Stress state along the Anninghe-Zemuhe fault zone, southwestern China, estimated from an array of stress orientation measurements with a new method
© The Society of Geomagnetism and Earth, Planetary and Space Sciences(SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2012
Received: 23 July 2010
Accepted: 22 August 2011
Published: 2 March 2012
Measurements of in-situ stress orientations at relatively shallow depths were conducted at 11 sites along the Anninghe-Zemuhe fault zone, southwestern China, with a newly developed method. The Anninghe fault in the fault zone has been considered to have a potential for a large earthquake with a magnitude of about 7.5, while the Zemuhe fault shows very little such potential. The present study has mainly two objectives: one is to show new data of the spatial distribution of the stress orientations around the fault zone, which is obtained directly from the measurements; the other is to examine the possibility to detect a stress pattern in relation to the past activities of earthquakes along the fault zone. The observed principal stress orientations are distributed between the NNW-SSE and NW-SE directions, which is consistent with the earthquake focal mechanism solutions near the fault zone. On the other hand, it is unclear whether the observed stress pattern is related to past activities of the fault zone, since the data fluctuation was too large to determine this. We note that the stress orientations are favorable for slip on the Anninghe fault, while being unfavorable on the Zemuhe fault, from the present experimental data.
Recent observations of stress states on other faults after large earthquakes fall into two viewpoints: one is that stress states are little affected by the main shock, which suggests that the stress drop of the main shock, relative to the tectonic stress level, is small (e.g. Kubo and Fukuyama, 2004). The other is the opposite case, suggesting that the stress drop of the main shock is comparable to the tectonic stress level (e.g. Hardebeck and Hauksson, 2001; Yamashita et al., 2004). If the conclusion of a relatively small stress drop is the case, then a stress measurement might not be directly useful for earthquake forecasts, because the measurement would not be able to resolve stress variations during an earthquake cycle. On the other hand, in the opposite case, stress measurements would be quite valuable for earthquake forecasts, because the stress accumulation process on the fault is likely to be observed by stress measurements. Considering these two conflicting observations, it is necessary to examine two aspects of the stress state of the Anninghe-Zemuhe fault zone: the first is whether the stress drop on the Zemuhe fault, relative to the tectonic stress at the latest earthquake in 1850, is detectable or not. The second is to determine what the stress state on the Anninghe fault is, relative to the tectonic stress.
The stress state around the two faults might be indicated by the focal mechanism solutions of micro-earthquakes. While Cheng et al. (2006) have shown that the average azimuths of the P-axis of the focal mechanism solutions are ESE-WNW over a relatively large regional area including the Anninghe fault, and NW-SE in an regional area including the Zemuhe fault, they do not estimate the local stress field near the two faults. The data of the azimuth of the P-axis seem to be rather scattered in figure 1 of Cheng et al. (2006), so that it is difficult to clarify the local stress fields along the two faults from this data. Cui et al. (2006) and Huang et al. (2009) present data concerning earthquake focal mechanisms of M > 4, and in-situ stress measurements with a stress relief method, or a hydraulic fracturing one, at several different sites along the Anninghe-Zemuhe fault zone. They show that the orientation of the maximum horizontal stress is roughly in the NW-SE direction. The data are, however, insufficient to depict a high-resolution stress field related to the difference of activities of the Anninghe and the Zemuhe faults. More data are thus necessary to clarify the stress state along the fault zone.
Usual stress-measurement techniques, such as stress relief and hydraulic fracturing experiments, are generally difficult to perform at many sites due to the high costs of the measurements. Kuwahara and Kiguchi (2006), Kiguchi and Kuwahara (2006) and Kiguchi et al. (2010) have recently developed a new relatively low cost method for measuring crustal stress orientations in Japan. The principle of the new method is to measure directly the creep deformation of the borehole just after drilling into bedrock under some crustal stress. An advantage of the new method is that it involves easier technical procedures at a relatively lower cost, leading to the possibility of many measurements in different locations. One disadvantage of this method is that it can provide only data relating to the orientation of the horizontal maximum principal stress, SHmax, at shallow depths down to about 20 m. On the other hand, Sbar et al. (1984) and Plumb et al. (1984) have shown that the majority of stress orientation data obtained at shallow depths of less than about 20 m are consistent with data at deeper depths, say, contemporary tectonic stresses. They also pointed out that the disturbance caused by thermally-induced stress from the surface is negligibly small at depths deeper than 10 m. We expect, therefore, to obtain an abundance of meaningful data to estimate the tectonic stress filed, with this new method. In the following section, we will discuss the effectiveness of only stress orientation data, without absolute values of stresses, for estimating the stress state of faults.
