Stress state of the Baoxing segment of the southwestern Longmenshan Fault Zone before and after the Ms 7.0 Lushan earthquake

Accepted Manuscript Stress State of the Baoxing Segment of the Southwestern Longmenshan Fault Zone before and after the Ms 7.0 Lushan Earthquake Manlu Wu, Chongyuan Zhang, Taoyuan Fan PII: DOI: Reference:

S1367-9120(16)30028-1 http://dx.doi.org/10.1016/j.jseaes.2016.02.004 JAES 2635

To appear in:

Journal of Asian Earth Sciences

Received Date: Revised Date: Accepted Date:

12 August 2015 8 January 2016 16 February 2016

Please cite this article as: Wu, M., Zhang, C., Fan, T., Stress State of the Baoxing Segment of the Southwestern Longmenshan Fault Zone before and after the Ms 7.0 Lushan Earthquake, Journal of Asian Earth Sciences (2016), doi: http://dx.doi.org/10.1016/j.jseaes.2016.02.004

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Stress State of the Baoxing Segment of the Southwestern Longmenshan Fault Zone before and after the Ms 7.0 Lushan Earthquake

Manlu Wua,b, Chongyuan Zhanga,b,*, Taoyuan Fana,b a

Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081,

China b

Key Laboratory of Neotectonic Movement and Geohazard, Ministry of Land and

Resources, Beijing 100081, China * Corresponding author at: No.11 Minzu University South Road, Haidian District, Beijing, China. Tel.: 86 010 88815175; fax: 86 010 68422326. E-mail address: [email protected] (Chongyuan Zhang).

1

Stress State of the Baoxing Segment of the Southwestern Longmenshan Fault Zone before and after the Ms 7.0 Lushan Earthquake

Abstract: In situ stress measurements were conducted with hydraulic fracturing and piezomagnetic overcoring method in a borehole at Qiaoqi of Baoxing region in the southwestern Longmenshan Fault Zone, to understand the current stress state and stress change after the Ms 7.0 Lushan earthquake. The stress regime of the Qiaoqi borehole is characterized by SH>Sh>Sv, indicating that the regional stress field is dominated by the maximum horizontal stress and this stress regime is prone to reverse faulting. Impression tests show that the orientations of the maximum horizontal principal stress are NW-NWW oriented. The magnitudes of the maximum horizontal principal stress at Qiaoqi are obviously higher than those before the Lushan earthquake, signifying that stress is still accumulating in this region. The real-time stress monitoring data shows that the stress in the NWW direction is increasing continuously before and after the earthquake. Authors have computed the frictional parameter, μm, using the derived stress data. The result demonstrates a high stress build-up level in the shallow crust before and after the Lushan earthquake. Adopting the Coulomb frictional-failure criteria, we conclude that Baoxing area, the southwestern section of the Longmenshan Fault Zone has already reached or exceeded a frictional limit equilibrium state. Evidence shows that the Lushan earthquake did not release the highly accumulated stress of the southwestern Longmenshan Fault Zone and the potential risk of earthquakes in this region still exists. Keywords: Southwestern Longmenshan Fault Zone, In situ stress measurements, 2

Coulomb frictional-failure criterion, Stress accumulation, The Lushan earthquake 1 Introduction On April 20, 2013, an Ms 7.0 earthquake occurred in Lushan, Ya'an, in the southwest segment of the Longmenshan Fault Zone. This event is one of the several strong earthquakes that have occurred since the devastating Ms 8.0 Wenchuan earthquake. Substantial knowledge has been gained recently by studying the focal mechanism, seismic rupture process, surface rupture, post-earthquake stress variation and correlations with the Wenchuan earthquake (Chen et al., 2013a; Dong et al., 2008; Ma et al., 2009; Wang et al., 2013; Xu et al., 2009; Zeng et al., 2013; Zhang et al., 2009; Zhang et al., 2013; Zhang, 2013a, 2013b). The focal mechanisms of the two events have been found to have similar thrust-type mechanisms. However, the Lushan earthquake differs obviously from the Wenchuan earthquake in the rupture process, spatial distribution of the aftershocks, and surface ruptures (Chen et al., 2013a). The Lushan earthquake did not produce surface ruptures, whereas the Wenchuan earthquake caused surface ruptures on both the central fault and the frontal fault of the Longmen Mountains. The long axis in the aftershock area of the Lushan earthquake extends approximately 40 km from NE to SW and is associated with a small-scale rupture. Nevertheless, the ruptures in the southwestern section of the Longmenshan Fault Zone are insignificant. In contrast, the aftershock area of the Wenchuan earthquake extended up to 304 km and is associated with a large-scale rupture that generated a lot of ruptures along the central-northern segment of the Longmenshan Fault Zone (Dong et al., 2008; Zhang et al., 2009; Zhang, 2013a). In situ stress measurements play an essential role in geodynamic process study and 3

