Tectonophysics 482 (2010) Contents lists available at ScienceDirect. Tectonophysics. journal homepage:

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1 Tectonophysics 482 (2010) Contents lists available at ScienceDirect Tectonophysics journal homepage: Localized rotation of principal stress around faults and fractures determined from borehole breakouts in hole B of the Taiwan Chelungpu-fault Drilling Project (TCDP) Weiren Lin a,,1, En-Chao Yeh b,2, Jih-Hao Hung c, Bezalel Haimson d, Tetsuro Hirono e a Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology, 200 Monobe-otsu, Nankoku, Kochi , Japan b Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology, Japan c Institute of Geophysics, National Central University, Chung-Li 32054, Taiwan d Department of Materials Science and Engineering and the Geological Engineering Program, University of Wisconsin, Madison, WI, 53706, USA e Graduate School of Science, Osaka University, Osaka , Japan article info abstract Article history: Received 12 September 2008 Received in revised form 10 April 2009 Accepted 15 June 2009 Available online 23 June 2009 Keywords: Stress orientation Stress perturbation Fault Fracture Borehole breakout To reveal details of stress perturbations associated with faults and fractures, we investigated the faults and large fractures accompanied by stress-induced borehole breakouts or drilling-induced tensile fractures in hole B of the Taiwan Chelungpu-fault Drilling Project (TCDP). Then, we determined the relationship between the faults and fractures and stress orientation changes. We identified faults and fractures from electrical images of the borehole wall obtained by downhole logging but also from photographs and descriptions of retrieved core samples, and measured the variations in the principal horizontal stress orientation ascertained from borehole breakouts observed on the electrical images in the vicinity of the faults and fractures. Identification of geological structures (faults, fractures, and lithologic boundaries) by electrical images only is difficult and may sometimes yield incorrect results. In a novel approach, therefore, we used both the electrical images and core photographs to identify geological structures. We found four patterns of stress orientation change, or no change, in the vicinity of faults and fractures in TCDP hole B: (i) abrupt (discontinuous) rotation in the vicinity of faults or fractures; (ii) gradual rotation; (iii) suppression of breakouts at faults, fractures, or lithologic boundaries; and (iv) no change in the stress orientation. We recognized stress fluctuations, that is, heterogeneous mesoscale ( 10 cm) stress distributions with respect to both stress orientation and magnitude. In addition, we found that stress state changes occurred frequently in the vicinity of faults, fractures, and lithologic boundaries Elsevier B.V. All rights reserved. 1. Introduction It is well known that stress and earthquakes are interrelated: stress triggers earthquakes and earthquakes alter shear and normal stresses on fault planes (Stein,1999; Seeber and Armbruster, 2000; Hardebeck, 2004; Ma et al., 2005). One effective and important approach to understanding how stress and faulting are interrelated is to examine relationships between stress state changes and the presence of faults and fractures. In this study, we focused on stress perturbations in the vicinity of faults and fractures by using data from the Taiwan Chelungpu-fault Drilling Project (TCDP), similar to a previous study (Lin et al., 2007a). The large, destructive Chi-Chi earthquake (Mw 7.6) occurred in westcentral Taiwan on 21 September 1999 as a result of convergence between the Philippine Sea and Eurasian plates (Fig. 1a and b; Kao and Chen, Corresponding author. Fax: address: lin@jamstec.go.jp (W. Lin). 1 Key Laboratory of Tectonics and Petroleum Resources of Ministry of Education, China University of Geosciences, Wuhan, China. 2 Now at Department of Geosciences, National Taiwan University, Taipei 106, Taiwan. 2000; Ma et al., 2000; Shin and Teng, 2001; Wang et al., 2002; Ji et al., 2003; Yue et al., 2005). To understand the physics of the earthquake and the mechanism of rupture propagation, the TCDP drilled two vertical holes 40 m apart (hole A to an approximate depth of 2000 m and hole B to an approximate depth of 1350 m) about 2 km east of the surface rupture (Fig. 1b d), near the town of Da-Keng (Ma et al., 2006). The Chelungpu fault dips gently to the east (30 ), and slips principally within and parallel to the bedding of the Pliocene Chinshui Shale (Fig. 1c). The TCDP holes penetrate three major fault zones (Fig. 1d; e.g., Hirono et al., 2006) within the Chinshui Shale, which, despite its formal lithostratigraphic name, in this area is composed mainly of siltstone (Lin et al., 2007c). Each of the three fault zones has components of wall rock, damage zone and fault core. By regional stratigraphic constraint, however, we believe they will merge into one below certain depth since we did not find distinct stratigraphic offset on the surface geology. A main objective of the TCDP was to determine the spatial distribution of the in situ stress and, in particular, to determine the stress state on and around the fault plane before, during, and after the earthquake. Previous stress-related studies in the area have reported the focal mechanisms of earthquakes occurring before the 1999 Chi-Chi /$ see front matter 2009 Elsevier B.V. All rights reserved. doi: /j.tecto

