault depression; The thickness of the crystalline basement in the Taihang mountain uplift is 3–5 km, and the Handan fault depression basin is thickened to 7–10 km. The thickness of the crust on the west side of Taihang mountain front fault is significantly greater than that on the east side. The thickness of the crust on the west side is decreased from 36– 40 km on the west side to 30–35 km on the east side and about 7–10 km on the east side. Due to the near east-west tension, the zone has disengaging movement, forming the characteristics of shovel-type normal fault combination. In the earth’s crust with high-speed and low-speed layer between configuration characteristics, seismic horizon of earthquake preparation 12–18 km deep in the earth’s crust, characterized by low speed and high speed layer mutation position, concentrated distribution of small earthquakes, the seismogenic layer a concentration distribution in the crust velocity structure conversion section. Seismic activity is concentrated in the west end of the Cixian-daming fault and the west side of the Xinhe fault, with an average depth of 12–18 km. Ó 2019 Published by Elsevier Inc.

This paper has been recommended for acceptance by ‘Luming Zhang’.

1. Introduction

⇑ Corresponding author at: China University of Mining & Technology, Beijing, Beijing 100083, China. E-mail address: [email protected] (J. Zhou). 1 He is mainly engaged in teaching and research on geological structure as well as the processing and interpretation of geophysical data. https://doi.org/10.1016/j.jvcir.2019.102612 1047-3203/Ó 2019 Published by Elsevier Inc.

Earthquake always happen in fractures of active structure as well as elastic medium. A large number of earthquake indicate that active fault not only is the root of earthquake, but also cause great

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damage to fault zone, and injury is also obviously more than areas on both sides of the fault. The exploration and study of the continental lithosphere structure and structure is the way for earth science to solve scientific problems in earthquake prediction. There are two distributions of north-north east and north-west to faults in this area. The Central north China is located on two first-order tectonic units. There are a series of fracture structures, such as Taihang fracture in north-south, Xinhe fracture in north-east, Cixiandaming fracture in east-west and Yongnian fracture et al. (Fig. 1). The Taihang mountain front fault is a boundary fault between the Taihang mountain uplift area and the North China Plain fault

area, and it is also an important tectonic belt in the North China and eastern China [7,8]. This fault rupture from north to south by the Huangzhuang – Gaoliying fault, Xushui fault, Baoding – Shijiazhuang fault, Handan fault, Tangdong fault and Tangxi fault and so on, more than 10 fractures. The Taihang Fault Zone is an important tectonic fault belt in northern and eastern China. This fault zone is not only a topographic boundary, but also an important boundary in the regional tectonic and geophysical fields. Some research supports that this fault belt is deep and large [1]; other work suggests it is an active fault and earthquake belt [1,12,15,17]. The Taihang Mountain piedmont fault is a large-

Fig. 1. The Regional tectonic zoning and seismic distribution. F1 – Taihang mountain front fault; F2 – Cixian-Daming fault; F3 – Linzhang fault; F4 – Xinhe fault; F5 – Anyang south fault; F6 – Longyao fault.

