1. Selection of evaluation indicators
Regional crustal stability is a comprehensive reflection of the modern activity level of the regional crust. It is affected by geological structure, seismic activity, terrain deformation field, in-situ stress field, Control of many factors such as geothermal flow fields, landslide flow geological hazards, and site stratigraphic lithology and rock mass structure. Therefore, in the process of comprehensive evaluation of regional crustal stability, the selection of indicators must not only better reflect the above factors, but also make the indicators as quantitative as possible, and avoid overlap between factors or indicators as much as possible. This is It is also one of the difficulties in quantifying geoscience issues. In view of this, combined with the characteristics of the study area and the feasibility of obtaining relevant indicators, corresponding evaluation indicators were selected for the above influencing factors: geological structural factors are mainly reflected by three indicators: fault level, activity age and activity rate, and seismic activity is reflected by potential The source area and its earthquake magnitude are represented, the lithology and rock mass structural characteristics are represented by regional engineering geological rock groups, the geostress field is represented by geostress accumulation, the terrain deformation field is mainly represented by vertical strain gradient, and the geothermal flow field is represented by geothermal changes. The gradient or the near-surface temperature of hot springs is represented, and the geological hazard factors of landslides and slides are represented by the susceptibility of geological hazards. Based on the characteristics of various indicators and through appropriate quantification, an indicator system for comprehensive evaluation of crustal stability in the study area is formed (Table 9-4).
Table 9-4 List of crustal stability evaluation index systems
2. Quantitative approach to evaluation indexes
As we all know, some geological elements can be expressed quantitatively, and some Descriptive, it is impossible to use absolute numerical values ??to express quantitatively, and only semi-quantitative methods can be used to reflect their differences in different areas. Therefore, the current quantification of geological research is actually still in the semi-quantitative-quantitative stage. As the degree of research continues to deepen, the degree of quantification will continue to increase. In accordance with the above guiding ideology, we aimed to evaluate the crustal stability along the Yunnan-Tibet Railway and quantified various factors (indicators) that affect regional crustal stability. The quantification of the evaluation indicators and the final evaluation analysis were completed on the ArcGIS 9.2 software platform. . In the quantification process, for indicators that can be directly quantified, such as in-situ stress values, vertical strain gradients, geothermal gradients, etc., contours are first drawn and then divided into different levels. For indicators that cannot be directly quantified, the method of scoring comparison is used to divide and classify the indicators according to the characteristics of the plane distribution.
(1) Active faults
1. Fracture scale
Faults can be divided into lithospheric faults, crustal faults, basement faults and capping faults according to their cutting depth. There are four levels of layer fractures, among which lithospheric fractures and crustal fractures are deep structures that have a certain control on the stability of the crust. They usually appear as structural suture zones in space. In the quantification process, fractures are divided into four levels according to the cutting depth, and assigned values ??or scores respectively: ① lithospheric fracture, assigned a value of 9 to 10 points; ② crustal fracture, assigned a value of 7 to 8 points; ③ basement fracture, assigned a value of 4 to 5 points ; ④ The capping layer is broken, and the value is less than 3 points. The score of the intersecting area affected by different faults is based on the principle of higher rather than lower, and the score is appropriately increased for the overlapping area. Generally speaking, as the distance from the fault zone increases, the degree of influence gradually decreases. Therefore, strips with different degrees of influence can be divided along the fault zone, which are divided into three levels: high, medium and low, respectively according to 100 %, 60%, and 30% decrease toward the periphery, and the strip widths are 5 km, 10 km, and 20 km from the fault line respectively; for active basement faults and caprock faults, the strip widths can be 2.5 km, 5 km, and 20 km, respectively. 10 km. From this, a quantitative result of the scale of active faults can be obtained, which is based on a layer in the GIS evaluation (Figure 9-2).
Figure 9-2 Comprehensive partitioning of active fault stability
2. Fault activity age and activity rate
The quantification of fault activity age is based on the study of active faults Obtained on the basis, the newer the age, the stronger the activity, which can be considered together with the fracture activity rate.
There are 157 faults of different sizes in the study area, 51 of which have activity rate records, with the rate ranging from 1.5 to 10.5 mm/a. Most of them are concentrated in the range of 1.5 to 5.7 mm/a, and only one fault has an activity rate. is 10.5 mm/a. Due to incomplete activity rate recording data, differences in measurement locations and measurement methods, the reliability is not very strong. In the evaluation of crustal stability in large areas, the activity rate is only used as a reference factor for assigning activity times. Therefore, the index of fracture activity intensity is obtained by focusing on the age of fracture activity and supplementing the activity rate. According to the age of fault activity, it is divided into 4 levels and assigned values ??respectively: ① Late Pleistocene-Holocene faults, assigned a value of 8 to 10 points; ② Early-Middle Pleistocene faults, assigned a value of 5 to 7 points; ③ Inferred active faults, assigned a value of 3 points . ④ If there is no active break, 0 points will be assigned. Take 20 km on both sides of the first three items as the influence area of ??the active fault, and the score of the intersection area of ??the influence area shall be higher rather than lower.
3. Comprehensive quantification of active faults
The two indicators of fault scale and activity intensity are superimposed according to the weight according to the spatial location. The weight of fault scale is 0.4 and the weight of activity intensity is 0.6. The weighted superposition results are stretched to a range of 0 to 10 points, and 2.5, 5, and 7.5 are respectively used as thresholds to comprehensively classify the impact of active faults on regional crustal stability into four levels (Figure 9-2).
(1) Strong, assigned 10 points; (2) Strong, assigned 6 points;
(3) Weak, assigned 3 points; (4) Weak, assigned 1 point.
