Comprehensive analysis and evaluation of regional stability

I. Selection and classification of quantitative evaluation indicators

This regional crustal stability evaluation adopts fuzzy mathematics comprehensive evaluation method. According to the evaluation indexes listed in Quantitative Evaluation of Regional Crustal Stability by Sun Ye and Tan Chengxuan, six evaluation indexes were selected. The total score is 10 according to the requirements of quantitative discrimination. Assign weights to different grades, and determine the scoring standards and grades (Table 2-3).

Table 2-3 Evaluation Index Weight, Fraction Line and Relatively Stable Graded Synthesis

sequential

(According to Sun Ye et al. 200 1, slightly modified)

Second, the analysis of regional stability characteristics

1. Surface crustal structure and geotechnical mechanical properties

The crust of Shandong Peninsula urban agglomeration area is composed of two geological blocks, eastern Shandong and western Shandong, and its boundary is located on the Yishu fault zone in the middle of the famous Tanlu deep fault. The eastern Shandong fault block is mainly mountainous and hilly, and the western Shandong fault block is mainly composed of Tai, Yi and Mengshan areas in central Shandong and the western Shandong sedimentary plain on the northeast edge of North China Plain. From the analysis of modern structural properties, the fault depression areas in the North China Plain in the west and north (mainly the Bohai Sea and its southwest coastal areas) are modern rift structures, and the uplift mountains in the middle and east of Shandong Province are rift ridges. Although the development history and structural characteristics of the two geological blocks are different, their crustal thickness, layered structure of deep media and physical conditions are not very different. Under the action of the above-mentioned regional tectonic stress field, the east and west blocks are squeezed and twisted along the unhealed Yishu fault zone, accompanied by uneven uplift differential activities. At the same time, there are different degrees of faults in the block, which leads to the new activities of the two groups of faults in the northwest and northeast, forming a grid image, and forming some triangular fault basins on the intersection side of the two groups of faults.

2. Deep crustal structure and deep faults

This area is bounded by Yishu fault zone, with Tai 'an and Yiyuan as the center, with high gravity value in eastern Shandong and low gravity value in western Shandong. The Yishu fault zone is generally located at the intersection of high gravity in eastern Shandong and low gravity in western Shandong. Generally, it is located in the high gravity area. On the background of high gravity, the northern Weifang shows local low gravity anomaly, and the overall distribution direction and shape of gravity anomaly are consistent with Jiyang Basin.

3. The rise and fall of the block and the current crustal activity rate.

The uplift of western Shandong and eastern Shandong is still dominated by rising, and the rising rate is generally 1 ~ 2 mm/a; the rising rate between Rongcheng and Qingdao in Jiaodong Peninsula is 3 mm/a; Yishu fault zone is a relative subsidence trough with an annual rate of 0 ~ 1 mm/a, and Yishui area forms a saddle.

4. Fracture and its activity

From the distribution map of main fault structures in Shandong Peninsula urban agglomeration area (Figure 2- 1) and the map of main fault structures in Shandong Peninsula urban agglomeration area (Table 2- 1), we can see that:

Changqing fault, Mengshan fault, Wen Zu fault, Bai Quan Zhuang-Wuseya fault, Jinshan-Yaojiayu fault, etc. Wen Zu fault is the main fault in Ji 'nan area, which has a large influence range.

There are Jinshan-Yaojiayu fault, He Zi fault, Shangwujing fault, Jiusan fault and Yidu fault in Zibo area, as well as Motianling-Dashan anticline, Songshugang-Daluoyu syncline, Lushan-Heifang-Kuaibao syncline and Lushan-Cao Shi-Zhangjiashao syncline (partially). Among them, the key faults in this area are He Zi fault, Shangwujing fault and Yidu fault. The surface has obvious mosaic structure, and the structure is complex.

Weifang area concentrates several main faults in Shandong Peninsula, namely Yishui-Tangtou fault, Anqiu-Juxian fault, Changyi-Dadian fault and Anqiu-Juxian depression. The main fault zone is combined with the structure, and the structure is complex.

The Shanxiangjia-Haoguanzhuang fault, Baichihe-Erwulikuang fault, Wadian Yolanda fault, Zhang Cang fault and Shimei-Rizhao fault in Qingdao and Rizhao area have the greatest influence, and the surface layer belongs to fragmented structure with medium development.

5. Current geostress and energy

Modern tectonic force field is the basis of crustal tectonic activity and seismic activity. North China is now in a stress field in which the northeast principal compressive stress and the northwest stress act nearly horizontally, and the direction and state of the stress field are basically stable. The direction of stress field in South China is different, characterized by NW compression and NE extension, and the action mode of force is almost horizontal. The eastern part of China lies between North China and South China, and the direction of principal compressive stress field is nearly east-west. It is considered that the direction of stress field is nearly north-south, and the action mode of force is close to horizontal and relatively stable. Shandong Peninsula urban agglomeration is located in the southern part of North China and the northern edge of East China. The stress field mainly follows the characteristics of North China, but it is also influenced by the stress field of East China. It is considered that the direction of stress field is slightly east than that of North China, and the average principal compressive stress axis is about 80 and the stress axis is about 350. The stress state is basically stable and the action mode is close to horizontal. At present, the intersection angle between the direction of the maximum principal compressive stress and the regional principal fracture is greater than 55.

