| Capital Region (39°-41°N,114°-119°E) is the national center of politics, culture and economy with developed economy and dense population. Capital Region is located at the intersection of Zhangjiakou-Bohai Sea structral belt and Fenwei Basin, where there exist intense and frequent earthquake activities. Historical seismic materials have recorded a series of intense earthquakes in the area. Examples are Sanhe-Pinggu earthquake at a scale of Ms8on September2nd,1679and Tangshan earthquake at a scale of Ms7.8on July28th,1976, which caused significant losses to the country and the people. Destructive and sudden earthquakes of this kind posed a great threat to the social stability of Capital Region and limited the rapid and steady development of economy in the area.The occurrence of endogenetic geological disasters, like earthquakes, is closely connected with the stress state of the crust. Major earthquakes’ formation and occurrence is the long-term process of stress’cumulation, concentration and strenthening and the sudden burst of strain energy of rock mass failure at stress concentration points. Mechanical properties and geostress of the crust materials have crucial significance on crust movement. On the basis of a good understanding of Capital Region and its neighboring area’s seismic geology, active structures and deep geophysics, it has quite important significance to deeply look into the Capital Region’s current geostress environment and evolution pattern for the purpose of researching fault activity, crust movement, dynamic mechanism of structure activity and earthquake occurrence mechanism.This paper systematically collected and analysed research results of regional geology, crust lithosphere dynamic characterastics, active faults and structural division, regional structural stress field and earthquake activities. Firstly, we focused analysis on hydraulic fracturing geostress measurement results of4deep holes (600-1000m) in Pinggu, Xifeng Temple, Miyun and Li Siguang Memorial in Beijing and aquired the current geostress changes with depth in the shallow crust of Beijing. And then, according to the crust structural characters, structural division and distribution pattern of active faults of the research area, Capital Region’s3D geological model was constructed based on linear elastic finite element method with ANSYS simulation software. A3D structural stress field numerical simulation research of current Capital Region was conducted with the measured geostress data and focal mechanism etc as stress target constraints. Finally, we chose1,20,40,60,80and100years as time scales for simulation analysis on the evolution patterns of horizontal principal stress’ relative values, orientations and elastic strain energy density variation values, which focused on100years scale horizontal principal stress evolution in the Beijing area and aquired a fitting curve of horizontal principal stress variation gradient by time. The horizontal principal stress’value, the maxium horizontal shear stress and their evolution by time of Beijing near-surface in2032,2052,2072,2092and2112were achieved by superimposed stress calculation. Furthermore, we investigated the horizontal movement style of current Capital Region and its influence on structural stress environment.Based on the research and analysis, this paper arrived at the following conclusions and understandings.1. Shallow crustal horizontal principal stress in the Capital Region shows linearly increasing trend with depth:the maximum horizontal stress increases with depth with a gradient of0.031MPa/m, and the minimum horizontal stress increases with depth with a gradient of0.0216MPa/m. The relationship among the three principal stresses is as follows: in about0-530m depth, showing σH> σh> av, as against the off-type stress state; within about530m-1000m depth, showing σ> σv> σh, as strike-slip stress state. The changes of stress characteristic parameters with depth are as follows:maximum horizontal lateral pressure coefficient σH/av=55.7/H+1.37, the minimum horizontal lateral pressure coefficient σh/σv=57.8/H+0.89, the average horizontal lateral pressure coefficient (σH+σb)/2σv=61.2/H+1.12, the ratio of the maximum and minimum horizontal stress σH/σh=1.47-4.37/H, and the horizontal shear stress relative value μm=0.19-2.33/H. The maximum horizontal principal stress dominance orientation of current Capital Region is NE～NEE, which is basically consistent with the P-axis principal compressive stress orientation obtained from the Capital Region deep crust focal mechanism and with the regional structural stress field orientation of principal stress of North China.2. Within0-40km crustal depth, the maximum horizontal principal stress of Capital Region is10.59-1027.66MPa, the minimum horizontal principal stress is6.37-1000.71MPa, and the vertical stress is5.04-1037.56MPa. The three principal stresses within about0- 30km depth increase linearly with depth; within30-40km depth, the three principal stresses slowly increase, presenting a trend to be stable, and the curve shape is nonlinearity. The relationship between the three principal stresses is as follows:within about0-15km depth, showing σH>σv≥σh, as strike-slip stress state; within about15-35km depth, showing σH>σv σh, as strike-slip stress state; within about35-40km depth, the relationship turned to σv>σH> σh, and the stress state is normal faulting. The stress characteristic parameters variation with depth are as follows:the minimum horizontal lateral pressure ah/σv=57.41/H+0.91, the maximum horizontal lateral pressure σH/σv=178.57/H+1.16; the maximum and minimum horizontal lateral pressure coefficients reach the maximum value level in the shallow crust, but present a decreasing trend with increasing depth; the maximum horizontal lateral pressure of about25km in depth tends to1.16, and the minimum horizontal lateral pressure coefficient of about10km in depth tends to0.91. The average of both horizontal lateral pressure coefficients is1.04, clearly suggesting the deep crustal stress environment in hydrostatic pressure. The maxium principal stress orientations within the crust of Capital Region vary little in the shallow crust and deep crust. Except that the principal compressive stress orientations are NW-NWW in Dalian and nearby area in eastern Shandong-Bohai block, the principal compressive stress orientations in most area of other structural units are NE-NEE.3. As a result of the influence of crust medium parameters’ differentiation in each sub-block and weakening effect of boundary faults, the principal stress of each secondary tectonic unit in Capital Region is discontinuously distributed both longitudinally and vertically, and-mainly concentrated in boundary faults. Whereas in more stable sub-blocks, the distribution of principal stress is also found to be somewhat similar this is reflected by stress in the same depth mostly keeping in a stable range. From the near-surface crust to the deep crust, it seems that each secondary tectonic unit in the Capital Region has an increasing trend in elastic strain energy density. Sub-blocks in North China show even distributions and relatively low values of elastic strain energy density and, elastic strain energy densities in active blocks or structural belt boundary faults are the largest from shallow to deep. Stress and strain accumulation and concentration occur more easily in an area of high elastic strain energy density. 4. In a100-year time scale, the cumulative displacement of the Capital Region at different times makes the relative value of horizontal principal stress increase. Affected by the medium differentiation of tectonic units in the study area and the faults, horizontal principal stress is not consistently and evenly distributed among tectonic units of different and even the same structure. The horizontal principal stress in each level structural unit is consistentin orientations and the horizontal principal compressive stress orientation is N70°-80°E, which is basically consistent with the maximum principal compressive stress orientation of North China. While the horizontal principal tensile stress orientations advocated mainly as N10°-20°W, which is basically consistent with the minimum principal compressive stress orientation of Noth China. Elastic strain energy density annual variation increases with the increasing displacement load. The variation in sub-block boundaries or primary active structural belt boundary faults is the largest. But the variation distributes evenly within secondary active blocks and primary active structural belts.5. The horizontal principal stress annual variation of Beijing under current horizontal movement rate increases with increasing displacement load and has a linear trend, but its increasing gradients with time have differences between different depth. In various time scales, the horizontal principal tensile stress variation is greater than the value of the principal compressive stress; the former is generally1.21to1.25times of the latter. Without regard to the influence of earthquake and other geological events, the minimum horizontal principal compressive stress will turn into tensile stress with time due to the current crustal movement pattern. The maxium horizontal principal compressive stress will increases the time, making the maximum horizontal shear stress increase, which thereby might cause the danger of large scale strike-slip fault failure in the study area. |
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