Analysis of in-situ stress field characteristics and tectonic stability in the Motuo key area of the eastern Himalayan syntaxis
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摘要:
为获取东构造结关键构造部位地应力特征、分析其构造稳定性，采用水压致裂法开展了墨脱断裂带西让段1个地应力孔、11个测试段的原位地应力测量工作。结果表明：61.43~121.34 m测试段最大、最小水平主应力值（SH、Sh）分别为3.05～14.50 MPa和2.16～9.87 MPa，垂向主应力值（Sv）为1.63~3.31 MPa，即SH>Sh>Sv；测点处应力场以水平挤压作用为主，均处于逆断层应力状态，且其主应力值随深度增加而逐渐增大，测点的最大水平主应力优势方位为北东东向；在整个地应力测量深度范围内，侧压系数值（Kav）为1.39~4.38，最大水平应力系数值（KHv）均大于1，且比值随深度的增加而增大，该关键部位区域应力场以水平应力为主导，方向性较强，所有测试段水平应力系数值（KHh）为1.23~1.66，与林芝−通麦段地应力特征参数计算结果基本相似；测点位置98 m以浅地层水平构造应力作用程度较小，应力积累水平较低，保持断层稳定所需的摩擦系数值小于实际断层的临界摩擦系数值，构造环境相对稳定，超过98 m深度地层由于水平构造应力起主要作用，保持断层稳定所需的摩擦系数值接近于实际断层的临界摩擦系数值，存在小概率发生断层失稳滑动的风险；区域强震在墨脱断裂带断层面上造成的左旋走滑方向上及逆冲方向上的库仑应力变化值的叠加量均为负值，抑制了断层的滑动，未能增加墨脱关键区域断层活动的危险性。
Abstract:In order to obtain the in-situ stress field characteristics and analyze tectonic stability in the Motuo key area of the eastern Himalayan syntaxis, the in-situ stress measurement of one in-situ stress hole and 11 test sections of the Xirang section of the Motuo fault zone were carried out by the hydraulic fracturing method. The results show that the maximum and minimum horizontal principal stress values (SH, Sh) in the test section of 61.43−121.34 m are 3.05−14.50 MPa and 2.16−9.87 MPa, respectively, and the vertical principal stress values (Sv) are 1.63−3.31 MPa, namely, SH>Sh>Sv. The in-situ stress field at the measuring point is dominated by horizontal compression, and all of them belong to the in-situ stress state of reverse fault. The principal stress values gradually increase with the increase of depth, and the dominant direction of the maximum principal stress is NEE. In the whole range of in-situ stress depth, the lateral pressure coefficients (Kav) are 1.39−4.38, the maximum horizontal stress coefficients (KHv) are greater than 1, and the ratio increases with the increase of depth. The regional stress field of this key area is dominated by horizontal stress and it is highly directional. The horizontal stress coefficients (KHh) of all test sections are 1.23−1.66, which are similar to the calculation results of in-situ stress characteristic parameters in Linzhi−Tongmai section. The horizontal tectonic stress of the shallow level at 98 m is relatively small, and the stress accumulation level is low. The friction coefficient required to maintain fault stability is smaller than the critical friction coefficient of actual fault, and the tectonic environment is relatively stable. When the depth exceeds 98 m, the friction coefficient required to maintain fault stability is close to the critical friction coefficient value of the actual fault due to the dominant role of horizontal tectonic stress, and there is a small risk of fault instability slip. The superposition of the Coulomb stress change in the sinistral strike-slip direction and the thrust direction caused by the strong regional earthquakes on the fault plane of the Motuo fault zone in the study area are all negative numbers, which inhibits fault slip and does not increase the risk of fault activity in the study area.
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图 1 墨脱地应力孔位置的构造与岩性
Figure 1. Structure and lithology surrounding the borehole for in-situ stress measurement in Motuo
图 2 墨脱地应力孔各测段水压致裂时间−压力曲线特征
Figure 2. Characteristics of the time–pressure curves by hydraulic fracturing method in each measuring section of the Motuo in-situ stress hole
图 4 墨脱地应力孔不同压裂段印模定向
a—107.53 m压裂段印模定向；b—121.34 m压裂段印模定向
Figure 4. Impression orientation of different fractured sections in the Motuo in-situ stress hole
(a) Impression orientation at the 107.53-meter fracture section; (b) Impression orientation at the 121.34-meter fracture section
图 5 测点地应力数据的库伦破裂准则计算结果与实测值的对比
Figure 5. Comparison between the calculated results of the Coulomb fracture criterion and the measured values based on ground stress data at the measuring points