6. Main internal dynamic geological disasters

Earthquake intensity is a comprehensive index to evaluate the degree of earthquake vibration and damage, and its influencing factors are many and complicated, but it is mainly influenced and restricted by earthquake magnitude, epicentral distance and foundation, as well as focal depth, dislocation mode, geological structure, topography and groundwater depth.

It can be seen from the seismic intensity zoning map of the State Seismological Bureau (Table 2-4) that the intensity zoning map of the study area is VI and VII, of which the northwest of Kenli-Dongying-Boxing-Zhoucun line is VI, and the southwest of Gaomi line is located in the Tanlu strong earthquake structural belt, which is VII; The northeast area of Laizhou-Zhaoyuan-Fushan-Rushan line is the VII degree area, and the southwest to Gaomi area of the line is the VI degree area.

Table 2-4 Distribution of Urban Seismic Intensity in Shandong Peninsula Urban Agglomeration Area

Thirdly, fuzzy comprehensive analysis and evaluation of regional stability.

1. Evaluation object set, factor set and evaluation set

Taking the main tectonic faults and neotectonic movements as the dividing line, the Shandong Peninsula urban agglomeration region is divided into 19 regions as evaluation units (Figure 2-5).

Figure 2-5 Schematic diagram of regional crustal stability fuzzy comprehensive evaluation of Shandong Peninsula urban agglomeration

2. Weight vector of evaluation factors

In the set of evaluation factors, the position and function of each factor in the "evaluation goal" are different, and the proportion of each evaluation factor in the comprehensive evaluation is also different. According to Quantitative Evaluation of Regional Crustal Stability (Sun Ye et al., 200 1), the weight distribution of evaluation indicators is as follows:

1) Surface crustal structure and geotechnical mechanical properties (F 1), with a weight distribution of 17%.

2) Deep crustal structure and deep fault (F2), with the weight distribution of 13%.

3) Block ups and downs and current crustal activity rate (F3), with a weight distribution of 10%.

4) Fracture and its activity (F4), with a weight distribution of 20%.

5) Current geostress and energy (F5), with a weight distribution of 15%.

6) Major internal dynamic geological disasters (F6), with a weight distribution of 25%.

Therefore, the weight vector is A={0. 17, 0. 13, 0. 10, 0.20, 0. 15, 0.25}.

3. Determination of membership degree of each index

In the evaluation of crustal stability, the number between 0 and 1 is used to reflect the influence degree (membership degree) of the j th influencing factor on different levels in the evaluation set. The membership degree of the same index changes with the change of numerical value, and the membership degree is different when the same index belongs to different levels. Thus, the functional relationship between representative membership degree and index value, that is, membership function, is established. The membership function used to determine the distribution characteristics of continuous variables in evaluation indexes should be normal distribution function.

According to the six main evaluation indexes, the overall quality matrix and score of each evaluation area are obtained according to the scoring standard, as shown in Table 2-5.

Table 2-5 Overview of Comprehensive Mass Matrix of Crustal Stability in Shandong Peninsula Urban Agglomeration to be Assessed

4. Fuzzy evaluation of regional stability

According to the membership values of different stability grades, the regional crustal stability grades are judged, as shown in Table 2-6. According to the calculation results of fuzzy evaluation of relative stability of each evaluation area, a comprehensive evaluation map of regional stability of Shandong Peninsula urban agglomeration is drawn, as shown in Figure 2-6.

According to the principle of maximum weight, the proportion of each grade of crustal stability in the administrative areas of eight cities is calculated, and the comprehensive evaluation map of regional stability of eight cities in Shandong Peninsula urban agglomeration is obtained, as shown in Figure 2-7 and Table 2-7.

Table 2-6 Determination of Crustal Stability Grade of Shandong Peninsula Urban Agglomeration

ⅰ: stable region; Ⅱ: basically stable region; ⅲ: unstable region; ⅵ: Unstable region.

Table 2-7 Comprehensive Evaluation of Crustal Stability of Eight Cities in Shandong Peninsula Urban Agglomeration Area

Figure 2-6 Comprehensive Evaluation of Regional Stability of Shandong Peninsula Urban Agglomeration

Figure 2-7 Comprehensive Evaluation of Regional Stability of Eight Cities in Shandong Peninsula Urban Agglomeration Area

Four. Regional stability evaluation results

Based on the above analysis, we can draw the overall evaluation conclusion of seismic environment and regional crustal stability in Shandong Peninsula urban agglomeration area as follows:

Comprehensive analysis and evaluation of (1) regional seismic environment

1) Shandong Peninsula urban agglomeration area is located in the southeast corner of North China seismic belt, mainly in the Tan-Lu seismic belt. There have been many destructive earthquakes in history, with the largest magnitude of 7.