knowledge of stress measurements provides the most useful information concerning the forces responsible for various tectonic processes, such as earthquakes (McGarr and Gay, 1978; Liao et al., 2003). Many in situ stress measurement campaigns along the Longmenshan Fault Zone have been conducted since the Wenchuan earthquake to understand what shallow stress level remains and how the stress field interacts with active faults (Chen et al., 2012; Feng et al., 2015; Guo et al., 2009; Meng et al., 2015; Qin et al., 2015; Sun et al., 2014; Wu et al., 2009, 2013). Stress data measured in 16 200-600 m deep boreholes was obtained until the Lushan earthquake (Meng et al., 2015). Given the obvious diversity in the in situ stress state, Meng et al. (2013) divided the Longmenshan Fault Zone into three segments along its strike, the northern segment, the middle segment and the southern segment. The middle segment is the transition of the other segments. The magnitudes of horizontal principal stress in the northern segment are apparently higher than those in the southern section in a depth of 50-100 m, while the characteristic is reversed at depth of 100-200 m. Generally, the magnitudes of horizontal principal stresses in the southern section are greater than those in the northern part and their difference becomes even larger when depth increases. Chen et al. (2012) adopted Coulomb frictional-failure criterion to find that, in the northeastern section of the Longmenshan Fault Zone, the in situ stress values at the hanging wall are much higher than those at the foot wall and the fault activity is apt to be triggered. Wu et al. (2009) and Qin et al. (2013) analyzed in situ stress data in some areas of the southwestern Longmenshan Fault Zone, such as in Kangding and Baoxing, and found that in situ stress had accumulated, reaching relatively high level. Abundant obtained data has shown that in the area of the southern 4

Longmenshan, especially at its southwestern section, in situ stress demonstrated a high stress level. Thus, the risk of the occurrence of moderate or strong earthquakes was believed increasing. Unfortunately, this prediction was confirmed by the Lushan earthquake (Parsons and Segou, 2014). Earthquake activity is a geodynamic process, in which stresses of lithospheric masses increase to produce instability of the crustal rock and ultimate failure and energy release (Liao et al., 2003). High stress level is an essential condition for earthquake generation, and the Lushan earthquake did not come as a surprise in the view of in situ stress study. Hence, after the Lushan earthquake, it is interesting and of necessary to measure in situ stress state in key part of the southwestern Longmenshan Fault Zone and investigate the potential risk of earthquakes. In this paper, we study this issue by combining the in situ stress data obtained before and after the earthquake with the real-time stress monitoring data before and after the Lushan earthquake. 2 Geology setting The Baoxing region makes up the southwestern section of the Longmenshan Fault Zone and is overlain by the Baoxing Massif which mainly contains the Precambrian crystalline rocks (Cook et al., 2013; Zhang et al., 2013). The Ms 7.0 Lushan earthquake occurred at the southeastern margin of the Baoxing Massif, while the Ms 8.0 Wenchuan earthquake at the southern margin of the Pengguan Massif (Cook, et al., 2013). The southwestern segment of the Longmenshan Thrust Belts consists of three approximately parallel NW-dipping active faults that are the Gengda-Longdong Fault, the Yanjing-Wulong Fault, and the Shuangshi-Dachuan Fault (Burchfiel et al., 1995; Zhan et al., 2013). The 5

Yanjing-Wulong Fault consists of many parallel sub-faults, trending N40°-50°E and with a total length of 160 km from south Yingxiu to east Luding. The Shuangshi-Dachuan Fault stretches from Dayi to Tianquan, about 140 km long, trending N40°-50°E (Chen et al., 2013) (Figure 1). Many researchers believed that the seismogenic fault of the Lushan earthquake is the Shuangshi-Dachuan Fault while some declared that the Lushan earthquake is a typical blind earthquake controlled by the buried Dayi Fault (Xu et al., 2013; Zhang et al., 2013). However, the results of focal mechanism solution and GPS reveal that the Lushan earthquake has a similar earthquake nucleation environment as that of the Wenchuan earthquake (Liu et al., 2015; Luo et al., 2015). The southwestern Longmenshan Belts characterize a series of high-dip imbricate thrust-nappe faults and indicate an inconspicuous movement rate across the entire Longmenshan Faults (Zhang et al., 2015). 3 In Situ Stress Measurement 3.1 Hydraulic fracturing Hydraulic fracturing is a borehole test method to obtain the state of in-situ stress in the Earth's shallow crust, including both the magnitude and orientation (Amadei and Stephansson, 1997). After firstly performed in oil field for stimulating production, it has been continuously improved in several decades, both theoretically and technically. It is one of two major in-situ stress estimation methods suggested by the International Society for Rock Mechanics (ISRM) (Haimson and Cornet, 2003). In a vertical borehole, the vertical stress is taken as being principal and equal to the overburden weight. Given that the rock mass of test interval is linearly elastic, 6

homogeneous, and isotropic, it can be deduced the maximum and the minimum horizontal stresses in the plane perpendicular to the borehole axis. The measurements are based on the principle of sealing off a test interval of 70 mm-100 mm in a borehole using a pair of inflatable straddle rubber packers that are sufficiently pressurized so that they can adhere to the borehole wall. Hydraulic fluid (water commonly) is pumped into the test interval from water tank using a pump with maximum pressure of 30 MPa-50 MPa, rapidly pressurizing the borehole wall until fractures are initiated in the rock. Typical fractures are two parallel traces that emerge on the borehole wall symmetrically. Generally, four cycles of re-fracturing tests are performed by injecting water into the test interval again until the previously closed fractures are fully reopened. An impression test is carried out with an impression packer with the same diameter as straddle packer when hydraulic fracturing tests are finished. High pressure squeezed sulfonated rubber layer into induced fractures and the orientation of fractures traced on the packer are estimated by electrical compass. In the actual field operation, the key pressure parameters, including the breakdown pressure (Pb), instantaneous shut-in pressure (Ps), and reopening pressure (Pr). The values of these parameters are taken on the ground, and the real pressures at test intervals should be corrected by adding the hydraulic pressure of the drill pipe height. The maximum and the minimum horizontal principal stress (SH and Sh), and the vertical principal stress (Sv) can be calculated using the following equations: SH  3Ps  Pr  Pp