2 W. Lin et al. / Tectonophysics 482 (2010) Fig. 1. Schematic diagram illustrating the tectonics, regional structures, geological section and two drill holes. (a) Tectonic setting of Taiwan and location of the 1999 Chi-Chi earthquake epicenter (after Lee et al., 2002). (b) Geological map showing the formation distribution and the several faults in the central portion of the western Taiwan. The Chelungpu fault (red line) ruptured during the 1999 earthquake (after Yeh et al., 2007). The Taiwan Chelungpu-fault Drilling Project (TCDP) site is indicated by a red star. The focal mechanism of the Chi-Chi main shock is located at the hypocenter of the Chi-Chi earthquake (Kao and Chen 2000). (c) Cross section through the drill site illustrates the relation between formations and major fault zones (after Yeh et al., 2007). (d) Diagram shows the three major fault zones belonging to the Chelungpu fault system penetrated by the two TCDP holes A and B. The dashed frames in (a) and (c) show the areas of (b) and (d) respectively; and the dashed line in (b) shows the orientation and range of the section map (c). earthquake (Yeh et al.,1991), and a stress tensor inversion of the Chi-Chi earthquake sequence (Kao and Angelier, 2001; Wang and Chen, 2001; Ma et al., 2005; Blenkinsop, 2006) revealed some regional stress information in the Taiwan area. Moreover, as part of the TCDP, Hung et al. (2009) determined stress magnitudes by hydraulic fracturing at four depths in hole B; Wu et al. (2007) and Lin et al. (2007a) obtained stress orientations mainly from borehole breakouts in hole A and hole B, respectively; Lin et al. (2007b) published preliminary results of threedimensional stress determination by anelastic strain recovery (ASR) of core samples from hole A; and Yabe et al. (2008) measured stresses by acoustic emission (AE) and deformation rate analysis (DRA) of core samples. In addition, Haimson et al. (2009) have estimated the

3 84 W. Lin et al. / Tectonophysics 482 (2010) maximum principal horizontal stress magnitude by using the combination of borehole breakout width and rock strength data from true triaxial compression tests, which are considered able to replicate in situ stress conditions reliably (see Haimson and Chang, 2002). Lin et al. (2007a) found clearly recognizable principal stress rotations in the vicinity of the shallowest major fault zone, at 1133 m logging depth in hole B, suggesting that this fault zone ruptured during the 1999 earthquake. Their study, however, focused on only this major fault zone. To reveal the details of stress perturbations associated not only with major fault zones but also with minor faults and fractures, in this study, following Lin et al. (2007a), we investigated all of the faults and large fractures accompanied by stress-induced borehole breakouts or drillinginduced tensile fractures and their relationship with stress orientation in TCDP hole B. We identified faults and fractures not only from Fullbore Formation MicroImager (FMI) electrical images of the borehole walls but also from photographs and descriptions of retrieved core samples, and then measured the variations in the principal horizontal stress orientations from the borehole breakouts identified in the electrical images in the vicinity of the faults and fractures. Identification of geological structures such as faults, fractures, and lithologic boundaries from only electrical borehole images is difficult and may sometimes yield incorrect results. Therefore, we used both electrical images and core photographs to identify the geological structures. This novel approach allowed us to examine stress perturbations resulting from the presence of the geological structures. InTCDP hole B we found four types of relationship between faults and fractures and changes of borehole breakout and drilling-induced tensile fractures in their vicinity, indicating principal horizontal stress orientation changes. From these observations, we recognized heterogeneous mesoscale ( 10 cm) stress distributions with respect to both orientation and magnitude. Moreover, we found that stress state changes were frequent in the vicinity of faults, fractures, and lithologic boundaries. core samples at core depths of approximately 1136, 1194, and 1243 m (Hirono et al., 2006). The depth differences arise from the drilling rod length and wire