Distribution of In Situ Stress in Northwest Jiaodong Peninsula

In order to study the present state of the tectonic stress eld in northwest Jiaodong Peninsula, the characteristics of the ground stress distribution along depth were analyzed with the method of regression analysis. A total of 164 items of in situ stress measurement data were collected.­ere are mainly two types of in situ stress states, one is σH>σh>σv and the other is σ>σv>σh, and the type of in situ stress state is related to the depth. ­e principal stresses σH, σh, and σv increase approximately linearly with depth, and the stress gradients are 0.0556, 0.0248, and 0.0346, respectively. ­e lateral pressure coecients KH, Kh, and Kav vary approximately hyperbolically with the increase in depth and approach 1.99, 1.12, and 1.56, respectively. ­e ratio of half of the maximum horizontal dierential stress to the average horizontal principal stress μd varies linearly with the increase of depth and approaches the value of 0.33. ­e maximum shear stress τm increases approximately linearly with depth, and the stress gradients are 0.0146. ­e ratio of horizontal principal stress dierence with vertical stress on 1000m–2000m is 0.5– 0.7. In addition, the maximum level of principal stress advantage direction in the northwest Jiaodong Peninsula is nearly E–W.


Introduction
In situ stress is the natural stress existing in the crust undisturbed by engineering, also known as the initial stress of rock mass, absolute stress, or original rock stress. e in situ stress state is an important index to characterize engineering geological disasters, earthquake preparation, and mine safety construction. e study of in situ stress measurement and its distribution characteristics is a basic and very important work in the process of underground engineering construction. e measured data of in situ stress can not only directly re ect the characteristics of the regional in situ stress eld but also help to explain the problems of fault activity, crustal movement, and the dynamic source of plate evolution. e northwest Jiaodong Peninsula is located in the easternmost part of Shandong Province, which is the third largest gold-producing region in the world. e complex geological structure of the area is not conducive to mining activities. Despite the particular importance of in situ stress measurement in this area, it is still vacant. e distribution law of regional in situ stress is the foundation for underground engineering construction. Scholars have carried out many studies on regional in situ stress. Sandiford et al., respectively, conducted in situ stress studies in Southeast Australia, Japan Island, South Africa, and the Qinshui Basin [1][2][3][4]. After the in situ stress measurement method was introduced in China, many earthquake-prone areas and resource-intensive areas have carried out a lot of work. Many scholars have also studied the local stress eld characteristics in the Shandong area. Zheng [5] carried out in situ stress measurements near the coast of the southern Bohai Sea. Liu [6] measured the in situ stress of the sea working face of Beizao mine. Cai and Qiao measured gold mines such as Sanshandao [7,8]. But in the Jiaodong Peninsula, especially in the northwest Jiaodong Peninsula where rare metal resources are enriched, the in situ stress characteristics are still blank. In order to fully grasp the distribution characteristics of the in situ stress eld in the northwest Jiaodong Peninsula, the distribution characteristics of the in situ stress eld in this area were analyzed based on the in situ stress measurement data of the author and others for many years. In particular, the data from the Sanshandao gold mine, Linglong gold mine, and Xincheng gold mine provide important support for this paper. e research results fill the gaps in the characteristics of in situ stress in the Jiaodong Peninsula and provide reliable data for mine engineering construction, tunnel construction, and the study of earthquake focal mechanisms in the area.