(1)

Sh  Ps

(2)

SV   H

(3) 7

where Pp is the pore pressure, γ is the unit weight of the overlying rock, and H is the depth below the surface. In low permeability rock formation, Pp is nearly equal to the hydrostatic pressure (Barton et al, 1995; Haimson and Doe, 1983; Moos and Zoback, 1990; Zoback and Haimson, 1982). The test equipment, a single-loop hydraulic fracturing system used for stress measurements were developed by the authors' institute. A pump with a maximum working pressure of 50 MPa is used to convey water to borehole. Commonly used packers have a diameter of 72 mm for boreholes with a diameter of 76 mm. For some boreholes with a big diameter of 134.5 mm, packers with a diameter of 130 mm are suitable (in this study). A mechanically controlled transfer valve on the top of the packer is used to switch from packer pressurization to injection into the test interval. A pressure is employed to give visual real-time information. A pressure transducer with an accuracy of 0.01 MPa is used to acquire the pressure data and then transmit to a recording unit, including a 12 bite A/D (analog/digital) board and a computer. Thus, pressure-time curves are recorded at the same time. 3.2 Data processing and interpretation of hydraulic fracturing The raw data of hydraulic fracturing measurements are pressure-time curves. Those critical pressure parameters, the breakdown pressure Pb, the re-open pressure Pr, and the shut-in pressure Ps are derived from the curves to calculate horizontal principal stresses. The Pb is taken as the peak pressure in the first pressure cycle. The Pr is marked as the pressure in a re-opening cycle where the test interval pressure versus time curve departs from its tangent slope. However, there are uncertainty factors in determining the 8

maximum principal stress by hydraulic fracturing using the re-opening pressure (Ito et al., 1999; Rutqvist et al., 2000). The apparent reopening pressure detects in the conventional manner is larger than the true reopening pressure. To minimize the error between the true and the apparent re-opening pressure in the tests, we used stiff drill rods to inject water into the test interval. Also, the test intervals are relatively shallow (Ito et al., 1999; Zoback, 2007). The Pr is the pressure reached when the hydraulically induces fracture closed (Haimson and Cornet, 2003). On one hand, the Pr is considered equal to the minimum horizontal stress for vertical boreholes (Eq. (2)); on the other hand, its estimation error will expand three times according the Eq. (1). Hence, it is of great importance to see that Pr and Ps are estimated correctly to the utmost. ISRM has suggested several methods to determine Ps value. It has to note that it seems make little sense for a classical pressure-time curve whose obvious and clear inflection point is detectable even to the naked eye, see Figure 2. When a curve of re-opening cycle is hard to determine the final Ps value, nevertheless, it is necessary to adopt a combination approach of more than two processing methods. After the first breakdown cycle is completed, four subsequent re-opening cycles are repeated to sufficiently induce the fractures and accommodate the local stress field. However, the last three cycles are slightly different in most cases and in this study, we use them all to better determine the Ps value and finally an average result is adopted. The dt/dP versus pressure (Cheung and Haimson, 1989; Hayashi and Haimson, 1991), dP/dt versus pressure (Zoback and Haimson, 1982; Lee and Haimson, 1989), and Muskat method (Aamodt and Kuriyagawa, 1981) which are the ones recommended by the ISRM 9

(Haimson and Cornet, 2003) have been all adopted, see Table 1. The Pr is also an average result calculated with the last three cycles (Table 2). 3.3 Piezomagnetic overcoring method Piezomagnetic overcoring method is a stress relief method which was first proposed by Hast (1958) and has been improved by the authors' institute ever since 1960s (Wang et al., 1986). It is a 2D stress measurement method and mainly applied to measure the stress in the subsurface to a depth less than 200 m. This method has been tested in the laboratory and in the field with measurement error of less than 10% for the magnitude and 3° for the direction of the principal stress (Wang et al., 1991). Piezomagnetic overcoring has been widely used to conduct shallow stress measurement in engineering construction activities such as tunneling, mining, and hydropower projects in China (Liao et al., 2003； Wu et al., 2005, 2009; Zhang et al., 2007). The key part of the piezomagnetic overcoring measurement system is a three-component piezomagnetic stressmeter that consists of three groups of permalloy cells that meet at 60° angle of neighboring cells. These cells are designed based on the principle of magnetostriction. The permalloy ferromagnetic material, its length and volume can change when magnetized, that is called linear magnetostriction and volume magnetostriction, respectively. Conversely, mechanical deformation can also make the magnetism of the ferromagnetic material changed, which is called piezomagnetic effect. The sensitive cell of piezomagnetic stressmeter, actually, works as a self-induction coil. If a stable alternating current is passed through the cell, when the pressure put on the core parallel to its axis changes, the magnetostriction of the core changes, and then the 10

potential across the core drops. Of course, the pressure change expresses as the change of voltage received by the acquisition instrument (Zhang et al., 2014). A biaxial test, also called calibration test, can determine the linearly relationship between voltage readings of three measurement cells and pressure acted by an oil pump. Therefore, the reading changes of stress measurement at a site can change into pressure variations at different directions. The theory principle of piezomagnetic overcoring is based on the borehole deformation in the plane stress state. In a test point of the vertical borehole, three pressure values and three directions together can calculate the maximum and the minimum horizontal principal stress, and the orientation of the maximum horizontal principal stress. If more than one successful overcoring test is done at near depth, for example, one meter, more accurate stress result can be calculated by least square method. 4 Results of In Situ Stress Measurements After the 2008 Wenchuan earthquake, we drilled two boreholes (BX and QQ) in this region to conduct stress measurement (Wu et al., 2009, 2013) and stress monitoring (Zhang et al., 2014). The results show that the stress is relatively high in this area and is still increasing in terms of stress monitoring data of the Qiaoqi station. To understand the stress state in this area after the Lushan earthquake and compare the pre- and post-earthquake stress states in the southwestern Longmenshan fault, we drilled the third borehole (QQ-1) for stress measurements at the Qiaoqi Power Plant, which is located 30 km from Baoxing County town, in December 2014 (Figure 3). The results of the in situ stress measurements from all three boreholes are shown below. 11