length of downhole logging tools which makes the logging depths usually shallower by approximately 3 m than the core depths. In addition to the three major fault zones, 30 minor faults with stress-induced breakouts or drilling-induced tensile fractures in their vicinity were recognized both in the FMI images and the core samples. In this study, we define minor faults as those less than 0.5 m wide and accompanied by fault gouge or showing displacement. Since both stress-induced borehole breakouts and drilling-induced tensile fractures depend on in situ stress conditions, we can use information on their geometry, observed in the borehole wall images, to estimate orientations of in situ principal stresses in the plane perpendicular to the borehole axis without knowledge of any other parameter values (Zoback et al., 1985; Barton et al., 1988; Moos and Zoback, 1990; Vernik and Zoback, 1992; Zoback et al., 2003; Haimson, 2007). The azimuth of the breakout is the same as the azimuth of the minimum principal horizontal stress S hmin but differs by 90 from the azimuth of maximum principal horizontal stress S Hmax ; whereas the azimuth of the drilling-induced tensile fracture is the same as the azimuth of S Hmax (e.g., Zoback et al., 2003). Many borehole breakouts and a few tensile fractures were observed within the surveyed depth range in TCDP hole B. Breakouts or tensile fractures are almost found in pairs, with one member of the pair opposite the other in the borehole wall. Thus, the difference in orientation between the two positions is always approximately 180. We recorded the azimuth data of breakouts and tensile fractures according to the criteria described by Lin et al. (2007a), and plotted the azimuth profile of the S Hmax (Fig. 2). Because we conducted the FMI logging in hole B approximately 5 years and 7 months after the last slip event, the results represent the post-seismic stress state at the time of drilling. In general, the azimuths of S Hmax were mostly distributed between N105E and N155E at all surveyed depths, except at around 1133 m. We 2. Profile of current principal horizontal stress orientations in TCDP hole B Lin et al. (2007a) carried out a stress analysis in TCDP hole B using both stress-induced compressive failures (borehole breakouts) and drilling-induced tensile fractures, focusing on the stress state in the vicinity of the shallowest major fault zone (1133 m logging depth). At depths near this shallowest major fault zone, they compiled the stress states, along with several independent stress measurements obtained by the ASR method (Lin et al., 2007b) and hydraulic fracturing tests (Hung et al., 2009). Importantly, the data suggest that the principal horizontal stress orientations rotated abruptly by approximately 90 in the vicinity of the major fault zone. This can be considered important scientific evidence that the Chelungpu fault ruptured at this major fault zone during the 1999 Chi-Chi earthquake. Furthermore, Lin et al. (2007a) proposed a constraint of possible magnitude range for the current principal horizontal stresses at depths near the fault zone. In this study, we ascertained borehole breakouts, drilling-induced tensile fractures and the orientation profile of the current principal horizontal stresses, determined at 1-m intervals, in TCDP hole B from the FMI images and calculated the statistics of the orientations. Then, we examined the characteristics of the stress orientation changes at 25-cm intervals in the vicinity of all minor faults and major fractures observed in the depth range of the FMI survey and retrieved core samples. We conducted downhole wireline FMI logging twice to obtain electrical images of the borehole wall from approximately 930 to 1330 m depth in hole B immediately after the target drilling depth was reached. In hole B, core samples were fully retrieved in the depth range from 950 to 1350 m (e.g., Hirono et al., 2007). Therefore, we set the investigation depth range in this study between 950 m and 1330 m, where both FMI images and core samples were available. Three major fault zones were observed in the FMI electrical images at logging depths of approximately 1133, 1191, and 1240 m, and in the Fig. 2. Profile of current principal horizontal stress orientations in TCDP hole B. The azimuth distribution of the maximum principal horizontal stress S Hmax determined from borehole breakouts ( and ) and drilling-induced tensile fractures (+ and ). The symbols and + and and refer to data obtained during the first and second FMI downhole logging runs (P1 and P2), respectively. The solid circles show the average azimuths of S Hmax from both breakouts and tensile fractures in each sub-range; vertical bars show the depths of the sub-ranges; horizontal error bars show the standard deviations. Arrows labeled Fig. 2 to Fig. 7 correspond to the locations shown in the respective figures. Modified from Lin et al., (2007a).

