Journal of Earth Science, Vol. 25, No. 5, p. 861–870, October 2014 ISSN 1674-487X Printed in China DOI: 10.1007/s12583-014-0473-z
The Stress State of the Beichuan-Jiangyou Segment of the Longmenshan Fault before and after the Wenchuan MS 8.0 Earthquake
Chengjun Feng, Qunce Chen*, Chengxuan Tan, Xianghui Qin, Peng Zhang, Wen Meng Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081, China; Key Laboratory of Neotectonic Movement & Geohazard, Ministry of Land and Resources, Beijing 100081, China
ABSTRACT: After the Wenchuan MS 8.0 Earthquake, which occurred on May 12, 2008, in Sichuan Province, China, we conducted a series of hydraulic fracturing stress measurements in three 200 m deep boreholes (ZK01, ZK02, and ZK03) drilled in Beichuan and Jiangyou regions near the northeas- tern segment of Longmenshan fault belt in 2009. These measurements revealed the near-surface stress
field in the fault region one year after the Wenchuan MS 8.0 Earthquake. However, the lack of the stress measurements before the earthquake in the same region makes it difficult to understand variations of the in situ stress field (near-surface) by comparative analysis. In order to determine the unknown hori- zontal principal stresses before the earthquake in Beichuan and Jiangyou regions, the following re- search method was tentatively applied. Firstly, we calculate the static co-seismic stress field by linear elastic finite element numerical simulation with ANSYS, based on the co-seismic static displacement
generated by the Wenchuan MS 8.0 Earthquake along the central Longmenshan fault plane in Beichuan and Jiangyou. Secondly, combining hydraulic fracturing measurements (after the earthquake) with the co-seismic stress (simulation), the magnitudes and orientations of horizontal principal stresses before the earthquake were calculated. Finally, the variation of the in situ stress (near-surface) in Beichuan
and Jinagyou, both before and after the Wenchuan MS 8.0 Earthquake, were obtained by comparative analysis. To do this the magnitude of SHmax was decreased on average by 13.01 and 6.54 MPa after the earthquake in ZK02 and ZK03, respectively and the magnitude of SHmin was decreased by 2.54 and 5.29 MPa in ZK02 and ZK03, respectively. Following the earthquake, the average direction of SHmax rotated anticlockwise by 42.5. KEY WORDS: Longmenshan fault belt, hydraulic fracturing, situ stress, co-seismic stress, numerical simulation.
0 INTRODUCTION of the maximum principal stress, which decreased from 12 MPa The magnitude and direction of the in situ stress in the before the earthquake to 3–4 MPa after the earthquake. After
Earth’s lithosphere have an important bearing on a host of the Tangshan MS 7.8 earthquake, which occurred on July 28, geophysical problems, such as plate driving and earthquake 1976, in the Hebei Province, China, Li et al. (1982) and Li and mechanisms and crustal movements (Haimson, 1978). The Wang, (1979) found the maximum principal stress (H<30 m), magnitude and orientation of maximum principal stress measured by overcoring method in the Phoenix Mountains Park (near-surface) can obviously change before and after large near the earthquake focal area, was oriented at N47W, which earthquakes in the vicinity of the earthquake focal area. For was not in accordance with the orientation of the regional tec- example, Liao et al. (2003) obtained the in situ stress measure- tonic stress field at N87W. The direction of the maximum ments (H<30 m) before and after the Kokoxili MS 8.1 earth- principal stress had greatly deviated after the earthquake. quake, which occurred on November 14, 2001, in Qinghai After the Wenchuan MS 8.0 Earthquake, which occurred Province, China. Using the overcoring method, in the central on May 12, 2008, in the Sichuan Province, China, in an attempt Qinghai-Tibet Plateau, they found a variation in the magnitude to investigate variation of the near-surface stress field, we con- ducted a series of hydraulic fracturing stress measurements in *Corresponding author: [email protected] three wells (~200 m) drilled in Beichuan and Jiangyou, near the © China University of Geosciences and Springer-Verlag Berlin northeastern segment of the Longmenshan fault belt in 2009. Heidelberg 2014 The purpose of this paper is twofold: to determine the near-surface stress field (~200 m) in the fault region one year
Manuscript received January 18, 2014. after the Wenchuan MS 8.0 Earthquake and to understand any Manuscript accepted June 15, 2014. variations of the in situ stress before and after the earthquake.