Geological Tectonic Environment in the Northwest of Jiaodong Peninsula
Shandong is located in the east of the north China block, in the Jiaodong Peninsula between the Bohai Sea and the Huanghai Sea, and is affected by the joint push of the Pacific plate and the Philippine plate. ere were a long evolutionary history and complex geological tectonic environment in the area, including the stable ancient land block of the Archean, the Paleozoic Proterozoic activity zone, and the stable Paleozoic land surface sea deposition in Claratong [9]. e NNE-trending and NE-trending faults are the main faults in the northwestern Jiaodong Peninsula. NE-trending faults have the characteristics of right-lateral strike-slip and mainly exhibit compressive-torsional activities including the Yishu fault zone (NNE-trending) and the Zhaoping fault zone (NE-trending). Figure 1 shows the basic pattern of active fracture structures in Shandong Province, some of which are still active at the present stage [11,12]. ese faults have an obvious control effect on the regional formation, tectonic movement, magmatic activity, and mineral distribution.
e Shandong section of the Tanlu fault zone runs between western Shandong, Jiaodong plots, and the Sulu super high voltage metamorphic zone, the fracture zone extending about 360 km towards N(10°∼25°)E. Affected by the Tanlu fault belt, the geological environment of the northwest Jiaodong Peninsula and the south Bohai Sea area is more complex. However, this area is an important mineral resource area in China, especially for rare and precious metals. e Jiaodong Peninsula is located at the intersection of mainland East Asia and the western Pacific plate. It is a tectonic shear belt related to the East Asian mainland and the western Pacific activity belt. It is controlled by the regional counterclockwise torsional stress field, forming an active region of the Neocathaysian system. e geotectonic is located in the Jiaodong uplift area of the second uplift zone of the giant tectonic zone of the Neocathaysian system, adjacent to the Tanlu fault zone in the west, the Bohai Bay Basin in the north, the Pacific plate in the east, and the Dabieshan-Sulu super high voltage metamorphic zone in the south [13,14]. Multistage tectonic magmatic activity in the Jiaodong region has developed a crisscross complex tectonic network. e base structure in the area is a Qixia anticlinorium and fracture structure composed of Precambrian formation, and its tectonic line direction is near the EW direction. Controlled by the collision orogenic and subduction region structural stress field, the fracture structure system is dominated by NE and NNE to the fracture structure. ere are 7 fault zones in the region, in turn from e first four are in the west of Jiaodong, and the overall trend is in the NE direction. e latter two are located in the eastern part of Jiaodong and generally move towards the near-SN direction.
e Muping-Jimo fault is in the junction zone, which is a boundary fault and generally towards the NE direction [15].

Source and Cause of In Situ Stress in the Northwest of Jiaodong Peninsula
In situ stress is a general term for stress within the Earth, which is the distribution of in situ stress within a spatial range [16][17][18][19][20]. e formation of in situ stress is mainly related to various dynamic processes of the Earth. e geological activities in the northwest Jiaodong Peninsula are frequent and the geological conditions are very special, which makes the source of regional in situ stress more complicated. But long after in situ stress was proposed, it was widely believed to be only related to overburden weight. Until 1950s, Hast proposed that the in situ stress is not only the vertical stress formed by the overlying strata but also the horizontal stress caused by tectonic stress [21]. For the first time in the tunnel, we proposed that the horizontal stress is much greater than the vertical principal stress, and it was gradually recognized that the tectonic movement is a major factor in forming the in situ stress [22][23][24]. A number of studies have found that influenced by the collision and extrusion of the Indian plate and the Eurasian plate, the distribution of the maximum principal stress in China is obviously regular. Ma [25] according to the unity and stability of the principal stress direction and the relation of three principal stresses with depth, with the obvious turning area and the principal stress value or stress gradient as the boundary, the crust stress area is divided ( Figure 2). e northwest Jiaodong Peninsula is on the eastern coast of China.
is region is a complex of geological records formed in important geological periods, which are preserved by ancient plots after multistage insertion, superimposition, and denudation, with a variety of structural combinations and styles [26][27][28][29]. e basic structural frame appears in the basin ridge, from north to south, Longkou Basin, Jiaobei uplift, and Jiaolai Basin. is structural pattern is the response to the Mesozoic mantle uplift and the lithosphere thinning in the crust. During the T3-K1 period, the region with a tectonic extrusion-extension effect was obvious [30], and the extrusion-extension model of the accordion structure in the Jiaodong area was formed.
After a large number of field investigations and experimental studies, the main tectonic activities of the Jiaodong Peninsula were divided into four periods. Stress characteristics are characterized by extrusion and extension interaction [31,32]. e first period is mainly extrusion, which can be divided into two stages: NW-SE in the early stage and near S-N in the late stage.

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forming associated NWW-SEE dextral strike-slip faults and nearly S-N dextral strike-slip faults [33,34]. e second period shows the transformation from NE-SW extrusion to NE-SW extension, which is reflected by the main positive fault in the Zhaoyuan-Pingdu fault band. e third period shows NW extension. e fourth period is near-E-W extrusion and near-S-N extension, and the NE direction fault in the region is mainly the right translation motion. In the field observation, a large number of E-W structures can be seen in the early stages of the extrusion structure. From the late Jurassic to the early Cretaceous period, Jiaodong plot shows the tectonic mechanical properties from extrusion to extension periodicity, the periodic change constantly changing and adjusting the stress field in the area [35,36].
rough statistical analysis of the dominant direction of the maximum horizontal principal stress in the Jiaodong block, combined with the existing research results, it is found that the dominant direction of the maximum horizontal principal stress in the region is the NEE-SWW direction (consistent with the focal mechanism solution [37][38][39]).