4 W. Lin et al. / Tectonophysics 482 (2010) Table 1 Statistics of the azimuths of maximum principal horizontal stress in sub-ranges of the survey depth range. Sub-range Depth range (m) Data number Average azimuth ( ) Standard deviation ( ) divided the whole surveyed depth range into several sub-ranges (shown in Table 1 and Fig. 2 by vertical bars) according to the presence or absence of breakouts and tensile fractures and also considering lithologic boundaries. Azimuthal standard deviations of all sub-ranges were between 9 and 22. The azimuthal standard deviations can be considered to include both measurement errors and stress orientation fluctuations. Thus, the precision of the azimuth data obtained from the borehole breakouts and the tensile fractures was generally less than the azimuthal standard deviations. The average azimuths of the subranges were concentrated within a relatively narrow range from N119E (near 1170 m depth) to N133E (near 1300 m). This azimuth of N E is consistent with the downdip direction of the bedding planes of the Chinshui Shale, and is also the same as the azimuth of rupture of the Chelungpu fault during the 1999 Chi-Chi earthquake (Lin et al., 2007c) approximately. Moreover, this orientation is in good agreement with the regional stress state at the TCDP drilling site obtained from fault slip analyses of the 1999 Chi-Chi earthquake data (Kao and Angelier, 2001; Blenkinsop, 2006) and tectonic stress data for central Taiwan (Yeh et al., 1991). As mentioned above, the stress orientation changed abruptly by approximately 90 in the vicinity of the fault zone at 1133 m depth. Even disregarding this abrupt change, the azimuths of S Hmax were not constant but varied abruptly or gradually. Most of these azimuthal variations were associated with faults, fractures, or lithologic boundaries. 3. Descriptions of stress orientation rotations and discussions 3.1. Abrupt changes in stress orientation in the vicinity of faults and fractures Our first example of an abrupt, drastic change in the orientation of the principal horizontal stresses is in the vicinity of a minor fault at approximately m logging depth (Fig. 3). The minor fault, which Fig. 3. An example of an abrupt stress orientation change in the vicinity of a minor fault at m logging depth. (a) FMI electrical image (unrolled electrical image of the borehole wall in TCDP hole B). The left edge corresponds to north. Dark colors in the image represent conductive areas and light colors resistive areas. (b) An optical image of the flat surface of a half-core split in the vertical plane approximately parallel to the downdip direction. The constriction ratio in the vertical and lateral directions is 2:1 in this figure and also in Figs. 3 7; the core diameter in Figs. 2 7 is approximately 83 mm. (c) An optical photograph of the cylindrical core taken on the drilling rig floor immediately after retrieval. (d) Plot of the S Hmax azimuth over a depth range of several meters around this minor fault.