Based on the above, an opportunity has arisen for cooperation between China and Japan to measure stress fields at 11 sites along the Anninghe-Zemuhe fault zone. The present paper describes the procedures of site selections and measurements. The effect of the actual landscape topography on the measured data is also discussed. We then compare the SHmax orientations measured applying the new method with theoretical stress fields calculated assuming various stress states, on both faults, related to the past activity of each fault, in order to examine the differences.
2.1 New method
We briefly describe here the new method used for measuring the stress orientation at shallow depths in the present study, following Kiguchi et al. (2010). The new method is based on the following principles: (1) A borehole drilled in stressed bedrock is expected to be quasi-statically deformed due to the viscous property of the rock just after drilling. (2) The orientation of SHmax coincides with the orientation of the minor axis of an ellipse fitted to the relative change of shape of the borehole cross-section during the deformation. The main advantage of this method is the notably lower cost to estimate the stress orientation, compared with existing methods, such as stress relief and hydraulic fracturing methods. One might expect that an absolute value of the crustal stress could be determined, if the viscosity of the rock could be measured. However, it is difficult to estimate the absolute value, since the measurement of viscosity is difficult under the low stress condition of this method corresponding to the shallow depth.
2.2 Selection of site locations
The technical reasons for choosing the sites mainly relate to the hardness of the rock and the landscape topography. To some extent, the rock should be hard, because stress cannot accumulate in soft rocks due to their ductility. We generally use the criterion of a P wave velocity in rock of larger than about 3 km/s as an index of hard rock (Sato et al., 2004). Regarding the landscape topography, a flat, or gently sloping, landscape topography is generally better for measurements, since steep slope topography especially affects the shallow stress (e.g. Liu and Zoback, 1992). Therefore, we avoid measuring near a steep slope. We will later show that the topography effect on the shallow stress state is not dominant, by carrying out a computer simulation ofa finite element modeling for the actual topography of a site in the present experiment.
We finally chose the 11 sites shown in Fig. 1 from the gentle-sloping topography sites from all the candidates, taking into consideration the above points. Each site is a different distance from the faults, or from the edge of the faults along the strike.
We conducted up to three measurements at different depths ranging from 10 m to 18 m for a given site. Drilling and measurements for a given site were basically carried out as follows: we first drilled a borehole down to 10 m or more to check the condition of the borehole rock by judging from the core samples about the hardness and smallness of the fracture density. If the condition was good enough to measure, a measurement of the stress orientation was made at that depth for 2 to 3 hours. We then retrieved the downhole tool and drilled at least 50 cm deeper. We then repeated the core samples check and measured the deformation again. Although a measurement time of a few hours was usually enough to estimate the stress orientation, we took measurements all night, because drilling work was impossible during the night-time. Thus, measurement times were about 2 to 3 hours during daytime and about 11 to 14 hours at nighttime. It took four weeks to conduct the measurements at 11 sites, from March to April, 2007, by using 3 boring machines. The measurements were successful at 7 sites, while for some sites only one measurement was meaningful. We could not measure at 4 sites due to the bad conditions of the boreholes.
The discarding of some measurements resulted mainly for three reasons. The first one was highly damaged rock in the borehole. We skipped sites of heavily damaged rock because it is disastrous if the downhole tool is trapped in the borehole. The second reason was a too enlarged borehole diameter. In some cases, it was impossible to measure the borehole deformation, because its diameter became larger than the maximum focus distance of the laser displacement sensor. This is probably due to the combined effect of the drilling technique and the rock properties. The final reason was muddy water in the borehole—caused by borehole-wall rock dust in the water. The laser beam of the laser sensor cannot be transmitted in very muddy water. Consequently, we analyzed data from 7 sites. Table 1 summarizes the locations of the measurements, the measurement depths at each location, and the reasons for unsuccessful measurements.
Summary of site parameters and the measurement results.
Elevation above sea level (m)
Remark for incomplete measurements
Borehole rocks were extremely fractured
Borehole diameter was too much enlarged
Borehole rocks were extremely fractured
Borehole water was muddy
Data processing for estimating the stress orientation is as follows: the relative change of the borehole shape during measurement was obtained from the difference between the borehole radii of the initial, and the final, parts of the measurement. The borehole radius just after drilling, and that of the final, part of the measurement were determined from the averaged data of the first 5 laps and of the last 10 laps, respectively. We used a smaller number of averaging data for the first part than for the last part, because the deformation rate in the first part is larger than in the last part. Then, we analyzed to fit an ellipse to the relative change of the borehole shape by least-square fitting. The unknown parameters for the fitting are the ellipse center coordinates of x0 and y0, the lengths of the minor and major axes, and the orientation of the major, or the minor, axis. The orientation of SHmax is determined to be the direction of the minor axis of the fitted ellipse.