The Law of In Situ Stress Distribution in the Northwest Jiaodong Peninsula
ere are many places similar to the characteristics of seismic activity in the Shandong and North China blocks, among which the seismic activity of the two has a relatively significant correspondence in time [11]. However, due to the existence of the Tanlu fault zone, the Shandong region has its own particularity. Especially in the Jiaodong region, the difference between this region and the western Shandong region not only exists in stratum distribution but also in in situ stress. e following will be analyzed for the in situ stress distribution law in the northwest Jiaodong Peninsula.

Type Laws of In Situ Stress Field Distribution.
rough the collection and measurement of data from the oil field, mine, geophysical research, and meteorological research, 164 items of in situ stress measurement data (depth from 4.7 m to 2300 m) were obtained by hydraulic fracturing methods and stress relief methods, as shown in Figure 3. e maximum horizontal principal stress in each group is greater than the vertical principal stress. It can be seen that the horizontal stress in the northwest Jiaodong Peninsula is dominant within the measurement depth, belonging to the typical structural stress field type. Among these, the magnitude relationship of three principal stresses in 40 groups was σ H >σ h >σ v . Accounting for 24.4% of the total groups, they belong to the inverse fracture stress state, which is conducive to the gestation and activity of the inverse fault. e magnitude relationship of the three principal stresses in the remaining 124 groups is σ H >σ v >σ h , accounting for 75.6% of the total number of groups, belongs to the slip stress state, and is conducive to the breeding and activity of the slip fault.
After analyzing the data by using the mathematical statistical analysis method, it is also found that when the depth is less than 300 m, the reverse-fault stress state dominates. In the depth range of 300 ∼ 450 m, the distribution of the reverse fracture stress state is roughly equal to that of the strike-slip stress state. When the depth exceeds 450 m, the stress state is mainly a striking slip. It can be seen with the increase of depth that the vertical principal stress gradually changes from the minimum principal stress to the intermediate principal stress, and the stress state type gradually changes from the σ H >σ h >σ v to the σ H >σ v >σ h .

4.2.
e Variation Laws of Principal Stress with Depth. Since the vertical stress in the hydraulic fracturing data is estimated by the weight of the upper rock layer, only the data from the stress relief method are used to analyze the laws of vertical principal stress with depth. e relationship between the maximum horizontal principal stress, the minimum horizontal principal stress, and the vertical principal stress with depth is fitted by linear. e results are as follows: In the formula, H is the depth (m) and R is the correlation coefficient. e variation laws of principal stress with depth are shown in Figure 4. Due to the different aspects of geological conditions, topography, and rock properties, there is some dispersion of in situ stress measurement data. Equations (1)- (3) show that the correlation coefficient R2 of the three principal stress fitting equations is approximately greater than 0.8, with a high degree of linear correlation, indicates the approximate linear growth of in situ stress with the increase of depth, and also reflects the obvious change of the growth law of in situ stress from the depth range of meters to a few kilometers.

e Varies Laws Lateral Pressure Coefficient with the Depth.
e lateral pressure coefficients are also widely used to characterize the in situ stress states at a point underground. Based on the ratio of σ H , σ h , (σ H + σ h )/2 with σ v (collectively referred to the lateral pressure coefficient), the variation of in situ stress state with depth in the northwestern Jiaodong Peninsula is analyzed, which is denoted as KH, Kh, and Kav, respectively. e equations (4)-(6) of the three lateral pressure coefficients in hyperbolic form (K � a/ H + , where a, b is the regression coefficient) are as follows: Figure 5 shows the distribution and fitting curves of the three lateral pressure coefficients with depth. As can be seen Overall, both the dispersion and values of the three lateral pressure coe cients tended to decrease with increasing depth, whose changes were nonlinear. e results show that the three lateral pressure coe cients will likely tend towards a stable value with increasing depth. When H < 1000 m, the value of K H is mostly greater than 2. When 1000< H < 3500 m, the range of K H from 1.2 to 2.0 and the range of K h from 0.5 to 1.0 are consistent with previous studies [40]. It can therefore be seen that in the shallow, tectonic stress is dominant, and as the depth increases, the in situ stress state gradually changes from the tectonic stress dominant state in the shallow part to the deep near hydrostatic pressure state.