Feng, C. J., Chen, Q. C., Tan, C. X., 2014. The Stress State of the Beichuan-Jiangyou Segment of the Longmenshan Fault before and after the Wenchuan Ms8.0 Earthquake. Journal of Earth Science, 25(5): 861–870. doi: 10.1007/s12583-014-0473-z 862 Chengjun Feng, Qunce Chen, Chengxuan Tan, Xianghui Qin, Peng Zhang and Wen Meng
1 NEAR-SURFACE STRESS FIELD ONE YEAR AF- SHmax=3Ps−Pr−P0 (1) TER WENCHUAN M 8.0 EARTHQUAKE IN BEI- S S =P (2) CHUAN AND JIANGYOU AREAS Hmin s
1.1 Experimental Sites Sv=rH (3) The locations of the three wells drilled in the Beichuan where S is the maximum horizontal principal stress, S is and Jiangyou areas are shown in Fig. 1. General information of Hmax Hmin the minimum horizontal principal stress, S is the vertical prin- the boreholes is further provided in Table 1. v cipal stress which is considered to be approximately equal to
the weight of the overburden at the test depth; P is the pore 1.2 Hydraulic Fracturing Test 0 pressure, r is the rock density (taken as 2 650 kg/m3), and H is The hydraulic fracturing technique is recommended as a the depth. frequently used method to determine rock stress by the Com- The azimuth of the maximum horizontal principal stress is mission on Testing Methods, International Society of Rock the same as the orientation of vertical hydraulic fracture strike, Mechanics (Haimson and Cornet, 2003). Moreover, it remains determined by impression packers and an electromagnetic the best known method for determining the magnitude and compass. direction of subsurface horizontal principal stresses (Hayashi and Haimson, 1991). From the pressure-time records during a 1.3 Measurement Results hydraulic fracturing test, three critical pressure parameters Hydraulic fracturing measuring results of all three wells (breakdown P , reopening P , and shut-in P ) can be determined. b r s are listed in Table 2. Note that we calculated the shut-in pres- The magnitude of principal stresses magnitude can be subse- sure P using dP/dt, dt/dP, and the point of intersection between quently calculated using the following equations (Haimson, s the tangents (Feng et al., 2012; Haimson et al., 1989), with the 1989, 1987; Hubbert and Willis, 1957). values corresponding well with each other.
Figure 1. Locations of the in situ stress measurement wells, Wenchuan MS 8.0 earthquake and some of its aftershocks (2008.05.12–08.10). F1, 2, 3 Longmenshan fault belt: F1-1. Gengda-Longdong fault; F1-2. Wenchuan-Maoxian fault; F1-3. Ping- wu-Qingchuan fault; F2-1. Yanjing-Wulong fault; F2-2. Beichuan-Yingxiu fault; F2-3. Chaba-Linansi fault; F3-1. Da- chuan-Shuangshi fault; F3-2. Guanxian-Jiangyou fault; F3-3. Jiangyou-Guangyuan fault. F4. the predmont concealed faults; F5. Longqushan fault; F6. Minjiang fault; F7. Huya fault; F8. Wenxian fault; F9. Bailongjiang fault; F10. Maduo-Gande fault; F11. Maerkang fault; F12. Xianshuihe fault; F13. Yulongxi fault; F14. Tianquan-Yingjing fault; F15. Ebian fault.
The Stress State of the Beichuan-Jiangyou Segment of the Longmenshan Fault before and after the Wenchuan MS 8.0 Earthquake 863
Table 1 General conditions of three boreholes
Borehole Latitude Longitude Altitude Depth Description of rock types Vertical distance from number (N) (E) (m a.s.l.) (m) and integrity the central fault ZK01 3149′09″ 10444′58″ 583 200 Standstone and mudstone of Jurassic 21.5 age; well intact ZK02 3157′46″ 10439′59″ 628 200 Mudstone and limestone of Devonian 4.5 age; well intact ZK03 3157′17″ 10440′20″ 624 200 Mudstone and limestone of Devonian 6.5 age; well intact
Table 2 Hydraulic fracturing measurement results of three wells Borehole Depth Characteristic pressures (MPa) Stress value (MPa) Azimuth of maximum number (m) Pb Pr Ps Po SHmax SHmin Sv horizontal principal stress ZK01 85.50 12.31 5.51 4.20 0.86 6.23 4.20 2.27 N60E 104.50 9.24 5.02 4.06 1.05 6.11 4.06 2.77 123.50 13.75 6.46 4.78 1.24 6.64 4.78 3.27 N53E 134.50 12.46 7.32 4.88 1.35 5.98 4.88 3.56 152.50 12.22 5.52 5.29 1.53 8.82 5.29 4.04 N71E 163.47 13.35 8.75 6.19 1.63 8.19 6.19 4.33 178.50 16.01 9.72 7.39 1.79 10.66 7.39 4.73 N61E ZK02 58.00 5.32 2.47 1.82 0.53 2.78 1.82 1.54 80.00 7.27 4.44 2.75 0.75 3.06 2.75 2.12 N33E 91.85 11.07 7.03 4.18 0.87 4.64 4.18 2.43 N55E 117.00 9.05 5.05 3.70 1.12 4.93 3.70 3.10 N53E 124.00 8.75 6.34 4.58 1.19 6.21 4.58 3.29 N87E 133.00 10.51 6.98 4.61 1.28 6.52 4.61 3.52 185.00 13.13 10.57 6.48 1.80 7.07 6.48 4.90 195.00 9.03 7.60 5.35 1.90 6.55 5.35 5.17 ZK03 77.00 7.59 6.02 4.25 0.74 6.00 4.25 2.04 N33E 86.00 4.58 3.38 2.78 0.83 4.14 2.78 2.28 95.50 7.79 4.08 3.10 0.92 4.31 3.10 2.53 N37E 105.50 8.67 6.50 5.54 1.02 9.09 5.54 2.80 N73E 123.00 9.26 7.23 5.48 1.20 8.02 5.48 3.26 133.00 8.78 6.47 5.41 1.30 8.45 5.41 3.52 144.00 9.91 7.13 5.77 1.41 8.76 5.77 3.82 166.00 11.49 10.08 7.59 1.63 11.05 7.59 4.40 176.78 10.55 7.57 6.85 1.73 11.26 6.85 4.68 193.00 14.35 9.47 9.19 1.90 16.19 9.19 5.11