5 86 W. Lin et al. / Tectonophysics 482 (2010) was interpreted as a left-lateral strike-slip fault with normal slip from the core description, has a large dip angle (dip/dip orientation: 70/050 approximately). The azimuth of the breakout abruptly rotates by approximately 90 (see Fig. 3a and d). Therefore, at the minor fault depth, the orientation of S Hmax changes roughly from the rupturing direction of the 1999 earthquake to one perpendicular to the rupturing direction. Within a narrow depth interval (centered at around 1133 m) below the minor fault, the S Hmax azimuth (N212E on average) differs by approximately 90 from its azimuth at other depths (Table 1 and Fig. 2). That is, the minor fault at m depth, where the S Hmax azimuth abruptly rotates, is the boundary between the normal stress orientation, one consistent with the regional stress orientation, and an anomalous stress orientation caused by the 1999 earthquake (Lin et al., 2007a). Moreover, the S Hmax azimuth (N212E on average) at approximately 1133 m in hole B is consistent with the stress orientation estimated by the ASR method at the depth of the shallowest major fault zone in hole A (Lin et al., 2007b). Yamashita et al. (2004) reported a similar example, in which the Nojima fault slip during the 1995 Kobe earthquake induced rotation of the principal stress axis orientation. Due to the thrust fault rupturing of the 1999 earthquake, it could be considered that both the maximum and minimum principal horizontal stresses dropped in the vicinity of the shallowest major fault zone. However, drops of the two principal horizontal stress magnitudes may differ from each other. Because the stress in the rupturing direction was the driving force, drop of the stress magnitude in this direction was more than that in the direction perpendicular to the rupturing direction (the fault strike direction). Therefore, it is possible that magnitude of the stress in the rupturing direction was more than that of the stress in the strike direction before the fault rupturing. That is, the maximum principal horizontal stress was consistent with the tectonic stress orientation before the earthquake. In addition, this stress state was also consistent with the reverse (thrust) fault regime which agrees with fault rupturing type of the Chi-Chi earthquake. As a result of the stress dropping during the fault rupturing, it is reasonable to infer that the stress in the rupturing direction changed from the maximum principal horizontal stress into the minimum principal horizontal stress through the earthquake; whereas the stress in the strike direction changed from the minimum horizontal stress into the maximum horizontal stress in the vicinity of the fault zone ruptured. Our second example (Fig. 4) is of an abrupt but minor orientation rotation of the principal horizontal stress across a fracture at approximately m logging depth (corresponding to the optical split core image in Fig. 4c). Across the fracture (60/050 approximately), the breakout (S hmin ) azimuth abruptly rotates by approximately 20, but it remains almost constant above and below the fracture. In the vicinity of this fracture, the retrieved core samples were broken, so the fracture attitude could not be identified by the core samples, but correlation between FMI image and core images of this fracture could be identified by taking into account the relationship between logging and core depths. Additionally, it is clear that the fracture is at the lithologic boundary between a layer of alternating sandstone and shale (upper side; Fig. 4a) and a layer of bioturbated sandstone (Fig. 4c). It could be inferred that the minor and abrupt stress rotation of across a fracture at approximately m logging depth may be caused by the mechanical properties such as Young's modulus change associated with the lithology change. Another example of an abrupt rotation in stress orientation (Fig. 5, in the vicinity of m) was identified on the basis of drillinginduced tensile fractures. Across two fractures (logging depth, around Fig. 4. An example of an abrupt but minor orientation change of the principal horizontal stress across a fracture (at m depth) and examples of no orientation change around several fractures are shown in the optical image in (a) and the lower fracture in the optical image in (c). (b) FMI electrical image. (d) A plot of the S Hmax azimuth over the corresponding depth range.

6 W. Lin et al. / Tectonophysics 482 (2010) Fig. 5. Examples of an abrupt but minor stress orientation change across two fractures (around m) determined from drilling-induced tensile fractures and breakout suppression at the lithologic boundary ( m depth). (a and c) Optical images of the flat surface of the half-core splits. (b) FMI electrical image m; 30/120; approximately parallel to bedding of the Chinsui Shale), the locations of the tensile fractures, which are the same as the azimuths of S Hmax (Zoback et al., 2003), abruptly rotated by 12 approximately. For this example, the main reason that induced abrupt stress rotation might be the presence of the fractures Gradual change in stress orientation in the vicinity of faults and fractures Immediately below a minor fault (approximately 3 cm thick; logging depth, m; 30/130 approximately) (Fig. 6a c), breakouts were identified that gradually rotate below the fault by 90 approximately over an interval of approximately 2.7 m, until reaching another fracture at m depth. Below this fracture (978.0 m), the breakouts do not rotate but maintain an almost constant azimuth consistent with the regional stress orientation. Because the gradual stress rotation is local (see the arrow labeled as Fig. 6 in Fig 2) and the lithologies in hanging and foot walls are almost the same (Fig. 6a and b), the rotation might be interpreted due to the presence of the minor fault. Similarly, in the SAFOD (San Andreas Fault Zone Observatory at Depth) pilot hole, Hickman and Zoback (2004) observed gradual rotations of the localized principal stress orientation around fault zones. For example, the apparent azimuth of S Hmax just below a minor fault gradually rotated by approximately 70 over an interval of only 5 m. Moreover, Shamir and Zoback (1992) and Barton and Zoback (1994) presented additional examples of stress rotation associated with fault movements from the Cajon Pass drill hole near the San Andreas fault and the KTB (Kontinentales Tiefbohprogramm der Bundesrepublik) drill hole, respectively.