J. Zhou et al. / J. Vis. Commun. Image R. 65 (2019) 102612

scale structure zone in north and east China which cross Beijing, with the NE-NNE extent spans approximately 620 km. It is very important to determine the fault zone activity due to the close relation of active structures and earthquakes. According to statistics, about 95% of earthquakes more than 6magnitude distributes fractures and tectonic basin formed in Cenozoic or active fractures on upheaval or hollow. The more intense the magnitude, the more obvious the relationship is. There are two active tectonic zones in the south of Hebei. An earthquake of 7.5-magnitude happened in Cixian on June 12th, 1830, which is caused by Cixian fracture in east-west. In March 1966, two earthquakes in Xingtai were related to the activities of the Xinhe fault [4,15,17,19]. We can only accurately know the fault positions beneath urban areas and appraise the possibility of the fault having an earthquake. Through the study of destructive earthquakes in the past, it is shown that there are activities in this section, which need to grasp the temporal and spatial distribution characteristics of seismic activity and the level of future earthquake. Although there is much significant information on the upper crustal mantle of northern China from many deep seismic surveys carried out in northern China, good seismic data of the North China Craton lithospheric structure is lacking because of short survey lines and inadequate explosion energy. The geological evolution, the composition and thermal condition of the crust, and the movement and deformation of the plate can lead to a non-uniform distribution of crustal velocity; the structure of crustal velocity thus records information on the long-term tectonic evolution. Regarding the Taihang Mountain piedmont fault activity, there are three different opinions: (1) it is a large deep fault zone; (2) it is an active fault zone and an earthquake structure belt; and (3) it is not an earthquake structure belt. According to the seismic data of the seismic network near the fault, the characteristics of the deep structure of those faults are analyzed by means of the joint wave velocity and the location of the seismic source, and the relationship between the small earthquakes and the fractured space structure. Statistics of National Seismic Network indicate that earthquakes of different magnitude happened around fractures. Method of velocity of longitudinal wave of earthquake and method of Joint inversion of earthquake focus are employed to deduce the structure and focal shock parameter of different stratum, thus to analyze variation of velocity of seismic wave and depth of seismic focus of different locations and different depths. According to which, the regulation of activity of fracture and distribution of earthquake in space and time can be studied. Thus the regularity of how the earthquake focus change with time can be obtained and the characteristics of fractures around Hebei province can be predicted. 2. Seismic tomography methods This method is mainly to add the source item in the seismic tomography process, and to determine the three-dimensional velocity structure and the seismic source parameters [2]. In theory and application, the main steps are: (1) the formulation of the problem (establishing the relationship between model and data); (2) Model parameterization; (3) To calculate the positive problem (ray tracing); (4) The reliability evaluation of the solution. 2.1. Methods From the location I to the earthquake station j, the time ray can be expressed as:

Z

earthquakestation

T ij ¼

uds location

ð1Þ

3

where m is the slowness (the reciprocal of the velocity), ds is the element of the length of the path, The actual observation of seismic waves can be expressed as.

tij ¼ si þ T ij

ð2Þ

Among them, si is the earthquake’s earthquake moment, the only known quantity in a certain sense, because the location and observation of the earthquake have unknown factors in some aspects. Earthquake source coordinates, starting time and ray path and slowness field is unknown (model parameters), according to the type (1); The initial source location and velocity model (a priori factors) theory as the T, can be calculated the seismic wave travel time residual according to a set of observations travel time by measuring t obs ij and formula (2). The difference between observed travel time and theoretical calculation travel time can be expressed as residual. cal rij ¼ t obs ij þ t ij

ð3Þ

In addition, the traveltime residual can also be approximately linearly represented by source parameters and velocity perturbations.

rij ¼

Z earthquakestation 3 X @T ij Dxk þ Dsi þ du du @xk location k¼1

ð4Þ

In the formula (4), the first term on the right is the source disturbance and the third item is the speed disturbance. It should be noted that the unknown source parameters are still a constraint in this equation. The partial derivative of the source parameters is proportional to the slowness of the source point, that is

@T ij 1 dxk ¼ ð Þ @xk v ds source

ð5Þ

Disperse the speed disturbance term in Eq. (4), then formula (2)–(4) can be written as

rij ¼

3 L X X @T ij @T ij Dxk þ Dsi þ Dmi @x @mi k i¼1 k¼1

ð6Þ

where mi represents the L point velocity parameters on this path. The partial derivative @T ij =@mi of the velocity model is the line integral between the velocity nodes on the given travel time reference. For multiple observations, Eq. (6) can be written as a linear inversion equation for the joint inversion of the seismic source location and velocity structure:

R ¼ HDh þ MDm

ð7Þ

where r is the traveltime residual, H and 4h are the partial derivative coefficients and perturbation of the source vector, and M and 4m are the velocity parameter partial derivative vector coefficient matrix and the velocity perturbation vector matrix, respectively. 2.2. Implementation First of all, we must parameterize the crustal structure model in the study area, mainly block method and grid method. Using approximate ray tracing and pseudo- bending joint inversion method, by connecting two points of the source and the receiving station, selecting arcs with different radii of curvature as rays, and superimposing the angles with different incident planes to obtain an initial ray path; then, Snell’s law is applied and the ray is perturbed according to the principle of minimizing the travel time of each ray along the path to obtain the final ray path.