Pre-earthquake measurements at Baoxing (BX) (Wu et al., 2009): This site is located at Muping Town, Xueshan Village in Baoxing on the footwall of the Wulong-Yanjing Fault (Figure 3 and 4). The borehole was drilled through granite to a depth of 400 m. The water level in the borehole is near the orifice. The diameter of the borehole is 130 mm at the depth of less than 120 m. Piezomagnetic overcoring measurements were only performed in the borehole segment from 18 m to 22 m due to poor rock integrity. At depths of more than 120 m, the borehole has a diameter of 76 mm and has good rock integrity. Therefore, hydraulic fracturing tests were performed in eight intervals, and impression tests were performed in three intervals. The results of the measurements are shown in Table 3. The maximum and the minimum horizontal principal stress at this location are determined to be 9.8 MPa and 7.9 MPa by piezomagnetic overcoring, respectively, in the intervals from 18 m to 22 m. The orientation of the maximum horizontal principal stress is N51°W. At a depth of 354 m, the maximum and the minimum horizontal principal stresses are 25.67 MPa and 15.75 MPa, respectively. The orientations of the maximum principal stress measured in all three impression tests are NW-NWW. The stresses measured in the joint position of the southwestern Longmenshan Fault Zone and the Xianshuihe Fault before the Wenchuan earthquake indicate that the maximum horizontal principal stresses are in the critical state to lead to reverse faulting (An et al., 2004). The maximum horizontal principal stress at this location is also estimated to be relatively high and is characterized by SH>Sh>Sv, implying that the region stress field is controlled by the maximum horizontal stress. 12

Pre-earthquake measurements at Qiaoqi (QQ) (Wu et al., 2013): This site is located at the Huaneng Qiaoqi Power Plant, which is approximately 30 km from Baoxing County town and is on the hanging wall of the Yanjing-Wulong Fault (Figure 3 and 4). The borehole is 220 m in depth and was drilled through gray black quartz schist. The water level is 3.5 m deep. The borehole has a diameter of 130 mm above 53 m and 76 mm below 53 m. Due to the existence of fractures, the rock integrity is worse above 70 m. Hydraulic fracturing measurements were performed in seven intervals below 100 m, and impression tests were finished in two intervals. The results of the measurements are shown in Table 4. The stress below 200 m is relatively high at this location. The maximum and the minimum horizontal principal stresses at the depth of 201 m are 18.63 MPa and 13.13 MPa, respectively. At 214 m, they are 25.67 MPa and 15.75 MPa, respectively. The orientations of the maximum horizontal principal stress are oriented between NW-NWW. The stress regime at this location is SH>Sh>Sv, stating that the horizontal stress plays a dominated role. Post-earthquake measurements at Qiaoqi (QQ-1): This site is located at the Huaneng Qiaoqi Power Plant as well, approximately 40 m away from the site of the pre-earthquake measurement. This site is located on the hanging wall of the Yanjing-Wulong Fault (Figure 3 and 4). The objective of the post-earthquake stress measurements here is to allow comparative analyses with the results from the pre-earthquake measurements at the Qiaoqi Power Plant (QQ) and in Muping Town (BX). The borehole is 200 m deep and was drilled in gray and white quartz schist. The water level in the borehole is 7 m. The hole is 168 mm in diameter at depths above 13 m and 134.5 mm in diameter below 13 m. The coring 13

recovery rate is near 100%, and the rock quality designation (RQD) values of the rock core at depths of 62 m-78 m, 85 m-110 m, 120 m-160 m, and 175 m-200 m are all greater than 95%. Piezomagnetic overcoring measurement was performed in only one segments at 174 m. Figure 5 shows the typical overcoring and calibration curve, which provides three groups of high-quality data and thus ensuring a reliable measurement result. Additionally, successful hydraulic fracturing measurements were conducted in five intervals (Figure 6), and impression tests were performed in four intervals. The final results are shown in Table 5. The maximum horizontal principal stresses obtained by hydraulic fracturing in five intervals between 128 m to 188 m are all greater than 21.00 MPa except the 136 m test interval, 19.60 MPa. The maximum and the minimum horizontal principal stresses at 182 m are 25.83 MPa and 18.47 MPa, respectively. Meanwhile, result of piezomagnetic overcoring measurement at 174 m shows that the maximum and the minimum horizontal principal stresses are 25.30 MPa and 14.90 MPa, which is close to the result of hydraulic fracturing test at 182 m. The stress regime is SH>Sh>Sv as well, indicating that the maximum horizontal stress still definitely dominates the stress field of Baoxing region. Compared with the pre-earthquake measurements from the Qiaoqi Power Plant and Muping Town, the maximum horizontal principal stresses at approximately 200 m are significantly higher. The orientations of the maximum principal stress obtained in the impression tests at this location are between NW-NWW, which is consistent with the direction determined before the earthquake. 5 Discussions 14