2) The earthquake damage and influence suffered by Shandong Peninsula urban agglomeration area not only comes from earthquakes with M ≥ 5 in this area, but also comes from surrounding areas, especially the earthquakes with M ≥ 7 and M ≥ 8 in the Tan-Lu earthquake zone. Historically, the highest earthquake impact intensity reached VII-VII in some areas, and VII-VII in some areas, reaching most other areas, which was an earthquake impact.

3) It is predicted that earthquakes of magnitude 5 and 6 will still occur in this area in the next 50 years, and the highest earthquake intensity in some areas can reach VII.

4) The north-central area of Shandong Peninsula urban agglomeration is a high-value area of future earthquake acceleration peak, which has higher earthquake destructive power compared with other areas, so special attention should be paid to earthquake prevention and earthquake resistance in urban planning of economic construction.

(2) Comprehensive analysis and evaluation of regional crustal stability.

Based on the comprehensive analysis of the earthquake environment and crustal stability in this area, six factors affecting and controlling the crustal stability in this area are determined, and the crustal stability in this area is comprehensively analyzed and evaluated by using fuzzy mathematics theory.

1. Regional crustal stability zone (Ⅰ)

The regional crustal stability zone is Jinan. Jinan is located in the north of Mount Tai. Geologically, the southern part is Luxi platform uplift, and the northern part is Jiyang depression in North China platform depression. The terrain is high in the south and low in the north. The southern part is a hilly area with an altitude of 500 ~ 1000m, and the northern part is a piedmont plain with an altitude of 25 ~ 32m m. Guangrao and Feicheng faults are distributed, adjacent to Liaokao fault zone in the west. In Jinan monoclinal fault block structure, the basement is metamorphic rocks of Taishan Group, and the caprock is Paleozoic strata, with undeveloped faults. North of Jiyang is a depression area, with thick Cenozoic loose layers and only Paleogene-Neogene active faults. There are few earthquakes in this area except Changqing and Pingyin, where there are earthquakes of magnitude 5 and 5.5. Therefore, Jinan is generally a stable area, and Changqing-Pingyin and Jiyang are basically stable areas.

2. Basic regional crustal stability zone (Ⅱ)

Cities with basically stable regional crust are Qingdao, Yantai, Weihai and Zibo.

The intensity and amplitude of fault activity in Qingdao during neotectonic period are getting weaker and weaker with the passage of time. Since the Quaternary, under the background of the overall intermittent rise, some parts of the NE-trending fault had a weak movement in the Middle Pleistocene. According to the analysis of historical earthquake data and modern earthquake data, there are no earthquakes with MS≥5 in Qingdao, but there are weak earthquakes around the urban area, both of which are 2.0≤ML≤2.9. Therefore, the overall fault structure activity level in this area is not high.

There are two groups of active faults in the surrounding area of Yantai (including the sea area) that strike NNE and NWW since Cenozoic; Quaternary active faults are Yishu fault in the northeast of Shandong and Yantai-Changshan Island-Bohai fault in the NWW direction, which are large in scale and frequent in activity. However, the scale of Quaternary active faults in the uplift area around Yantai is small. According to the nature, landform and material composition of neotectonic movement, except Huangxian fault basin, most of Yantai area is a relatively stable and slowly rising hilly area, and its southern and northern parts are different in movement form and activity range. The neotectonic movement in Yantai basically inherited the characteristics of long-term rise based on the east-west anticline, but the rise on the north side was stronger than that on the south side, and the whole area tilted from northeast to southwest due to the activity of the NW-trending fault. The multi-level nature of marine erosion platforms and planation planes in coastal areas shows that these inclinations are intermittent and tremulous.

Generally speaking, the Cenozoic active faults in Weihai and Zibo areas are undeveloped, and the weak earthquake density is low.

3. Regional crustal instability (Ⅲ)

The regional crustal instability areas are Dongying City and Weifang City. Most of this area belongs to the northwest plain of Shandong. There are many Paleogene-Neogene and Quaternary active faults in this area, which cut and cross each other, such as Guangnan fault, Yidu fault and He Zi fault. Divided into seismic resolution ⅵ degree and local ⅶ degree. There have been earthquakes of magnitude 5 in history, with high weak earthquake density.

4. Regional crustal instability area (Ⅳ)

The regional crustal instability area is Rizhao City. Affected by the Tan-Lu fault zone, there are frequent weak earthquakes in this area. It is predicted that there will be an earthquake of magnitude 6, and the intensity of earthquake zoning is VIII degrees. There are Changyi-Dadian fault, Yishui-Tangtou fault and Anqiu-Juxian fault in the area, which are generally distributed in the northeast direction, with horizontal and vertical displacement of Quaternary uplift or fault, Quaternary active fault and shallow Moho surface in some parts of the crust.