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2.3. Calculation effectiveness evaluation The evaluation of the inversion solution is mainly the resolution analysis of the solution, which is measured by the diagonal matrix of the resolution matrix [16,18,20]. Generally, the numerical analysis method is used to perform the resolution analysis. In the joint inversion method, the resolution can be analyzed by the number of rays in the grid, the resolution diagonal elements (RDE), the total number of partial weights (DWS), and the spread function. The resolution diagonal element shows the independence of the solution of a model parameter. DWS is used to quantitatively analyze the relative ray density that affects each node. The distance of the ray distance model node is used to measure the importance of the length of the ray. The previous person gave the lowest DWS value of about 50, and the resolution matrix above 0.4 can reflect the results of this area is relatively reliable. 3. The joint inversion 3.1. Data All travel time data of earthquake wave are taken from observation reports of seismic earthquakes in the city of Handan and its surrounding areas. The study area ranges from east longitude 113.5° to 115°, and north latitude 36° to 37.5°. Based on 8 seismic network in southern hebei region from 1992 to 2010 of the 2396 original seismic data (magnitude greater than 1.0) records, using seismic velocity structure and focal position of the joint inversion method [2,11,13] conducted a joint inversion of the velocity and position of the seismic longitudinal wave, and determined the characteristics of the seismic longitudinal wave velocity structure around the Handan fault zone. Using seismic events, require three or more stations recorded seismic phase and the source parameters (longitude, latitude and depth) of the earthquake, there are 796, overall seismic events belong to I observation precision class, selected to participate in the inversion of P wave observation data, 6798 (Fig. 2). 3.2. Grid division and speed model establishment Based on the research results of the crustal velocity structure in this area and its adjacent areas [1,14], a reference velocity structure model was established. Using the grid method, a grid of 0.25°  0.25° is adopted, and the velocity value in the longitudinal depth is changed (Table 1). In this model, the velocity structure is represented by continuous function, and velocity at any point in the grid can be calculated by the method of interpolation.

Fig. 2. Travel time of P-wave involved in inversion vs. epicentral distance.

Table 1 The original P-wave velocity of the crust in the study area. Depth/km P-wave Velocity/kms1

0 4.8

5 5.6

10 6.2

15 6.35

20 6.5

33 6.8

60 8.0

In the process of inversion of location and velocity of earthquake, the time difference is caused by disturb of earthquake parameters and velocity, which can be showed as the following equation:

dt ¼ Dt þ

N X @t @t @t @t Dx þ Dy þ Dz þ Dv @x @y @z @ vn n n¼1

ð8Þ

where Dt, Dx, Dy, Dz and Dv n is disturb of the original time of earthquake, longitude, latitude, depth, and velocity respectively. N is the sum of velocity parameters. As to l earthquakes and j stations, the Eq. (8) can be rewritten as the following form:

dt ¼ Adv þ Bdx

ð9Þ

where dt is travel time vector of m dimension, dv is velocity disturbance of panel point of n dimensions, dx is parameter disturbance vector of earthquake focus of 4 dimensions, A is the deviation derivative matrix of travel time to velocity, which is m  n dimensions, B is the deviation derivative matrix of travel time to earthquake focus parameters, which is m  4 dimensions. Employing the rectangular projection operators, and resolve Eq. (9) into the following two formulas to solve velocity parameters and earthquake focus parameters respectively:

ðI  P B ÞAdv ¼ ðI  P B Þdt

ð10Þ

Bdx ¼ PB ðdt  Adv Þ

ð11Þ

According to Eqs. (10) and (11), determine velocity parameters first, and then determine earthquake focus parameters, thus eliminate the accuracy of location affected by uncertainty of velocity structure. 3.3. The validity of inversion results This inversion mainly uses the seismic wave velocity inversion seismic velocity tomography imaging method, and uses the velocity structure and the source position to jointly invert. The travel time residual error uses the orthogonal projection operator proposed by Liu Futian, adopts a stepwise iterative inversion method, and To reduce the instability of the solution, the parameter adjustment after each iteration of inversion is controlled within 10% of the model parameters [11,13,3,18,20]. The least squares method is used to determine the velocity structure of the network coverage area and the location of seismic events. After 5 iterations of inversion, the root mean square residual (RMS) of P-wave travel time decreases from 0.877 s before inversion to 0.372 s after inversion, and the positioning error is 0.098 km in EW direction, which is in the NS direction. The average is 0.096 km, with an average of 0.042 km in the vertical direction. In order to improve the positioning accuracy and reduce the inversion error, the diagonal elements of the resolution matrix are used to illustrate the reliability of the solution. In the depth of 0 km, the value of the diagonal element is basically about 0.5, and only a few regions can reach 0.6 and above, which is the average resolution effect. At the depth of 10 km, the resolution matrix of most regions is greater than 0.9, and the value of the edge region can reach about 0.7. At the depth of 20 km, the Angle element value of the middle eastern resolution matrix in the research area is about 0.9, while the value of the diagonal element in the west is around 0.5. At the depth of 30 km, only the diagonal element value