5.1 Magnitude of the maximum horizontal principal stress The pre- and post-earthquake in situ stress measurements in Baoxing region demonstrate that the stress state changes significantly (Tables 3-5). It is notable that because of the effect of topography and the existence of rock microfissures and anisotropy, the results of intact rock at shallow depth can show discreteness to some extent. Therefore, only the data deeper than 100 m are considered reliable tectonic stress values and are adopted for the comparative analysis (Zang and Stephansson, 2010). In the pre-earthquake measurements, the maximum horizontal principal stresses at Muping Town and the Qiaoqi Power Plant are 25.67 MPa and 23.73 MPa, respectively, whereas in the post-earthquake measurements at the Qiaoqi Power Plant, all the maximum horizontal principal stresses measured between 128 m and 188 m from the two methods are greater than 21 MPa except the 136 m test interval of 19.60 MPa. In fact, this magnitude is also relatively high for such a shallow depth. The maximum horizontal principal stress at 182 m is as high as 25.83 MPa. The magnitudes of in situ stresses in two nearly boreholes at Qiaoqi site, the hanging wall of the Yanjing-Wulong Fault, before and after the Lushan earthquake manifest that in the similar depths, the maximum horizontal principal stresses increase significantly. Note that at Qiaoqi borehole before the earthquake, the magnitudes of the maximum horizontal principal stress above 174 m are 5-7 MPa, and relatively low in such depths. The existence of fractures or broken zones may influence the magnitudes of in situ stress. However, after the earthquake, at nearby borehole (40 m away), the magnitudes of the maximum horizontal principal stress have exceeded those before the earthquake in the whole borehole depth. It seems that the Lushan earthquake did not relieve the stress 15

concentration in this region. In situ stress measurements of this fault zone had been completed for approximately 20 years before the Lushan earthquake in tens of boreholes, which were mainly concentrated in Kangding, Erlangshan, and Feixianguan of Lushan (Qin et al., 2013; Wang et al., 2015). These sites are situated near south of the southwestern Longmenshan Fault Zone and closer to the joint part of Longmenshan Fault Zone and the Xianshuihe Fault Zone (Figure 4). At depths from 150 m to 400 m, the maximum horizontal principal stresses are found to be ~18 MPa, <20 MPa, and ~25 MPa at Erlangshan, Feixianguan, and Kangding, respectively, suggesting a moderate-high in situ stress level in the southwestern Longmenshan Fault Zone. After the Wenchuan earthquake, a four-component piezomagnetic stressmeter was installed in the Qiaoqi borehole to monitor the variation of long-term stress after the in situ stress measurement campaign was finished. Being different from piezomagnetic overcoring stress measurement, piezomagnetic stress monitoring does not need a overcoring, but only installs a measuring stressmeter into borehole at a proper pre-stress and then records the reading changes of four groups of cells (Zhang et al., 2014). The system consists of a four-component piezomagnetic monitoring stressmeter with its diameter of 90 mm, stressing system, orienting systems, controlling system, and data transmission system. The piezomagnetic monitoring system used at Qiaoqi site was introduced in detail by Zhang et al. (2014). The measuring cells of this pre-stressing instrument contact the borehole wall directly after stressing. The orientations of the four measuring components are in N60°E, N75°W, N30°W, and N15°E directions for cell 1-4, 16

respectively (Figure 7). The hour data of this monitoring station from March 1, 2013 (before the Lushan earthquake) to June 25, 2013 (after the earthquake) are analyzed, suggesting that the crustal stress in NWW direction (cell 2) shows an obvious increase before and after the Lushan earthquake (Zhang et al., 2014). In Figure 7, we just obtained and showed the stress changes in a short period. The reading of NWW direction cell has shown a relatively high increase rate before and after the earthquake, indicating that there was a compressional force in the NWW direction. Of course, this force cannot increase all the time but show a background of stress change before and after the earthquake. Further study about stress changes of piezomagnetic stress monitoring in Qiaoqi station is necessary. 5.2 Orientation of the maximum horizontal principal stress Large earthquakes can lead to the change of stress state in a region, including the significant variation of the orientation of the maximum principal stress from that of regional tectonic stress field before the earthquake (Liao et al., 2003; Lin et al., 2013). After adjustment over a period of time, the orientation of the maximum principal stress will revert to the original stress field (Li et al., 1982). In situ stress measurements were performed before the Wenchuan earthquake in the region of Wenchuan and Erlangshan in the middle-southern of the Longmenshan Fault Zone and determined the orientations of the maximum horizontal stresses to be about N60°W (An et al,. 2004). After the Wenchuan earthquake, NW-NWW orientations were measured in Baoxing, Qiaoqi and Kangding region of the southwestern Longmenshan Fault Zone. After the Lushan earthquake, impression tests conducted in Qiaoqi borehole 17

show that the directions of maximum horizontal stress are oriented between NW-NWW (Wu et al., 2013). The focal mechanism solution of the Lushan earthquake was calculated, stating that the event is a high-angle thrust earthquake with a principal compressional stress axis orientation of ~120° (i. e. N60°W ) and a dip angle of ~10° (Chen et al., 2013b; Wang et al., 2013). It can be inferred that the orientation of the principal stress of the southwestern Longmenshan faults remains relatively stable before and after the Lushan earthquake, and still dominated by NW-NWW stress, reflecting background stress state controlled by the southeastward extrusion of the Tibetan Plateau (Clark and Royden, 2000; Royden et al., 2008). 5.3 Analysis of fault activity The Earth's crust is in a critical equilibrium state as the faults slip or earthquakes happen （Townend and Zoback, 2000）. According to Coulomb frictional-failure criterion, Byerlee (1978) carried out laboratory experiments on a lot of different type rocks and derived the frictional coefficient, μ, of 0.6-1.0, commonly known as Byerlee' law. The strength of the critically stressed fault can be estimated by evaluating the limited frictional coefficient. It is suggested that the stress states obtained from in situ stress measurements in the upper crust accord with the failure equilibrium (Zoback, 2007). The ratio of the maximum effective principal stress to the minimum effective principal stress, that corresponds to the case in which a critically orientated fault is at the frictional limit is given by Jaeger and Cook (1979) S1  Pp S3  Pp