Among these three aspects, we chose the flattening parameter of the ellipse as an indicator of the data reliability, because the flattening parameter is likely to be directly related to the anisotropic stress state. The data coverage, and the short-wavelength perturbations, will be used as auxiliary indicators. Thus, we have introduced a parameter F (= (a − b)/a) of the nominal flattening of the ellipse fitted to the relative change of the borehole shape as the degree of borehole deformation, where a and b are the lengths of the major and minor axes, respectively. It should be noted that a and b are given not by the actual shape of the borehole, but by the nominal shape assuming the original borehole shape is a true circle with a diameter of 116 mm. The parameter F is given at the lower right of each figure. The larger is the value of F, the more reliable is the estimation. The orientation determined with a small value of F is considered to be sensitive to data noise. In the case, for example, of F < 1/400%, which means a − b < 2.5 µm, roughly a data error of measurement as indicated in Fig. 7, the estimation is unlikely to be reliable. The results of the SHmax orientations, and the value of F estimated from the data, are summarized also in Table 1.
4. Effect of Landscape Topography on the Stress State at Shallow Depths
Physical constants applied to each element of the FEM.
Results for the four cases are shown in Table 3 for each site where the stress orientation data are obtained. All the results of stresses and the azimuths of SHmax are calculated at a depth of 17.5 m from the surface. It is noted that the stress fields are tensional at sites No. 1 and No. 5 for all four models. SHmax is regarded as a principal stress with a smaller absolute value than another principal stress in the tensional stress case. The results show noticeable variation, according to site location. If the topography were flat, SHmax and the other horizontal principal stress, S H min, were always (ν/(1 − ν))S zz where ν is Poisson’s ratio and (ν/(1 − ν)) = 1/3 when ν = 0.25 (e.g. Savage et al., 1992). On the other hand, we can see a clear topography effect from the results, sometimes showing SHmax larger than S zz , or the tensional stresses.
Stresses and the azimuths of the SHmax at a depth of 17.5 m calculated from the FEM model.
S zz (kPa)
S H max* (kPa)
S H min* (kPa)
S zz (kPa)
S H max* (kPa)
S H min* (kPa)
The results of the computed topography effect for the four models at each site are generally within a narrow range of 10°–20°. A mean azimuth at each site is plotted with a dotted line in Fig. 9, with the result of actual measurement. We can see a general tendency that the calculated SHmax azimuths, due to the effect of topography, are parallel to a valley at the foot of a mountain, and the azimuth is parallel to the slope of the mountain in the middle of the mountain. We can also see that the measured results are not likely to coincide with the calculated ones. Thus, we conclude that the topography effects are negligibly small in the present experiment.
More sophisticated theoretical calculations concerning, for example, the nonuniform stress on the fault might be necessary in order to evaluate quantitatively the stress field along the fault zone. However, the causes of error in the data is not necessarily clarified, at present, because stress orientations are affected by many unknown nonuniformities such as small fractures, and elastic constants in and around the borehole. Therefore, more data are necessary to perform a more sophisticated analysis. It might be difficult to conclude that the measured stress pattern is related to the past activity of each fault. The present experiment, so far, indicates that the orientation of SHmax of the tectonic stress is in the range N45°W–N60°W and that the orientations of SHmax are almost parallel to the fault strike of the Zemuhe fault, and favorable to slip of the Anninghe fault, respectively.
We measured the stress orientations at 7 sites around the Anninghe-Zemuhe fault zone to test the detectability of spatial variations in the stress state caused by the difference of the stress state on the two faults, using a newly-developed method. The results show that the observed orientation is roughly NW-SE, which is consistent with the tectonic stress field estimated by other methods of stress measurements, and of earthquake focal mechanism solutions. This indicates that the new method gives promising data. Further, the shear stress observed on the Zemuhe fault seems to be small compared with the tectonic stress. The present results indicate that the stress acting on the Anninghe fault is favorable to slip on the fault, while the stress acting on the Zemuhe fault is not.
This work was supported by the Ministry of Science and Technology of the People’s Republic of China under contract no. 2006DFA21660. DEM data around the studied area were downloaded from http://www.ersdac.or.jp/GDEM/J/1.htm. We appreciate Dr. K. Omura and Dr. Y. Yabe for their constructive comments and suggestions on the revisions of the manuscript.
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