Significant characteristics of these data are as follows. (c) In borehole ZK03 (at 50.00–200.00 m) (1) Both horizontal principal stresses for all tests were SHmax=0.081 9H−1.920 3 MPa R=0.906 6 (8) larger than the vertical stress (SHmax>SHmin>Sv) (Fig. 2), sug- gesting a thrust faulting regime within the relatively shallow SHmin=0.041 5H−0.265 4 MPa R=0.916 0 (9) depths attained by our tests. where stress is in MPa, H is depth in meters, and R is the cor- (2) The magnitude of S varied between 5.98 and 10.66, Hmax relation coefficient. 2.78 and 6.55, and 4.14 and 16.19 MPa for ZK01, ZK02, and (3) The horizontal shear stress (being equal to half the dif- ZK03, respectively; S was in the range of 4.06–7.39, Hmin ference between the horizontal principal stresses) varied from 1.82–6.48 and 2.78–9.19 MPa, respectively. The magnitude of 0.55 to 1.64, 0.16 to 0.96, and 0.61 to 3.50 MPa in ZK01, the horizontal principal stresses clearly increased with depth ZK02, and ZK03, respectively. (Fig. 2), and the linear regressions with respect to depth are as Using equations (4)–(9), the magnitude of S and S follows. Hmax Hmin at 200 m in ZK01, ZK02, and ZK03 can be calculated. The
(a) In borehole ZK01 (at 50.00–200.00 m) magnitude of SHmax was 10.48, 7.58 and 14.46 MPa, respec- tively, while S was 7.41, 6.31, and 8.75 MPa. The horizon- S =0.045 4H+1.403 9 MPa R=0.845 5 (4) Hmin Hmax tal shear stress at 200 m, derived from a plot of
SHmin=0.032 9H+0.828 6 MPa R=0.919 2 (5) [½(SHmax–SHmin)], was 1.54, 0.63, and 2.85 MPa in ZK01, ZK02, and ZK03, respectively. It should be noted that the horizontal (b) In borehole ZK02 (at 50.00–200.00 m) shear stress at 200 m near the central Longmenshan fault gen-
SHmax=0.030 6H+1.458 8 MPa R=0.895 5 (6) erally increased with distance from the fault (on perpendicular planes to the strike of fault). This trend is in agreement with SHmin=0.027 7H+0.774 8 MPa R=0.915 4 (7)
864 Chengjun Feng, Qunce Chen, Chengxuan Tan, Xianghui Qin, Peng Zhang and Wen Meng results near the San Andreas fault belt (USA) (Zoback et al., situ stress field around the vicinity of the earthquake focal area, 1980) and Tancheng-Lujiang fault belt (China) (Li et al., 1982) which predominantly manifests as a fraction of maximum prin- (Fig. 3). cipal stress or a deflection of its orientation after the earthquake (4) Orientation of the measured maximum horizontal prin- (Liao et al., 2003; Li et al., 1982; Li and Wang., 1979). The cipal stress varied from N33E to N87E, with an average of Wenchuan MS 8.0 Earthquake ruptured two NW-dipping re- N60E±27 for depths of 50–130 m in ZK02 and ZK03, and in verse faults along the Longmenshan fault belt at the eastern ZK01 from N53E to N71E, with an average of N62E±9 for margin of the Tibetan Plateau (Li et al., 2009; Chen et al., depths of 80–180 m (Fig. 4). 2008). It further generated a 240 km long surface rupture along
The directions of SHmax in ZK01, ZK02, and ZK03 were the Beichuan-Yingxiu fault, characterized by right lateral obli- not in agreement with the directions measured by hydraulic que faulting, with maximum vertical and horizontal co-seismic fracturing techniques in Pingwu, Maoxian, and Wenchuan areas displacements of 6.2 and 4.9 m, respectively. Along the Guan- in 2004 (An et al., 2004) (Fig. 4), and with directions indicated xian-Jiangyou fault, characterized by a dip-slip reverse faulting, by focal mechanism solutions from the Wenchuan MS 8.0 it generated a 90 km long surface rupture with a maximum Earthquake and most of its aftershocks (MS 6.0–6.9, MS 5.0–5.9, vertical co-seismic displacement of 3.5 m (Wang et al., 2013; and MS 4.0–4.9) (Cui et al., 2010; Kusky et al., 2010) (Fig. 4). Li et al., 2008; Xu et al., 2008). There are 109 aftershocks of the Wenchuan MS 8.0 earthquake Assuming that the near-surface crustal stress at the foot- shown