(2) Seismic activity
Seismic activity can be expressed in different ways. Through historical earthquake analysis and seismotectonic research, we have compiled a potential source area division map along the Yunnan-Tibet Railway. , on this basis, the seismic activity intensity in the study area is divided into 4 levels (Figure 9-3).
Figure 9-3 Seismic activity intensity zoning map of the study area
(1) Earthquake magnitude Ms>7.0 or seismic intensity I≥X area, assigned 9 to 10 points, including research 7.5-magnitude and 8-magnitude earthquake areas within the district are assigned 9 points and 10 points respectively;
(2) Earthquake magnitude 6.0≤Ms≤7.0 or seismic intensity I=VIII, IX area, assigned 6 ~8 points; including earthquake areas of magnitude 6, 6.5 and 7 within the study area, which are assigned 6 points, 7 points and 8 points respectively;
(3) Earthquake magnitude 5.0≤Ms<6.0 or The earthquake intensity I=VII degree area is assigned 5 points;
(4) The earthquake magnitude Ms<5.0 or the earthquake intensity I≤VI degree area is assigned 1 point.
(3) Grading of in-situ stress accumulation intensity
Today, the degree of in-situ stress concentration is mainly based on the finite element numerical simulation results. The maximum shear stress is selected as the evaluation index and is divided into four levels. Assign values ??respectively (Figure 9-4).
(1) High value area, the maximum shear stress value is greater than 15MPa, assigned a score of 8 to 10 points;
(2) High value area, the maximum shear stress value is greater than 10 to 15MPa , assigned a value of 6 to 7 points;
(3) Medium area, the maximum shear stress value is greater than 5.0~10MPa, assigned a value of 3 to 5 points;
(4) Low value area, the maximum If the shear stress value is less than 5.0MPa, the value is <3 points.
Figure 9-4 Zoning diagram of the maximum shear stress field in the study area
(4) Ground strain gradient grading
Since the active fault mainly considers horizontal displacement, Try to avoid overlap between indicators as much as possible, and mainly select vertical strain gradient indicators to reflect changes in ground strain (Figure 9-5). According to the vertical strain gradient, it can be roughly divided into 4 levels, and assigned values ??respectively.
Figure 9-5 Vertical strain gradient grade zoning map of the study area
(1) High value area, vertical strain gradient >0.06 mm/km, assigned a score of 9 to 10 points;
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(2) Median value area, vertical strain gradient 0.04~0.05 mm/km, assigned 7~8 points;
(3) Low value area, vertical strain gradient 0.02~0.03 mm/ km, assigned a value of 4 to 6 points;
(4) Extremely low value area, vertical strain gradient <0.02 mm/km, assigned a value of 1 to 3 points.
(5) The geothermal flow field
The geothermal gradient can reflect the geothermal flow field. However, the geothermal parameters available in the study area are not uniform, and through comparative analysis, the geothermal parameters in the study area are not uniform. The near-surface temperature contours of hot springs are basically consistent with the geothermal gradient. Therefore, the near-surface temperature contours of hot springs can be used to replace the geothermal gradient to generate a geothermal zoning map (Figure 9-6), which can then be divided according to the near-surface temperature of hot springs. levels and assign values ??respectively.
Figure 9-6 Geothermal field zoning map of the study area
(1) High-value area, the near-surface temperature of hot springs is 75-100°C, and is assigned a value of 10 points;
(2) Median value area, the near-surface temperature of hot springs is 50-75℃, and is assigned 8 points;
(3) Low-value area, the near-surface temperature of hot springs is 25-50℃, and is assigned 6 points;
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(4) Extremely low value area, the near-surface temperature of hot springs is <25°C, and is assigned a value of 4 points.
(6) Lithological Characteristics
According to the distribution characteristics of engineering geological rock groups in the study area, they are divided into 4 levels (Figure 9-7), and are given according to different rock types. Assignment (Table 9-5).
Table 9-5 Main lithology characteristics and grading scale
Figure 9-7 Engineering geological properties grading map of geological bodies in the study area
(1) Extreme Poor, assigned a value of 8 to 10 points, including fault zones, ophiolites, clastic complexes, various special rock masses and Neogene (N) strata;
(2) Poor, assigned a value of 5 to 7 It includes Paleogene (E) clastic rocks, soft and hard alternating rock formations mainly composed of mudstone, weak strata with coal in the Cretaceous (K), Jurassic (J), and Triassic (T), and slopes. Quaternary (Q) in steep areas;
(3) Medium, assigned a score of 2 to 4, including lithological distribution areas dominated by carbonate rock, gneiss, volcanic rock, and thick sandstone , most areas where the Permian (P) and Triassic (T) are distributed, the Quaternary (Q) area in the gentle zone, etc.;
(4) Good, assigned 1 point, mainly including Various granite rock bodies, intrusive bodies in sedimentary rocks, such as diorite dikes (γ), granite dikes (δ), etc.
(7) Susceptibility to geological disasters
Geological disasters such as collapses, landslides and debris flows are very common in the study area and are important factors affecting the stability of ground buildings. On the basis of previous data analysis and geological disaster investigation, the susceptibility to geological disasters such as collapses, landslides, and debris flows is taken as one of the evaluation factors, and its correlation with internal dynamic effects is considered as much as possible. It is also given according to four levels. Assignment (Figure 9-8).
(1) Highly prone areas, mainly located in mountainous areas and strongly differentiated lifting zones, where large-scale landslides, collapses, debris flows and other geological disasters develop, are assigned 10 points;
(2) relatively Highly prone areas, which develop small to medium-sized landslides, collapses, ground fissures, and debris flows, are assigned 8 points;
(3) Medium prone areas, which mainly develop various small geological disasters, are assigned 6 points;
(4) Low-risk areas, which are basically free of geological disasters but belong to mountainous areas, are assigned 4 points.