e Variation Law of Relative Magnitude of Horizontal
Di erential Stress with Depth. e relative size of the ) is a parameter related to the crust destruction state, which indicates the relative size of the maximum shear stress in the horizontal surface, which can re ect the crust shear stress state in the region to a certain extent. It is reasonable as the mechanical basis for judging the active fracture instability sliding. It can help better understand the characteristics of the tectonic stress eld within the region. e results are as follows: Figure 6 shows the distribution law of μ d with depth. As can be seen from Figure 5, the distribution is relatively discrete. e range is 0.09 ∼ 0.65, with an average value of 0.33, mainly concentrated in the range of 0.09 ∼ 0.49. e range of value is 0.34 ∼ 0.52 in the depths greater than 1500 m, with an average of 0.41. e range of value is 0.09 ∼ 0.65 in the depth range of 0 ∼ 1500 m, with an average of 0.32. It can be seen that the uctuation range of the value less than 1500 m is larger, and the uctuation range of the value gradually decreases with the increase in depth. Relevant research [41] shows that if the value in the crust exceeds 0.5 ∼ 0.7, shear sliding failure may occur. Except for some data points, the value of the northwest Jiaodong Peninsula is basically 0.1∼0.5. It can be inferred that under the current in situ stress state, the possibility of shear sliding failure of the faults in the region is small most of the time.

e Variation Law of Ratio of Horizontal Principal Stress Di erence with Vertical Stress with Depth.
e horizontal principal stress di erence determines the shear stress in the rock mass. e relationship between the ratio of principal stress di erence and vertical stress and depth is shown in ere are 26 data points where 1.1 < K H-h ≤1.7, accounting for 15.9%.
As shown in the above analysis and Figure 6, the K H-h of shallow ground is very discrete. With the development of the deep ground, K H-h is mostly concentrated between 0.7 and 1.0 in the range of 400 ∼ 1000 m. e discreteness of K H-h gradually decreases in the range of 1000 ∼ 2000 m, basically stable between 0.5 and 0.7. It is predicted through analysis that within a certain period of time, the rock mass can be guaranteed to be in a relatively stable state.

e Variation Law of Maximum Shear Stress with Depth.
e maximum shear stress is 1/2 of the di erence between the maximum and minimum principal stress, and the relationship with depth is shown in Figure 8. e relationship between maximum shear stress and depth is obtained by data regression, As can be seen from the gure, the maximum shear stress generally increases with depth. At the same time, an obvious discrete occurs. For all that, the discretization tends to decrease with depth. e reason for this phenomenon may be the scattered distribution of measuring points in the shallow, and these points are a ected by the surface environment, coupled with the measurement results obtained by di erent methods which are quite di erent. So, the shallow shear stress distribution discretization is large. Although the number of measuring points in the deep is small, the distribution in the deep is more concentrated than that in the shallow, which greatly reduces the dispersion of the shear stress distribution.   Figure 9 shows the rose chart of the direction of principal stress advantage in the northwest Jiaodong Peninsula.
e gure shows that the overall direction of principal stress advantage in the northwest Jiaodong Peninsula is near E-W, and NWW-SEE also occupies a considerable proportion. is result is similar to the direction of the maximum horizontal principal stress in Shandong province, as shown in Figure 10 [10].

Conclusion
(1) By processing the 164 groups of data obtained by hydraulic fracturing methods and stress relief methods, the distribution law of the in situ stress eld in the northwest Jiaodong Peninsula is got. e relationship among maximum principal stress, minimum principal stress, and vertical principal stress is analyzed by using the mathematical statistics method. In this area, the σ H >σ h >σ v gradually develops into the σ H >σ v >σ h below −450 m.
(2) e variation relationships of the maximum horizontal principal stress, the minimum horizontal principal stress, and the vertical principal stress with depth, respectively, are σ H 0.0556H − 0.7772, σ v 0.0346H−1.6137, and σ h 0.0248H + 1.0504, respectively. e distribution laws of maximum, minimum, and average lateral pressure coe cients are in accordance with K H 573.07/H + 0.2819,  Advances in Civil Engineering stress with depth concentrated in 0.7-1.0 at 400-1000 m and stabilized in 0.5-0.7 at 1000-2000 m. It is inferred that although the shear stress increases linearly with the increase of depth, there will be no shear dislocation failure in the rock stratum within 1500m. (4) rough analysis, it is found that the directions of maximum horizontal principal stress in the northwestern Jiaodong Peninsula are EW and NEE-SWW, accounting for about 76.2% of all data [42].

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that they have no conflicts of interest.