in Fig. 4, including 6 aftershocks with an MS 6.0–6.9, 21 wall of central Longmenshan fault was consistent before the aftershocks with an MS 5.0–5.9 and 80 aftershocks with an MS Wenchuan MS 8.0 Earthquake, the great co-seismic surface 4.0–4.9. displacement generated by the earthquake will certainly effect However, note that the compressive stress indicated by the in situ stress environment along and around the rupture focal mechanism solutions from 15 aftershocks (MS 4.0–4.9) zone. It is our inference that the direction of SHmax (near-surface) (located at the hanging wall of the central Longmenshan fault may have deflected after the Wenchuan MS 8.0 Earthquake, near the Beichuan area) is orientated in the NE-NEE direction, having been influenced by the co-seismic surface rupture in the as measured in ZK01, ZK02, and ZK03 (Fig. 4). Beichuan and Jiangyou areas. Cui et al. (2010) and Hu et al. (2008) concluded that these In the subsequent sections we study the influence of the
15 aftershocks (MS 4.0–4.9) may not have occurred along the earthquake on near-surface stress fields in Beichuan and central Longmenshan fault but some unknown secondary faults Jiangyou, and further analyze the variation of in situ stress near near Beichuan area, with an NW-NWW strike. Hence, the the Longmenshan fault belt before and after the Wenchuan MS orientation of the compressive stress from these earthquakes, as 8.0 Earthquake. indicated by focal mechanism solutions, differed from the di- rection of the regional tectonic compressive stress field. 2 CALCULATION OF NEAR-SURFACE IN SITU The boreholes used in this study (ZK01, ZK02, and ZK03) STRESS IN THE BEICHUAN AREA BEFORE THE were located at the footwall of the central Longmenshan fault. WENCHUAN MS 8.0 EARTHQUAKE Our field investigations found no secondary faults with a strike 2.1 Research Methods of NW-NWW in the Beichuan and Jiangyou areas. Therefore, it Because of a lack of in situ stress measurement results in is uncertain that whether or not these 15 aftershocks will have Beichuan and Jiangyou areas before the Wenchuan MS 8.0 any influence on the near surface stress fields in the Beichuan Earthquake, it is difficult to analyze the variation of the and Jiangyou regions at the footwall of the fault. near-surface stress field before and after earthquake by a com- A large earthquake generally has a greater impact on the in parative analysis. To solve this problem, we applied the
Figure 2. Linear regressions with respect to depth in three wells.
The Stress State of the Beichuan-Jiangyou Segment of the Longmenshan Fault before and after the Wenchuan MS 8.0 Earthquake 865
Figure 3. Horizontal shear stress at a depth of 200 m as a function of distance from the fault.
Figure 4. Direction of the maximum principal compressive stress indicated by hydraulic fractures (2004, 2009) and focal me- chanism solutions of the Wenchuan MS 8.0 earthquake and some of its aftershocks (MS 4.0–6.9) (2008.05.12–8.10). following research methods. (c) Normal compressive stress is assigned a positive value, (1) Some necessary and idealistic assumptions were used while tension stress is negative. Shear stress, which turns the in this approach. rock finite element in a clockwise rotation, is positive, while (a) It is assumed that the rock is homogeneous, isotropic, rotation in the opposite direction is negative. and initially impermeable. (d) A planar rectangular coordinate system x′oy′ is estab- (b) The vertical stress is assumed to be one of the principal lished, where the positive direction of x′ and y′ axis refers to the stresses, which is unchanged before and after the earthquake. direction of east and north, respectively (Fig. 5b). The other two principal stresses are horizontal. (2) Stress field (~200 m) in ZK02 and ZK03 before the
866 Chengjun Feng, Qunce Chen, Chengxuan Tan, Xianghui Qin, Peng Zhang and Wen Meng
Figure 5. Plane right-angle coordinate xoy and components of stress tensors. earthquake (unknown). b b b b b ( x' y' ) x' y' 2 b 2 (15) b min ( ) x'y' Assuming that σ max is the maximum horizontal principal 2 2 b stress, σ min is the minimum horizontal principal stress and θ0 is b b the angle between σ max and the x′ axis in x′oy′, the in situ stress and 2 x'y' . (16) tan20 b b tensor before the earthquake under plane stress conditions can x' y' be expressed as follows.