图 6 墨脱关键区域活动断裂和1970—2013年M > 3地震空间分布图
Figure 6. Spatial distribution of active faults and earthquakes with M > 3 from 1970—2013 in the key areas of Motuo
图 7 研究区及附近区域断层面在区域强震影响下不同深度处左旋走滑和逆冲库仑应力变化图（沙滩球表示地震的震源机制解）
Figure 7. Variation of sinistral strike-slip and thrust Coulomb stress at different depths (a–h) on the fault plane in the study area and nearby area under the influence of regional strong earthquakes (The beach balls represent the source mechanism of the earthquake)
表 1 水压致裂地应力测量结果
Table 1. Results of hydraulic fracturing in-situ stress measurement
测段深
度/m破裂压力
Pb/MPa重张压力
Pr/MPa瞬时关泵压力
Ps/MPa抗拉强度
T/MPa孔隙水压力
P0/MPa最大水平主应力
SH/MPa最小水平主应力
Sh/MPa垂向主应力
Sv/MPa最大主应力
方向61.43 5.03 4.48 2.22 0.55 0.60 4.58 3.42 1.63 70.65 2.73 2.45 1.47 0.68 0.69 3.05 2.16 1.87 79.87 3.85 3.43 2.11 0.42 0.78 3.68 2.89 2.12 89.09 7.48 4.61 2.45 3.97 0.88 3.72 3.03 2.38 98.31 8.64 7.52 3.86 3.62 0.98 7.54 4.84 2.65 102.92 16.27 14.25 8.84 3.22 1.03 14.50 9.87 2.78 107.53 6.55 5.41 2.39 1.74 1.08 8.84 5.47 2.91 NE79.6° 112.14 8.91 6.35 4.09 2.56 1.13 10.06 6.22 3.04 116.74 9.81 6.16 4.13 3.65 1.18 10.42 6.31 3.18 118.84 13.46 6.54 6.49 6.92 1.20 11.13 6.69 3.24 121.34 12.07 8.46 6.53 4.01 1.23 12.74 7.75 3.31 NE70.5° 注：地应力测量孔静水位为9.60 m 
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表 2 墨脱地应力孔水压致裂主应力间的变化规律
Table 2. Variation law of hydraulic fracturing principal stress in the Motuo in-situ stress hole
H/m Kav KHh KHv 61.43 2.46 1.34 2.82 70.65 1.39 1.41 1.63 79.87 1.55 1.27 1.73 89.09 1.42 1.23 1.57 98.31 2.34 1.56 2.85 102.92 4.38 1.47 5.21 107.53 2.46 1.62 3.04 112.14 2.68 1.62 3.31 116.74 2.63 1.65 3.28 118.84 2.75 1.66 3.44 121.34 3.10 1.64 3.85
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表 3 研究区的附近区域强震的震源机制
Table 3. Focal mechanism of the strong earthquakes in the vicinity of the study area
日期 经度 纬度 深度 震级 走向1 倾角1 滑动角1 走向2 倾角2 滑动角2 破裂面长度 位错量/走滑 位错量/逆冲 1985-08-01 95.53°E 29.24°N 40.0 km MW5.7 176° 15° 153° 292° 83° 76° 6.49 km 0.102 m 0.408 m 1988-01-25 94.87°E 29.80°N 33.0 km MW5.2 37° 69° 159° 135° 71° 23° 3.84 km 0.164 m 0.063 m 2003-08-18 95.91°E 29.26°N 33.0 km MW5.5 65° 77° −6° 156° 84° −167° 6.10 km 0.237 m 0.025 m 2004-09-27 95.70°E 29.78°N 31.1 km MW4.9 126° 79° −176° 35° 86° −11° 2.41 km 0.129 m 0.009 m 2005-06-01 94.72°E 28.81°N 19.0 km MW5.8 209° 6° 26° 93° 87° 95° 7.57 km 0.040 m 0.451 m 2013-04-16 95.12°E 28.67°N 39.6 km MW4.9 300° 39° 90° 120° 51° 90° 1.88 km 0.000 m 0.236 m 2017-11-17 95.14°E 29.69°N 12.0 km MW6.5 119° 24° 72° 318° 67° 98° 22.34 km 0.105 m 0.746 m 2019-04-23 94.67°E 28.35°N 16.1 km MW6.1 208° 9° 21° 97° 87° 99° 12.04 km 0.088 m 0.557 m 2022-11-10 94.41°E 28.38°N 15.0 km MW5.6 226° 10° 43° 93° 83° 97° 5.56 km 0.048 m 0.389 m
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表 4 通过地应力实测结果推算研究区墨脱断裂带断层面上的应力值
Table 4. Estimation of the stress values on the fault plane of the Motuo fault zone in the study area based on the measured results of in-situ stress
深度/
m最大水平主
应力SH/MPa最小水平主
应力Sh/MPa垂向主应力
Sv/MPa正应力
σ/MPa剪应力
τ/MPa剪应力
τ’
/MPa61.43 4.58 3.42 1.63 3.16 4.04 0.71 70.65 3.05 2.16 1.87 2.16 2.70 1.14 79.87 3.68 2.89 2.12 2.80 3.24 1.22 89.09 3.72 3.03 2.38 2.95 3.27 1.41 98.31 7.54 4.84 2.65 4.60 6.71 1.28 102.92 14.50 9.87 2.78 8.86 12.87 0.47 107.53 8.84 5.47 2.91 5.20 7.88 1.37 112.14 10.06 6.22 3.04 5.86 8.97 1.34 116.74 10.42 6.31 3.18 5.98 9.30 1.44 118.84 11.13 6.69 3.24 6.32 9.94 1.41 121.34 12.74 7.75 3.31 7.23 11.37 1.27
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表 5 研究区断层面在区域强震影响下库仑应力变化
Table 5. Variation of Coulomb stress on the fault planes in the study area under the influence of regional strong earthquakes
深度/km 正应力
变化/bar剪应力
变化/bar
（左旋走滑）剪应力
变化/bar
（逆冲）库仑应力
变化/bar
（左旋走滑）库仑应力
变化/bar
（逆冲）0.10 −0.013 −0.004 0.003 −0.0086 −0.0016 1.00 −0.012 −0.003 0.003 −0.0072 −0.0012 5.00 −0.010 0.001 0.002 −0.0025 −0.0015 10.00 −0.008 0.002 0.002 −0.0008 −0.0008
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