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of the resolution matrix in the central region of the research area can reach about 0.7, while in other regions, it is basically around 0.3 (Fig. 3). 4. Results 4.1. The distribution of epicenter before and after inversion The epicenter distribution before and after the inversion shows that the epicenter of the repositioning is clearly concentrated in the active fault zone (Fig. 4). The location of the earthquake is mainly concentrated on both sides of the active fault zone. And mainly earthquake distributed in the broken under the plate. The epicenter distribution before and after the inversion shows that the epicenter of the repositioning is clearly concentrated in the active fault zone (Fig. 4). The location of the earthquake is mainly concentrated on both sides of the active fault zone. And mainly earthquake distributed in the broken under the plate. Earthquake focuses of small earthquakes in the studied area are mainly distributed in the upwelling area of Taihang mountain western of fracture in the front of the mountain, while there are few earthquakes in the plain. Epicenters mainly distribute as two

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belts: one distribute in the direction of NE is Xinhe fault (F4); the other is the Cixian-daming fault (F2) in EW direction, and the fracture in the front of Taihang divide Cixian into two sections, the west and the east section, and minor shock happens frequently in the west section, which indicates that the west section is still active, while there are few minor earthquakes in the east section. Furthermore, few minor shocks happen in the front of the Taihang mountain, and there is barely minor shock along the fracture from Handan to Cixian, which indicates that this section is at peace; there are minor shocks in the east of south section (south of Cixian-Daming fracture). 4.2. The distribution of P-wave velocity in different depth The deep structure of the study area is mainly from the north China plain seismic zone, and the main fracture forming mechanism is the tension, which forms the crustal uplift and subsidence of different scales. Employing calculation method mentioned above, distribution parameters of earthquake are obtained, and distribution of small earthquakes located is shown in Fig. 4. Fig. 5 shows that the velocity of the P-wave is significantly different from the east and west sides of the Taihang mountain front

Fig. 3. The Contour map of diagonal element isogram in different depths (a-0 km; b-10 km; c-20 km; d-30 km).

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Fig. 4. The earthquakes distribution map before (a) and after (b). F1 – Taihang mountain front fault; F2 – Cixian—Daming fault; F3 – Linzhang fault; F4 – Xinhe fault; F5 – Anyang south fault.

Fig. 5. Velocity change in the study area in difference deep (a) 5 km; (b) 10 km; (c) 15 m; (d) 20 km.

fault, and the velocity structure in different depth ranges is obvious. In the depth of 5 km (Fig. 5a), the faulted western plate is a high-speed layer, which is characterized by the ancient metamorphic rocks, quartz sandstone and quartzite and other metamorphic rocks in the Taihang mountain uplift area, which are high velocity media, (Fig. 5b), the longitudinal wave velocity of the West plate in Taihang mountain front fault is higher than that in the east plate, but the velocity difference between the two is smaller. The velocity of the longitudinal wave is higher than that of the east plate. The vertical wave velocity in the depth of 15 km (Fig. 5c)  20 km (Fig. 5d) is basically the same on both sides of the east and west of Handan fault, and the difference is weak, which indicates that the lithology of the upper and lower faults is not very different. It is worth noting that the depth of 20 km and below, pitch wave velocity is lower than that of the east plate, which indicates that

the crustal thickness of the western plate (Taihang mountain uplift area) is obviously thickened, and the crustal thickness of the broken east plate (upper plate) is thinner. 4.3. P-wave velocity and source profile near the active fault To analyze characteristics of minor shocks on the main fracture zone, 2 vertical sections are made along or perpendicular to the main fault (Fig. 4b). For hade in this area is comparatively large, just earthquakes of which the depth of focus less than 10 km of the vertical sections are project to the section, and earthquakes on the section are mainly considered caused by faulting. In Fig. 6 (AA0 and BB0 are mainly perpendicular to Taihang fracture and the supposed fault strike, nearly parallel to Cixian-daming fault), 2 sections are numbered A-A0 , B-B0 (Fig. 6).