bb 2.2 Calculation of Stress Tensor B= xxy''' (10) bb 2.2.1 Numerical simulation of co-seismic stress tensors in yx'' y ' ZK02 and ZK03 (3) Co-seismic stress field (~200 m) in ZK02 and ZK03 (a) Three-dimensional finite element model 3D geological generated by the Wenchuan MS 8.0 Earthquake (can be obtained modeling, with a length of 10 km (from west to east) and width by numerical simulation): of 12 km (from south to north), was performed using ANSYS c Assuming that △σ max is co-seismic maximum horizontal by employing a 1 : 500 00 digital elevation model (Fig. 6a). c principal stress, △σ min is co-seismic minimum horizontal prin- The bottom surface of the model was at an altitude of -2.5 km. c cipal stress and θ1 is the angle between △σ max and the x’ axis The model was simplified as follows. in x’oy’, the co-seismic stress tensor under plane stress condi- ① The model included the locations of two boreholes tions can be expressed as follows. (ZK02 and ZK03) at the footwall of the central Longmenshan
c c fault. △ x' △ x'y' ② The central Longmenshan fault was regarded as a plane C= c c (11) △ y'x' △ y' with a strike of N40E, inclination of 310° and inclination an- (4) Stress field one year after the earthquake (～200 m) in gle of 60°. ③ ZK02 and ZK03 (Known). Heterogeneity of rock materials with respect to depth a was not considered in the model. However, the model was di- Assuming that σ max is the maximum horizontal principal a vided into two parts with different horizontal physical proper- stress, σ min is the minimum horizontal principal stress and θ2 is a ties (Fig. 6a). the angle between σ max and the x′ axis in x′oy′, the stress tensor ④ one year after the earthquake under plane stress conditions can From reference to previous experimental result of rock be expressed as follows. mechanics, the elastic modulus for each rock mass was 18 200 and 20 000 MPa, respectively. The Poisson ratio was 0.22 and a a x' x'y' 0.21, respectively. A= a a (12) y'x' y' A tetrahedral element with six nodes was used in a three dimensional finite element model, producing a total of 229 842 a a where σ max, σ min is equal to SHmax and SHmin, respectively, in elements and 48 719 nodes (Fig. 6b). ZK02 and ZK03. (b) Boundary conditions (5) The relation between each stress tensor: B + C = A The normal displacement on the northern, southern, east-
b b c c a a ern, western, and bottom boundaries of the model was equal to x' x' y' △ x' △ x'y' x' x'y' b b + c c = a a (13) y' y' zero. y'x' △ y'x' △ y' y'x' (c) Load conditions Thus, components of the stress tensor before the earth- Co-seismic vertical and horizontal displacement on the b b b b quake (σ x’, σ x’, τ x’y’, and τ y’ x’) can be calculated using equa- central Longmenshan fault plane in the Beichuan area was b b tion (13). Furthermore, σ max, σ min and θ0 (~200 m) in ZK02 about 2.5 and 3.0 m, respectively. As the faulting acted as a and ZK03 can be also determined from equations (14), (15), reverse and right-lateral strike-slip fault in Beichuan, a dis- and (16), respectively. placement of 3.0 and 2.5 m was therefore loaded on the central
b b b b Longmenshan fault plane along its strike and inclination, re- b ( x' y' ) x' y' 2 b 2 max ( ) x'y' (14) spectively (Fig. 6a). 2 2
The Stress State of the Beichuan-Jiangyou Segment of the Longmenshan Fault before and after the Wenchuan MS 8.0 Earthquake 867
Figure 6. Three dimensional finite element model (a) and division of the finite element mesh (b).