7 88 W. Lin et al. / Tectonophysics 482 (2010) Fig. 6. Examples of a gradual change in the stress orientation below a minor fault (975.4 m), of breakout suppression at the same minor fault and of no change in the stress orientation across a fracture (978.0 m). (a and f) Optical photographs of the cylindrical core. (b and e) Optical images of the flat surface of the half-core splits. (c) FMI electrical image. (d) A plot of the S Hmax azimuth over the corresponding depth interval Suppression of breakouts at faults or fractures or lithologic boundaries Breakouts were absent in the hanging wall of the minor fault at m logging depth described in Section 3.2, but breakouts were present in the foot wall of the fault (Fig. 6c). This suggests that breakouts were suppressed at this minor fault. It is clear from the optical core images that the lithology of the hanging and foot walls is almost the same (Fig. 6a and b), suggesting that their compressive

8 W. Lin et al. / Tectonophysics 482 (2010) strengths are likely to be approximately equal. Thus, we can infer that the stress state, especially the magnitude of the maximum principal horizontal stress, differs greatly between the two parts. Another example of suppression of breakouts at a minor fault (thickness, b5 mm with thin and soft gouge; dip, 40/260 approximately; a right-lateral oblique strike-slip fault) similarly suggests that the magnitude of the maximum principal horizontal stress differs between the hanging and foot walls (Fig. 7). It is likely that the stress magnitude changes stepwise across the fault. An example of breakout suppression at a lithologic boundary can be recognized at around m logging depth (Fig. 5b and c). Above the boundary is pure sandstone (i.e. without bioturbation), and below it is sandstone with strong bioturbation. The porosity, measured from the core samples from TCDP hole B, of the pure sandstone is 22.7±3.5% (average±standard deviation, n=95) and that of the sandstone with bioturbation is 10.3±3.6% (n=98) (Lin et al., 2008). Because of the higher porosity of the pure sandstone, it is reasonable to infer that its compressive strength is lower than that of the sandstone with bioturbation. As a suite of useful experimental data, Chen (2005) conduct uniaxial compression tests with the TCDP hole A core samples under water-saturated conditions. The author showed that the average porosity and uniaxial compressive strength of a pure sandstone specimen were 15.1% and 6.4 MPa, respectively; whereas the values of sandstone with bioturbation were 6.4% and 30.4 MPa (average, n=3), respectively. Therefore, the prediction that the sandstone with bioturbation is stronger much than the pure sandstone can be considered to be reliable. Thus, the presence of breakouts in the stronger rock, and not in the weaker rock, is interesting, because in general breakouts occur easily in rock with lower strength if the stress magnitudes are the same. Therefore, a possible interpretation is that the magnitude of the maximum principal horizontal stress in the sandstone with bioturbation is higher than that in the pure sandstone. If this interpretation is true, then Young's modulus of the sandstone with bioturbation is much higher than that of the pure sandstone, which suggests that the harder or stronger rock formation bears a high load due to gravitational or tectonic loading. These observations again indicate that the stress distribution is not homogeneous in actual, natural formations. In addition, the azimuth of the breakout below the lithologic boundary differs by approximately 90 from that of the tensile fractures above the boundary (Fig. 5b). That is, the orientation of S- Hmax is the same above and below the boundary, although it fluctuates around some fractures. Taken together, these observations of stress orientation and our interpretation of a stress magnitude change, mentioned above, suggest that the magnitude of S Hmax changed stepwise, but its orientation remained the same, across this lithologic boundary No change in stress orientation across faults or fractures Stress orientations do not always change around faults and fractures. For example, the breakouts remain at almost the same azimuth in the vicinity of a minor fault (logging depth, m; 30/ 120 approximately) (Fig. 8) inferring S Hmax orientation does not change. Other examples of an approximately constant stress orientation across fractures occur around several fractures between and m depth (Fig. 4a) and around a fracture at m depth (Fig. 4c), all of which are parallel to the formation bedding with 30/ 120. Also, around a fracture at m depth (30/120 approximately), no stress orientation change was recognized (Fig. 6c, e, and f). The lack of a stress orientation change suggests that the stress state remains mostly continuous around faults and fractures if the lithologies of the Fig. 7. An example of breakout suppression at a fracture ( m depth). (a) FMI electrical image. (b) Optical image of the flat surface of the half-core split. (c) Optical photograph of the cylindrical core.