1  2  



2

(4)

Where S1 and S3 are the maximum and the minimum principal stresses, respectively. 18

According to Anderson's faulting theory, Eq. (4) should change into the following form for thrust fault stress regime such as stress state of the southwestern Longmenshan Fault. Then the thrust fault will slip when S H  Pp SV  Pp





1  2  



2

(5)

If the crustal stress action reaches the frictional strength of the critically stressed fault, the frictional sliding will occur along the optimally orientated plane, which can be given by

1      tan 1   2 2 

(6)

where  is the angle between the normal of a fault plane and the direction of the maximum horizontal principal stress. For a cohesionless fault, if it reaches the critically stressed state depends on how large the ratio of effective shear stress to effective normal stress. We can use a non-dimensional parameter, μm, the ratio of the maximum effective shear stress to the mean effective principal stress to describe the failure state of the crust (Jamison and Cook, 1980). Wang et al. (2015) considered that μm has a similar physical meaning with the frictional coefficient, μ and suggested adopting μm to reflect the capability and stress level of the upper crust as well as analyze stress build-up in the shallow. The greater the μm is, the higher the shear stress accumulates, and thus the stress state is easier to cause fault sliding. Hence, μm can be regard as a frictional parameter to weigh the build-up level of crustal stress and then study fault activity (Jamison and Cook, 1980). When taking account of pore pressure and then μm can be described by 19

m 

S1  S3 S1  S3  2 Pp

(7)

Figure 8 shows the variation of the in situ stresses with depth at three boreholes on the southwestern Longmenshan Fault Zone in Baoxing before and after the Lushan earthquake. All the maximum horizontal principal stresses increase with depth and are characterized by SH>Sh>Sv, indicating that the regional stress field is controlled by the horizontal stress. According to Anderson fault theory (1951), this stress regime is propitious to thrust fault activity. Zoback and Townend (2001) observed from deep boreholes at several locations worldwide, indicating that hydrostatic pore pressures persist to depths of as much as 12 km in the upper crust, and the brittle crust is in a state of failure equilibrium according to Coulomb frictional-failure theory. As a result, the brittle crust is stronger than it would be under near-lithostatic pore pressure conditions. So the hydrostatic pore pressure plays an important role in evaluating fault activity, in other words, appraising the fault strength. Ignoring of hydrostatic pore pressures, most of the maximum horizontal principal stresses at the Qiaoqi site are below the lower critical stress limit (μ=0.6) for the occurrence of frictional sliding on the well-oriented fault but those deeper than 200 m exceed the limit line. However, after the Lushan earthquake, the maximum horizontal principal stresses of the whole Qiaoqi borehole have exceeded the lower limit of the critical stress and some even have exceeded the upper limit of the critical stress (μ=1.0) for frictional sliding on the optimally orientated fault (Figure 8). Taking account of hydrostatic pore pressures, the maximum horizontal principal stresses at the Qiaoqi borehole deeper than 170 m have exceeded the lower critical stress limit. However, after the earthquake, all those stresses 20

are above the upper limit of critical stress. Basically, stress state at Baoxing site is safe at above two conditions. Hence, we can believe that (1) stress in Qiaoqi changed to even greater after the Lushan earthquake, and (2) hydrostatic pore pressures deceases the critical stress limit line determined by Coulomb frictional-failure criterion and Byerlee' law, thus making faults more dangerous. Because Baoxing borehole is located at foot wall of Yanjing-Wulong Fault while the two Qiaoqi boreholes at its hanging wall, we do not compare in terms of activity of this fault. When it comes to the Shuangshi-Dachuan Fault, all three boreholes lie at its hanging wall and can make comparative analysis. From another perspective, μm represents the level of stress accumulation in a region. Wang et al. (2015) used μm values based on the stress measurements before and after the Wenchuan earthquake to analyze stress build-up level for a time scale about 21 years and evaluate frictional strength of the different segments of the southwestern Longmenshan fault. It was proposed that the mean value of μm has been up to 0.56, suggesting a high stress build-up degree (Wang et al., 2015). In this study, we can calculate the mean value of μm of the three boreholes in Baoxing region (Table 3-5). Before the Lushan earthquake, the mean values of μm of the Baoxing and Qiaoqi boreholes are about 0.40 and 0.50, respectively. However, after the Lushan earthquake, the mean value of μm of Qiaoqi borehole comes up to 0.77. It is notable to emphasize that the minimum and the maximum value of μm are 0.71 and 0.81, respectively, revealing a significantly high stress build-up level at present (Figure 9). Note that the parameter μm and μ have a similar geophysical implication because they are derived from the same theories. So, we can replace μ with μm to perform fault friction 21