(d) Simulation results ( ) ( ) x y x y sin2 sin2 (19) ① Co-seismic maximum principal stress: Co-seismic b 2 2 xy maximum principal stress (~200 m) in ZK02 varied between ( ) 11.37 and 15.66 MPa at N70W (Figs. 7 and 8a), and was in the x y ab sin 2 xy cos 2 (20) range of 6.80–7.80 MPa at N65W in ZK03 (Figs. 7 and 8b). 2 The co-seismic maximum principal stress was tension stress, where the angle θ between a (or b) and x is positive when the with a dip of near 5 (noting that in ANSYS tension stress is direction of rotation from x to a is anticlockwise. Conversely, positive and compression stress is negative). Generally, the when the rotation is in the alternate direction it is negative. co-seismic maximum principal stress was approximately equal If the positive direction of the x axis is the same as maxi- to ᇞσc . max mum horizontal principal stress (SHmax) and the positive direc- ② Co-seismic intermediate principal stress: co-seismic tion of the y axis is also the same as the minimum horizontal intermediate principal stress, which is perpendicular to the principal stress (SHmin) (Fig. 5b), the stress tensor in xoy can be co-seismic maximum principal stress (~200 m), varied between expressed as follows 1.70 and 2.64 MPa at N20E in ZK02 (Figc. 9 and 8a), and a SHmax 0 max 0 x xy 3.83 and 4.12 MPa at N25E in ZK03 (Figs. 9 and 8b). The P= = 0 S = a (21) yx y Hmin 0 min co-seismic intermediate principal stress is also a tension stress, The components of the stress tensor in x′oy′ (σax′, σay′, with a dip of near 10. Generally, the co-seismic intermediate and τax′y′) can be then calculated using equations (18), (19), principal stress was approximately equal to ᇞσc . min and (20). The results are given in Table 4. ③ Components of co-seismic stress tensors in a plane stress state for the hydraulic fracturing tests with impression 2.2.3 Stress tensor and horizontal principal stresses before experiments in ZK02 and ZK03 were obtained from simulation the earthquake in ZK02 and ZK03 results (shown in Table 3). Using equations (13)–(16), the components of the stress
tensor in x′oy′ (σbx′, σby′, and τbx′y′) and the horizontal prin- 2.2.2 Calculation of stress tensor one year after the cipal stresses before the earthquake can be calculated. The re- earthquake in ZK02 and ZK03 sults are given in Table 5. It is assumed that the stress tensor at one point within the Significant characteristics of these data from Table 5 are as body, under a plane stress state, in the rectangular coordinate follows. system xoy can be expressed as (Fig. 5a). b a) With respect magnitude, the vertical stress (σ v) was overall the smallest stress at the ZK02 and ZK03 test locations, x xy P= (17) yx y suggesting a thrust faulting regime within the relatively shallow depth. Normal and shear stresses on an arbitrary oblique section b b b) At 80–124 m in ZK02, the magnitude of σ max and σ min with the outer normal direction of a or b (Fig. 5a) can be calcu- varied between 16.57 and 19.08 MPa and 5.24 and 6.86 MPa, lated using stress component transformation equations (18), b respectively. At 75–110 m in ZK03, the magnitude of σ max and (19), and (20) b σ min varied between 10.96 and 15.93 MPa and 8.27 and 10.49 ( ) ( ) MPa, respectively. These results reveal that the average of the x y x y cos2 sin2 (18) a 2 2 xy maximum horizontal principal stress in ZK02 closer to the fault is greater than the stress in ZK03 further from the fault.
868 Chengjun Feng, Qunce Chen, Chengxuan Tan, Xianghui Qin, Peng Zhang and Wen Meng
Figure 7. Co-seismic maximum principal stress near the fault (ZK02-ZK03 profile).
Figure 8. Directions of co-seismic maximum and intermediate principal stresses near (a) ZK02 and (b) ZK03.
Figure 9. Co-seismic intermediate principal stress near the fault (ZK02–ZK03 profile).
c) The horizontal shear stress varied between 5.34 and d) The maximum horizontal principal stress calculated 6.11 MPa, with an average of 5.69 MPa at 80–125 m in ZK02, before the earthquake in ZK02 and ZK03 was in a N70–85W and was between 1.06 and 2.72 MPa, with an average of 1.71 direction, in good agreement with the direction (N60W) de- MPa at 75–110 m in ZK03. This suggests that the average of termined by the hydraulic fracturing technique in Pingwu, horizontal shear stress in ZK02 closer to the fault is similarly Maoxian, and Wenchuan areas in 2004. It was also in agree- greater than the stress in ZK03 further from the fault. ment with the orientation indicated by the focal mechanisms