J. Zhou et al. / J. Vis. Commun. Image R. 65 (2019) 102612

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Fig. 6. The seismic P-wave velocity with main active faults (a: A-A0 ), and. (b: B-B0 ). F1 – Taihang mountain front fault; F3 – Linzhangfault.

Fig. 6 shows that the small earthquakes in the Taihang Mountain uplift area on the west side of the Handan fault are significantly stronger than those of the piedmont plains on the eastern side of the fault. The small earthquakes in the southern section of the Handan fault are significantly stronger than those in the middle In the vertical direction, the Handan fault is gradually extended from the shallow layer to the deep layer, and the small earthquake is mainly concentrated in the lower plate of the Handan fault, the distribution of the small fault on the fault is small, and the development depth of the small earthquake is 0–35 Km within, but mainly concentrated in the range of 5–15 Km, accounting for 68% of the overall. The analysis of the P-wave velocity structure in the strike direction of the Cixian-daming earthquake Fig. 6 B-B0 profile shows that the thickness of the crust on the south side of the Cixian-daming fault is smaller than that on the north side, which is basically consistent with the depth of the two sides of the hypocenter. It is inferred that the Taihang Mountain is in Cixian. The south side of the fault rises and the thickness of the crust thickens. In the uplift area of the Taihang Mountains, the P-wave velocity of the Taihang Mountains shows a gradual increase towards a certain depth in the ground. At the depth of 10–18 km, there is a zone with a decrease in the P-wave velocity, and this zone is a small-shock-centralized distribution zone. The analysis indicates that the fault zone is currently at a comparatively low level. Analyze the small earthquakes in Cixian County, the epicenter of the Taihangshan uplift that is concentrated on the west side of the fault is thickened in the crust thickness. In the plain area, there are few earthquakes and is mainly concentrated near the fault zone. The focal depth is concentrated in the range of 12–18 km. The low velocity body is the evolution process of the lower lithosphere thinning and asthenosphere material upwelling, heating the top mantle and the crust and causing the crust thinning. Comprehensive analysis of the seismic Pwave velocity structure section shows that there are obvious differences in the crust structure between the east and west sides of the Taihang mountain piedmont fault. The crystalline basement surface is characterized by ‘‘uplift in the uplift zone and depression in the fault depression zone”, and the thickness of the uplift in the sediment cover decreases. The characteristics of thicker fault zones

are thicker; the structure of the crust below the uplift zone is complex, and there is a phenomenon that the P-wave velocity is uneven at the depth of 12–18 km, and there is a low-velocity body. The activity field of the small earthquake occurs here. The existence of the crust structure is considered Horizontally changing lots. 5. Discussion The variation of the longitudinal wave velocity of the seismic wave on both sides of the Handan fault shows that the thickness of the sedimentary caprock on the west side of the fault (the Taihang Mountain uplift) is thinner and the thickness of the sediments in the east (plain) is thickened, The forces of the west and the crustal material are squeezed upward and downward at the same time, resulting in the uplift of the Taihang Mountains and the thickening of the crust, which is in line with the direction of the tensile stress in the eastward direction [4], indicating that the Handan fault Mainly developed in large-scale detachment fracture system [9]. The longitudinal wave velocity and crustal thickness of the seismic wave on both sides of the Handan fault are obviously changed, which is the characteristic of the deep fault [6]. 5.1. Force source analysis The changes of the P-wave velocity in the profile reflect the changes of the rock mass structure and matter at different depths. In order to analyze the characteristics of the extension of the deep tectonic space in the Handan fault, the velocity profile of the seismic wave in the 36.6° azimuth is obtained near the vertical direction of Handan fault. Fig. 6 shows that the velocity of the P-wave is rapidly changing in the Taihang mountain front fault zone, and the western plate (lower plate) high-speed area is the Taihang mountain uplift area. The low velocity area is the Quaternary and Tertiary caprocks, Thickness of 2 km, while the eastern side of the cap layer thickness increased to about 8 km. In the vertical direction, the Handan fault is located in the transitional area of the deep crust structure, which