analysis, see Figure 9. Based on the value of μm calculated by the measured stress data in the southwestern Longmenshan Fault Zone, most data of Qiaoqi borehole after the earthquake exceed the critical value of μ=0.6; that is, some well-oriented faults, may reach or approach a frictional equilibrium theoretically according to Byerlee's law and the possibility for slipping may occur. However, we need discuss an issue here that no matter μ or μm is utilized to evaluate fault strength or stress accumulation degree of the southwestern Longmenshan, the representativeness of shallow stress state need an appraisal. Firstly, the stress data shallower than 100 m was not used so that the effect of topography may be small enough. Moreover, we have stated that the key parameters for principal stress computation were weighted and evaluated with the average value of three re-opening cycles, thus the magnitudes of stress measured at Qiaoqi borehole are safely reliable for the shallow depth. We postulate a preliminary understanding that the maximum principal stress directions are driven by weak crustal material flow caused by eastward extrusion of the Tibetan lithosphere based on previous studies (Clark and Royden, 2000; Royden et al., 2008). It can be inferred that the stress field in this region is dominated by deep movement (Meng et al., 2015). So we can deduce the stress orientations from the shallow crust to seismogenic depth are consistent. As for the stress magnitude, Liao et al. (2003) has proved that the shallow stress value can be representative. About three months before the 2001 Ms 8.1 Kunlun earthquake, the magnitude of the maximum horizontal principal stress measured by piezomagnetic overcoring at 18 m below the surface near the Kunlun Strike Fault was 12.9 MPa surprisingly; after the earthquake, the maximum horizontal 22

principal stress at the same site decreased to 3.5 MPa, only about one-third of that before the earthquake (Liao et al., 2003). It can be infer that the shallow stress reflected how highly the crustal stress accumulated before the Kunlun earthquake. Theoretically and in most cases, the principal stress values increase linearly with the depth. However, because the in situ stress state in deeper crust is still not clear due to the limited measurement depth, the argument for this hypothesis is inadequate and further research is necessary to distinguish the deep stress field in the Baoxing segment of the southwestern Longmenshan Fault Zone. 6 Conclusions The present-day stress state of the southwestern Longmenshan Fault Zone, mainly the Yanjing-Wulong Fault, in Baoxing region is studied using in situ stress data that were collected before and after the Lushan earthquake from the three boreholes following piezomagnetic overcoring method and hydraulic fracturing method. In situ stress measurements in conjunction with the data of stress real-time monitoring, post-earthquake stress accumulation and fault activity of the Baoxing area are analyzed. The conclusions are as follows: (1) The in situ stress measurements in Qiaoqi Power Plant near Yanjing-Wulong Fault indicate that the maximum horizontal principal stresses after the earthquake are higher than those of the nearby borehole and those of Muping Town before the earthquake. In addition, analysis of stress monitoring data shows that crustal stress has an increasing trend in the NWW direction before and after the earthquake. Therefore, the post-earthquake stress seems not being released, but still accumulating in this region. 23

(2) The orientations of the maximum horizontal principal stresses of the southwestern Longmenshan Fault Zone before and after the earthquake are between NW-NWW, which are consistent with those in other regions of the southwestern Longmenshan fault after the Wenchuan earthquake as well as the orientation of the principal compressive stress axis (P axis) from the focal mechanism of the Lushan earthquake. (3) The analysis of the fault activity adopting the Coulomb frictional-failure criteria implies that Baoxing area, the southwestern section of the Longmenshan Fault Zone has already reached or exceeded a frictional limit equilibrium state. The frictional parameter, μm, has a high value, suggesting the high stress build-up level before and after the Lushan earthquake. The Lushan earthquake seems not relieve the stress concentration of the southwestern Longmenshan fault and the potential risk of the earthquakes in this area still exists in the future. Acknowledgements The excellent collaboration of all participants is highly appreciated. This research was supported by Project of Geological Survey “Dynamic Monitoring and Analysis of Present-day stress and strain field in the East Margin of Tibetan Plateau” (Grant No.: 12120114002401), Special Fund Research in the Public Interest (Grant No.: 201211076), and Chinese Government's Executive Program (Grant No.: SinoProbe-06-01(201311179)). Critical and constructive comments and suggestions offered by Prof. Weiren Lin and an anonymous reviewer greatly improved this manuscript. The authors thank Miss Irene Yao for her kindly help in manuscript revision.

24

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33

Figures:

Figure 1 (a) Tectonic map showing main structures of the Tibetan Plateau. (b) Geology map of the Longmenshan Fault Zone and its vicinity, modified after Burchfiel et al (1995) and Tian et al (2013). F1, Guanxian-Anxian Fault; F2, Yingxiu-Beichuan Fault; F3, Wenchuan-Maowen Fault; F4, Yanjing-Wulong Fault; F5, Shuangshi-Dachuan Fault; F6, Dayi buried Fault; BM, Baoxing massif; PM, Pengguan massif.

34

Figure 2 Typical re-opening cycle curve for shut-in pressure determination. One can easily estimate shut-in pressure from the clear inflection point at the circle place.

35

Figure 3 Topographic map of Baoxing region

36

Figure 4 In situ stress measurements conducted in the southwestern Longmenshan fault zone. YX, Yingxiu site; QQ, Qiaoqi site, QQ-1, Qiaoqi site (after the Lushan earthquake); BX, Baoxing site; FXG, Feixianguan site; ELS, Erlangshan site; KD, Kangding site.

37

Figure 5 Stress relief and calibration curves of piezomagnetic overcoring at 174 m.

38

Figure 6 Hydraulic fracturing curves of Qiaoqi borehole after the Lushan earthquake.