solutions from the Wenchuan MS 8.0 Earthquake and most of Table 3 Components of co-seismic stress tensor in ZK02 its aftershocks (MS 4.0–6.9). and ZK03 3 CONCLUSIONS Borehole Depth Stress tensor By comparative analysis of the horizontal principal c c c number (m) ᇞσ x’ (MPa) ᇞσ y’ (MPa) ᇞτ x’y’ (MPa) stresses in ZK02 and ZK03 before and after the Wenchuan MS ZK02 80.00 −12.45 −3.55 3.73 8.0 Earthquake, changes of the near-surface stress field in Bei- 91.85 −12.27 −3.50 3.68 chuan near the central Longmenshan fault were as follows. 117.00 −11.90 −3.38 3.57 (1) The near-surface stress state, being a thrust faulting re- 124.00 −11.80 −3.35 3.55 gime, did not changed before and after the earthquake. ZK03 77.00 −7.14 −4.74 1.43 (2) The magnitude of SHmax decreased after the earthquake 95.50 −7.12 −4.70 1.44 by an average of approximately 13.01 (74%) and 6.54 MPa 105.50 −7.11 −4.68 1.44 (51%) in ZK02 and ZK03, respectively. Whereas, the magni-
The Stress State of the Beichuan-Jiangyou Segment of the Longmenshan Fault before and after the Wenchuan MS 8.0 Earthquake 869
tude of SHmin decreased on average by 2.54 (41%) and 5.29 (4) The same was true for the average of horizontal shear MPa (56%) in ZK02 and ZK03, respectively. For the hori- stress (on a plane perpendicular to the strike of the Long- zontal shear stress, it decreased after the earthquake by an menshan fault). Before the earthquake it also decreased with average of 5.24 (92%) and 0.62 MPa (36%) in ZK02 and the distance from the central fault, however, increased with ZK03, respectively. the distance after the earthquake. (3) The average maximum horizontal principal stress (on (5) Direction of the maximum horizontal principal stress a plane perpendicular to the strike of the Longmenshan fault) before the earthquake varied from about N70W to N85W, decreased with the distance from the central fault before the averaging N77.5W±7.5. We conclude that the average di- earthquake, however, after the earthquake increased with rection of SHmax rotated anticlockwise by 42.5 after the distance from the fault. earthquake.
Table 4 Components of the stress tensor one year after the Wenchuan MS 8.0 Earthquake in ZK02 and ZK03
Borehole Depth Horizontal principal stress Stress tensor a a a a a a number (m) σ max (MPa) σ min (MPa) Direction of σ max θ2 σ x’ (MPa) σ y’ (MPa) τ x’y’ (MPa) ZK02 80.00 3.06 2.75 N33°E -57° 2.84 2.97 0.14 91.85 4.64 4.18 N55°E -35° 4.49 4.33 0.22 117.00 4.93 3.70 N53°E -37° 4.48 4.15 0.59 124.00 6.21 4.58 N87°E -3° 6.21 4.58 0.09 ZK03 77.00 6.00 4.25 N33°E -57° 4.77 5.48 0.80 95.50 4.31 3.10 N37°E -53° 3.54 3.87 0.58 105.50 9.09 5.54 N73°E -17° 8.79 5.84 0.99
Table 5 Components of the stress tensor and horizontal principal stresses before
the Wenchuan MS 8.0 Earthquake in ZK02 and ZK03
Borehole Depth Stress tensor Principal stresses b b b b b b b number (m) σ x’ σ y’ τ x’y’ θ0 Direction of σ max σ max σ min σ v (MPa) (MPa) (MPa) (MPa) (MPa) (MPa) ZK02 80.00 15.29 6.52 -3.59 -19.66° N70.34°W 16.57 5.24 2.25 91.85 16.76 7.83 -3.46 -18.90° N71.10°W 17.94 6.65 2.43 117.00 16.39 7.53 -2.98 -16.98° N73.02°W 17.30 6.62 3.10 124.00 18.00 7.93 -3.46 -17.25° N72.75°W 19.08 6.86 3.29 ZK03 77.00 11.91 10.22 -0.63 -18.40° N71.60°W 12.12 10.01 2.04 95.50 10.66 8.57 -0.86 -19.75° N70.25°W 10.96 8.27 2.53 105.50 15.89 10.53 -0.45 -4.78° N85.22°W 15.93 10.49 2.80
ACKNOWLEDGMENTS Pekinensis, 47(6): 1063–1072 (in Chinese with English We would like to thank Changxing Long, Shuwen Dong, Abstract) Chunting Liao, and Qimei An for their helpful discussions Feng, C. J., Chen, Q. C., Wu, M. L., et al., 2012. Analysis of and suggestions. This work was supported by the National Hydraulic Fracturing Stress Measurement Data— Scientific Program of China-Experimental Study on the Discussion of Methods Frequently Used to Determine Technique of In-Situ Stress Measurements and Monitoring Instantaneous Shut-In Pressure. Rock and Soil Mechan- (No. SinoProbe-06) and China Special Research Grant for ics, 33(7): 2149–2159 (in Chinese with English Ab- Non-profit Public Service (No. 201211096). stract) Haimson, B. C., 1978. The Hydrofarcturing Stress Measuring REFERENCES CITED Method and Recent Field Results. Int. J. Rock Mech. An, Q. M., Ding, L. F., Wang, H. Z., 2004. Research of Min. Sci. & Geomech. Abstr., 15: 167–178 Property and Activity of Longmen Mountain Fault zone. Haimson, B. C., 1987. Measurement of In Situ Stress. Me- Journal of Geodesy and Geodynamics, 24(2): 115–119 thods Exp. Phys., 24 (Part B): 377–408 (in Chinese with English Abstract) Haimson, B. C., 1989. Hydraulic Fracturing Stress Mea- Chen, G. H., Xu, X. W., Zheng, R. Z., et al., 2008. Quantita- surements. Int. J. Rock Mech. Min. Sci. & Geomech. tive Analysis of the Co-Seismic Surface Rupture of the Abstr., 26: 647–685 2008 Wenchuan Earthquake, Sichuan, China, along the Haimson, B. C., Cornet, F. H., 2003. ISRM Suggested Me- Beichuan-Yingxiu Fault. Seismology and Geology, thods for Rock Stress Estimation-Part 3: Hydraulic 30(3): 723–738 (in Chinese with English Abstract) Fracturing (HF) and/or Hydraulic Testing of Cui, X. F., Hu, X. P., Yu, C. Q., et al., 2010. Research on Pre-Existing Fractures (HTPF). Int. J. Rock Mech. Min. Focal Mechanism Solutions of Wenchuan Earthquake Sci. & Geomech. Abstr., 40: 1011–1020 Sequence. Acta Scientiarum Naturalium Universitatis Haimson, B. C., Tunbridge, L. W., Lee, M. Y., et al., 1989.