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is the transitional area of the Taihang Mountain uplift high velocity layer material and plain low speed medium. Through the change of the velocity structure, Handan fault is inclined to the east, with the inclination of 70° from the shallow layer to 60° in the middle layer and in the range of 5–10 km, the inclination angle is changed from 50° to 40°. 5.2. Seismogenic depth According to calculation results, depths of earthquake focus of main tectonic earthquake are statistically analyzed as Figs. 7 and 8. In the studied area, depths of earthquake focus are from 6 km to 10 km, the peak value is about 10 km. 18% earthquakes with depths of focus are 10 km, 66% earthquakes with depths of focus are from 6 to 10 km, and 93% earthquakes with depths of focus are from 4 to 22 km. Distribution of depths of earthquake focus indicates that earthquakes in the studied area focus on some advantage layers, and 2 km, 22 km can be considered as upper and lower bounds of depth of earthquake focus respectively. The focal depth of the small earthquakes within the scope of the study ranges from 0 km to 40 km, the depth distribution peaks at about 10 km, the 6–10 km depth earthquakes account for 66% of the total, and the 5–22 km depth earthquakes account for 93% of

Fig. 7. The distribution of earthquake depth since 1992–2010.

the total (Fig. 8). The depth distribution of these earthquakes indicates that the earthquake rupture in the study area is concentrated in a certain advantageous stratum, and 12 km and 18 km can be used as the focal depth of the superior strata. This is consistent with Zhao Yan-lai et al. [10], who studied earthquakes in the Beijing-Tianjin-Tangshan area with focal depths ranging from 5 to 25 km, with dominant depths ranging from 8 to 15 km, suggesting that the seismogenic horizons in southern Hebei are slightly deeper than in the northeast and north of Hebei province. Comprehensive analysis of the crustal structure of southern Hebei province, from shallow to deep is composed of five speed variation layer, above the top of the crystal base and sedimentary cover, and the average seismic p-wave velocity in the 3.1 km/s, the Cenozoic and Mesozoic and palaeozoic era and has the ancient boundary strata. The thickness of the Taihang uplift zone thickness is thinner, the average in 3–5 km, and in the west and the east uplift area are characterized by thickening of the thickness of the sedimentary cover, thickening of 7–9 km, characterized by Handan graben position, in the east of graben sedimentary cover thickness thinning, a 5–7 km. Crystalline basement level by the end of the middle layer interface is two speed is higher, the top speed of 5.80–6.20 km/s, its top position in Handan graben recessed deep about 8 km, east and west on both sides of the raised area becomes shallow is about 5 km; However, the base interface is relatively gentle, and the average depth is 13–15 km, and it deepens slightly downward to the eastern sub concave. The lower velocity is 6.2–6.5 km/s, the bottom interface is 17–25 km deep, and the lower part of the east sub concave, the interface is raised up. The interface of the middle layer to the Moho surface has three velocity layers. The most outstanding one is a low speed layer with a speed of 5.2 km/s, which is 3– 4.5 km thick. On top of the low speed layer is the shell layer of 6.5–7.0 km/s, which is about 5 km thick. Below the low-velocity layer, the velocity is 7.0 km/s, the thickness is 5–7 km, and the depth of Moho is 32–36 km. The small earthquakes with magnitude greater than 2.0 in this area are frequent, and the large earthquakes are mainly concentrated near the Xinhe fault and the Cixian-Daming fault. For the seismogenic structure of the 7.2-magnitude earthquake in March 1966, Xu Jie et al. [5] pointed out the strong Seismic activity and fractures and faulted basins in the shallow crust may not be considered as a simple correspondence relationship. The seismogenic fault is not a single gentle dipping fault main fracture or a high-angle fracture below it, but a combination of both. The high-angle fracture is the main part of the seismogenic fault. The repositioning of this small earthquake fully illustrates this point. Since the Pliocene in southern Hebei, the activity of the Taihang Mountain piedmont fault has been weakened overall since the Pliocene. The early period is characterized by strong normal fault activity, controlling the deposition of the Pliocene and the Quaternary, and the property of the late normal fault. The weakening shows a right-lateral strike-slip nature; in space, the main activities of the Taihang Mountain piedmont fault are reflected in the northern and southern sections, and the middle section is relatively weak, especially in the middle north of the middle section, and basically stops in the Quaternary.

6. Conclusion

Fig. 8. Distribution frequency of focal depth since 1992–2010.

Through a network observation report near the active fault structure in southern Hebei, the tomographic imaging method was used to reposition small earthquakes, and a comprehensive analysis was made of the fault structure and spatial distribution characteristics of small earthquakes in the southern section.