39

Figure 7 Changes of element readings of Qiaoqi piezomagnetic stress monitoring instrument. Star on the curve represents the Lushan earthquake.

40

Figure 8 Schematic illustration of the fault activity of three boreholes in Baoxing region. We use an average water level of 5.0 m in the borehole to calculate Pp.

41

Figure 9 Dependence of differential stress on effective mean stress of three measurement sites in Baoxing region.

42

Tables: Table 1 Calculation results of shut-in pressures at Qiaoqi site (QQ-1) PS(MPa)-dP/dt method

PS(MPa)-dt/dP method

PS(MPa)-Muskat method

Cycle3

Cycle4

Cycle5

Cycle3

Cycle4

Cycle5

Cycle3

Cycle4

Cycle5

128

9.94

9.59

9.40

10.35

10.82

9.77

9.98

9.82

9.35

9.89

136

8.87

8.83

8.85

9.38

9.30

9.27

9.20

9.10

9.04

9.09

159

9.92

/

9.39

10.00

9.88

10.39

10.89

10.49

9.94

10.11

182

17.80

17.82

17.94

16.62

16.04

15.56

15.21

16.18

16.54

16.63

188

9.35

9.12

8.90

10.91

10.89

10.11

9.22

9.28

8.81

9.62

Depth(m)

43

Avg PS(MPa)

Table 2 Calculation results of re-opening pressures at Qiaoqi site (QQ-1) Depth(m)

Pr(MPa)-single tangent method

Avg Pr(MPa)

Cycle3

Cycle4

Cycle5

128

9.19

9.10

9.05

9.11

136

9.02

9.34

9.05

9.14

159

10.04

10.03

10.05

10.04

182

26.16

25.93

25.88

25.99

188

10.24

9.87

9.33

9.81

44

Table 3 Results of hydraulic fracturing measurement at Baoxing site (BX) (Wu et al., 2009) μm

Depth(m)

Pp(MPa)

Pb(MPa)

Pr(MPa)

Ps(MPa)

SH(MPa)

Sh(MPa)

Sv(MPa)

128

1.28

6.67

5.96

3.53

3.63

3.53

3.39

0.05

156

1.56

6.11

5.26

3.44

3.50

3.44

4.13

0.16

189

1.89

11.71

10.32

7.36

9.87

7.36

5.01

0.44

219

2.19

12.12

8.33

6.45

8.83

6.45

5.80

0.30

258

2.58

12.53

12.25

9.81

14.60

9.81

6.84

0.48

283

2.83

16.62

15.76

12.27

18.22

12.27

7.50

0.53

N80°W

317

3.17

17.29

12.82

11.09

17.28

11.09

8.40

0.46

N74°W

354

3.54

18.56

18.04

15.75

25.67

15.75

9.38

0.58

387

3.87

11.86

11.48

10.08

14.89

10.08

10.26

0.27

Note: The mean rock density is assumed to be 2650 kg/m3 to estimate the vertical stress.

45

SH Ori.

N23°W

Table 4 Results of hydraulic fracturing measurement at Qiaoqi site (QQ) (Wu et al., 2013) Pp(MPa)

Pb(MPa)

Pr(MPa)

Ps(MPa)

SH(MPa)

Sh(MPa)

Sv(MPa)

μm

117.50

1.14

9.86

7.86

5.22

6.66

5.22

3.11

0.47

135.00

1.31

9.17

7.63

4.73

5.25

4.73

3.58

0.27

167.00

1.63

9.53

8.17

5.09

5.47

5.09

4.43

0.16

174.50

1.71

21.85

13.76

9.51

13.06

9.51

4.62

0.59

N49°W

192.07

1.88

23.36

19.12

12.09

15.27

12.09

5.09

0.61

N60°W

201.27

1.97

19.55

18.79

13.13

18.63

13.13

5.33

0.66

214.37

2.10

19.48

18.51

14.78

23.73

14.78

5.68

0.72

Depth(m)

3

Note: The mean rock density is assumed to be 2650 kg/m to estimate the vertical stress.

46

SH Ori.

Table 5 Results of hydraulic fracturing and piezomagnetic overcoring measurement at Qiaoqi site (QQ-1) Depth(m)

Pressure/Stress Parameters(MPa)

μm

SH Ori.

3.39

0.81

N85°W

10.47

3.60

0.77

N63°W

11.71

4.21

0.76

25.30

14.90

4.61

0.77

N85°E

25.83

18.47

4.82

0.77

N73°W

21.02

11.51

4.98

0.71

Pp

PH

Pb

Pr

Ps

SH

Sh

Sv

128

1.21

1.29

19.62

10.40

11.18

21.93

11.18

136

1.29

1.38

25.83

10.52

10.47

19.60

159

1.52

1.60

21.34

11.64

11.71

21.97

174

1.68

182

1.75

1.84

32.73

27.83

18.47

188

1.81

1.89

19.41

11.70

11.51

Note: The final values of Pb, Pr , and Ps are the sum of PH.and original breakdown pressure Pb, Avg Pr, and Avg Ps, respectively. PH is equal to the hydrostatic pressure with the full drill-pipe water. The mean rock density is assumed to be 2650 kg/m3 to estimate Sv.

47

Highlights: 1 We characterize in situ stress state in Baoxing region before and after the Lushan earthquake. 2 Stress magnitudes and stress accumulation at present are significantly greater than those prior to the Lushan earthquake. 3 Fault activity in the southwestern Longmenshan fault is discussed based on Coulomb frictional-failure criterion, implying that earthquake risk seems still exist.

48