870 Chengjun Feng, Qunce Chen, Chengxuan Tan, Xianghui Qin, Peng Zhang and Wen Meng
Measurement of Rock Stress Using the Hydraulic Frac- with English Abstract) turing Method in Cornwall, U. K. Part II. Data Reduc- Li, H. B., Fu, X. F., Jerome, V. W., et al., 2008. Co-Seismic tion and Stress Calculation. Int. J. Rock Mech. Min. Sci. & Surface Rupture and Dextral-Slip Oblique Thrusting of the
Geomech. Abstr., 26(5): 361–372 MS 8.0 Wenchuan Earthquake. Acta Geologica Sinica, Hayashi, K., Haimson, B. C., 1991. Characteristics of Shut in 82(12): 1623–1643 (in Chinese with English Abstract) Curves in Hydraulic Fracturing Stress Measurements and Li, Y., Huang, R. Q., Zhou, R. J., et al., 2009. Geological the Determination of the In Situ Minimum Compressive Background of Longmenshan Seismic Belt and Surface Stress. J. Geophys. Res., 96: 18311–18321 Ruptures in Wenchuan Earthquake. Journal of Engineer- Hu, X. P., Yu, C. Q., Tai, K., et al., 2008. Focal Mechanism ing Geology, 17(1): 4–18 (in Chinese with English Ab- Solutions of Wenchuan Earthquake and Its Strong After- stract) shocks Obtained from Initial P Wave Polarity Analysis. Liao, C. T., Zhang, C. S., Wu, M. L., et al., 2003. Stress Change
Chinese J. Geophys., 51(6): 1711–1718 (in Chinese with near the Kunlun Fault before and after the MS 8.1 Kunlun English Abstract) Earthquake. Geophys. Res. Lett., 30(20): 2027–2030 Hubbert, M. K., Willis, D. G., 1957. Mechanics of Hydraulic Wang, L., Tian, W. B., Masoud, A., et al., 2013. Relationship Fracturing. Trans. Aime., 210: 153–166 between Remotely Sensed Vegetation Change and Frac- Kusky, T. M., Abduwasit, Ghulam, A., Wang, L., et al., 2010. ture Zones Induced by the 2008 Wenchuan Earthquake, Focusing Seismic Energy along Faults through China. Journal of Earth Science, 24(2): 282–296
Time-Variable Rupture Modes: Wenchuan Earthquake, Xu, X. W., Wen, X. Z., Ye, J. Q., et al., 2008. The MS 8.0 Wen- China. Journal of Earth Science, 21(6): 910–922 chuan Earthquake Surface Ruptures and Its Seismogenic Li, F. Q., Sun, S. Z., Li, L. Q., 1982. In Situ Stress Measure- Structure. Seismology and Geology, 30(3): 597–629 (in ments in North China and Tancheng-Lujiang Fault Zone. Chinese with English Abstract) Chinese Journal of Rock Mechanics and Engineering, 1(1): Zoback, M. D., Tsukahara, H., Hickman, S., et al., 1980. Stress 73–86 (in Chinese with English Abstract) Measurements at Depth in the Vicinity of the San Andreas Li, F. Q., Wang, L. J., 1979. Stress Measurements in North Fault: Implications for the Magnitude of Shear Stress at China. Acta Geophysica Sinica., 22(1): 1–8 (in Chinese Depth. J. Geophys. Res., 85: 6157–6173