J. Zhou et al. / J. Vis. Commun. Image R. 65 (2019) 102612

(1) The small earthquakes are concentrated in the distribution of the Xinhe fault in the northeast and the Cixian-Daming fault in the northwest part of the west, showing the characteristics of strip distribution. In the active fault structure, the small earthquakes on the active fault structure are concentrated and the distribution strips are narrow, which indicates that the fracture surface has a steep slope, and the epicenter of the earthquake is concentrated on both sides of the fracture surface. Small earthquakes on both sides of the Cixian-Daming fault and the Xinhe fault zone are concentrated, and the earthquakes in the Taihang Mountain uplift area are more intense than the small earthquakes in the North China Plain area, and the small earthquakes are concentrated in the 8–18 km below the surface. (2) For the collected seismic data of magnitudes greater than 2.0 from 8 earthquake observation network stations in southern Hebei Province from 1992 to 2010, the joint inversion method of P-wave velocity and depth was used to analyze the velocity structure of seismic P-waves to understand and master the deep geological structure of the Tanlu fault belt. The characteristics of the change; the upward and downward simultaneous compression forces on the west side of the Taihang Mountain piedmont fault are deformed to form the fault, which is characterized by thicker crust thickness on the Taihangshan uplift zone on the west side of the fault and thinning of the crust thickness on the east side of the fault subsidence zone. The thickness of the cover on the west side of the fault is 3–5 km, while the thickness of the cover on the east side suddenly increases to about 7– 9 km. (3) The crustal structures in the southern part of Hebei Province and northern Zhangbei region of Hebei Province and the eastern parts of Beijing-Tianjin-Tangshan region are different, and there are earthquake-prone layers with different depths. The seismogenic layers in the southern part of Hebei are mainly distributed in 12–18 km, ie the middle crust, which is more than that of other regions. Deep seismogenic horizons provide important constraints for the determination of the thickness of the seismogenic zone of the southern Hebei crust, the lower boundary of the active land mass, the clarification of the cause and mechanism of the earthquake, and the seismic hazard analysis. (4) The southern crust of Hebei Province is composed of 5 velocity variability layers: 1 Sedimentary caprock: P-wave velocity is 3.1 km/s and consists of the Cenozoic, Mesozoic, Paleozoic, and Upper Proterozoic strata. The average thickness of the Taihang Mountain uplift is 3–5 km, and the thickness of the fault depression is 7–9 km. In the east of the mantle, the Luxi uplift has a thickness of 5–7 km. There are obvious differences in the crustal structures on the east and west sides of the Taihang Mountain piedmont fault. The crystalline basement shows the characteristics of ‘‘uplift in the uplift zone and depression in the fault depression zone”. The depth of the fault depression zone is about 8 km, and the uplift zones on the east and west sides It becomes about 5 km. The middle crust is a high-speed layer with a depth of 12–18 km and the low-velocity layer is interphased. The lower layer velocity can reach 6.2–6.5 km/s. The most prominent is a low-velocity layer with a velocity of 5.2 km/s. 4.5 km, with a depth of 17–25 km. 4 The lower crust is a high-speed layer of 7.0–8.0 km/s, with a thickness of 7–10 km and a Moho depth of 36–42 km.

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Declaration of Competing Interest There is no conflict of interest. Acknowledgements This research was partially supported by the National Key R&D Program of China (Grant No. 2018YFC0807801), the China Geological Survey Bureau (Grant No. DD20160267), the National Science Foundation of China (Grant No. 41604069). Thanks to peer review experts valuable advice and suggestions! References [1] Jin-li Huang, Da-peng Zhao, Three-dimensional P wave velocity structure of the crust and deep structure environment of the Strong Earthquakes in Capital Region, Chin. Sci. Bull. 50 (4) (2005) 348–355. [2] Fu-tian Liu, Simultaneous inversion of earthquake hypocenters and velocity structure (I) – theory and method, Geophysics 27 (2) (1984) 167–175 (in Chinese). [3] Mingliang Xu, Fuhai Chen, Lu Li, Chen Shen, Pei Lv, Bing Zhou, Rongrong Ji, Bio-inspired deep attribute learning towards facial aesthetic prediction, IEEE Trans. Affective Comput. (2018), https://doi.org/10.1109/TAFFC.2018. 2868651. [4] Jing Wu, Yuan Gao, Yu-tao Shi, Seismic anisotropy in crustin southwestern Capital Circle, China, Earthquake 28 (2) (2008) 115–122 (in Chinese). [5] Jie Xu, ZHong-jing Fang, Li-hua Yang, Tectonic background and causative fault of 1966 Xingtai Ms7.2 earthquake, Seismol. Geol. 10 (4) (1988) 51–59 (in Chinese). [6] Jie Xu, Zhan-wu Gao, Hang-qing Song, The structural characters of the piedmont fault zone of TAIHANG mountain, Seismol. Geol. 22 (2) (2000) 111–122 (in Chinese). [7] Xi-wei Xu, Wei-min Wu, Xian-kang Zhang, et al., in: The Latest Crustal Deformation and Earthquakes in Beijing Area, Science Press, Beijing, 2002, pp. 150–159 (in Chinese). [8] Xiao-ping Yang, Bao-jin Liu, Yan Zhan, et al., Survey of crustal structure and fault activity around southern Shijiazhuang in the eastern margins of Taihangshan Mts, Chinese J. Geophys. 59 (2) (2016) 528–542 (in Chinese). [9] Xue-min Zhang, Gui-ling Diao, Ying-ping Zhao, et al., Study on mantle shearwave velocity structures in North China, Chinese J. Geophys. 49 (6) (2006) 1709–1719 (in Chinese). [10] Yan-lai Zhao, Rruo-mei Sun, Shi-rong Mei, The change of seismic parameters in Bohai area, Earthquake China 3 (1993) 129–137 (in Chinese). [11] Long-quan Zhou, Fu-tian Liu, Xiao-fei Chen, Simultaneous tomography of 3-D velocity structure and interface, Geophysics 49 (4) (2006) 1062–1067 (in Chinese). [12] Junwei Han, Ji Xiang, Hu Xintao, Zhu Dajiang, Li Kaiming, Jiang Xi, Cui Guangbin, Guo Lei, Liu Tianming, Representing and retrieving video shots in human-centric brain imaging space, IEEE Trans. Image Process. 22 (7) (2013) 2723–2736. [13] Long-quan Zhou, Fu-tian Liu, Jin-song Liu et al., Determination of the crustal velocity model of Dongsha islands using the inversion of s-p wave field, Prog. Geophys. 20(2), 503–506 (in Chinese). [14] ZHi-ping Zhu, Xian-kang Zhang, Yu-jie Gai, et al., Study on the slow velocity structure of the Earth’s crust in Xingtai areas, Acta Seismol. Sin. 17 (3) (2006) 328–334 (in Chinese). [15] Junwei Han, Dingwen Zhang, Gong Cheng, Lei Guo, Jinchang Ren, Object detection in optical remote sensing images based on weakly supervised learning and high-level feature learning, IEEE Trans. Geosci. Remote Sens. 53 (6) (2015) 3325–3337. [16] Mingliang Xu, Jiejie Zhu, Pei Lv, Bing Zhou, Marshall Tappen, Rongrong Ji, Learning-based shadow recognition and removal from monochromatic natural images, IEEE Trans. Image Process. 26 (12) (2017) 5811–5824. [17] Junwei Han, Dingwen Zhang, Xintao Hu, Lei Guo, Jinchang Ren, Feng Wu, Background prior-based salient object detection via deep reconstruction residual, IEEE Trans. Circuits Syst. Video Technol. 25 (8) (2015) 1309–1321. [18] Mingliang Xu, Hao Su, Yafei Li, Xi Li, Jing Liao, Jianwei Niu, Pei Lv, Bing Zhou, Stylized aesthetic QR code, IEEE Trans. Multimedia (2019), https://doi.org/ 10.1109/TMM.2019.2891420. [19] Junwei Han, King Ngi Ngan, Mingjing Li, Hong-Jiang Zhang, Unsupervised extraction of visual attention objects in color images, IEEE Trans. Circuits Syst. Video Technol. 16 (1) (2006) 141–145. [20] Mingliang Xu, Hua Wang, Shili Chu, Yong Gan, Xiaoheng Jiang, Yafei Li, Bing Zhou, Traffic simulation and visual verification in Smog, ACM Trans. Intell. Syst. Technol. 10 (1) (2019), 3:1–3: 17.