9 90 W. Lin et al. / Tectonophysics 482 (2010) Fig. 8. Example of consistent stress orientation across a minor fault ( m). (a) FMI electrical image. (b) Optical image of the flat surface of the half-core split. (c) Optical photograph of the cylindrical core. hanging and foot walls are similar and if shear stresses on the faults or fractures are less than their frictional strengths. 4. Distribution and quantity of faults and fractures according to stress orientation change patterns We determined the numbers of faults and fractures in the target depth range of m with respect to the four patterns of stress orientation change (Table 2). Only those faults and fractures accompanied by stress-induced borehole breakouts or drillinginduced tensile fractures, where it was possible to examine any rotations of stress orientation, were counted. Thus, not all faults and fractures in the depth range were included. The proportion of minor faults with stress changes, including abrupt or gradual orientation rotations or suppression of breakouts, was clearly higher than that of fractures (Table 2). Therefore, the stress orientation more often rotated in the vicinity of minor faults than in the vicinity of fractures. The reason may be that faults with gouge slip more easily than fractures. In detail, the stress state can remain continuous across a plane such as a fault or a fracture if the shear stress on the plane is not larger than the product of the frictional coefficient and the normal stress on the plane. In general, a fracture surface is likely to be rougher than a fault surface; on the other hand, fault gouge, which has a relatively low frictional coefficient, is usually present in a fault. Therefore, the frictional coefficient of a fracture might be higher than that of a minor fault (Byerlee, 1978; Mizoguchi et al., 2008). Consequently, a stress change is more likely in the vicinity of minor faults compared with fractures. Another interesting observation is that suppression of breakouts was relatively common for both minor faults and fractures. Breakouts often occurred and then disappeared at faults or fractures or lithologic boundaries. This situation might reflect a stress state change in the vicinity of faults and fractures. On the other hand, in addition to a stress change, at a lithologic boundary, a strength change on either side of the boundary might also be an important factor. 5. Conclusions To understand stress perturbations associated with minor faults and fractures, we investigated all of the faults and large fractures accompanied by borehole breakouts or drilling-induced tensile fractures and their relationship to stress changes in TCDP hole B. We identified faults and fractures not only from the FMI electrical images of the borehole wall but also from photographs and descriptions of drilling core samples, and measured the variations in the principal horizontal stress orientation in the vicinity of the faults and fractures ascertained from the breakouts identified in the electrical images. Identification of geological structures (faults, fractures, and lithologic boundaries) from electrical images alone is sometimes difficult. Therefore, we used both electrical images and core photographs to identify the geological structures. Table 2 Minor faults and fractures accompanied by breakouts or drilling-induced tensile fractures. Total number of faults or fractures Stress orientation change patterns a Number of faults or fractures Minor faults b 30 c Abrupt change 8 27 Gradual change 4 13 Suppression of breakouts 6 20 No change Fractures 65 Abrupt change 9 14 Gradual change 2 3 Suppression of breakouts No change a b Percentage See the text for detailed definitions of the stress orientation change patterns. The number of minor faults with stress-induced borehole breakouts or drillinginduced tensile fractures in their vicinity between approximately 950 and 1330 m depth in TCDP hole B. Thus, not all minor faults in the depth range are included. c The minor fault at approximately m, shown in Fig. 5, was counted as exhibiting both a gradual change and suppression of breakouts. Therefore, the sum of the third column exceeds this total, and the sum of the percentages does not equal 100%.

10 W. Lin et al. / Tectonophysics 482 (2010) We found four patterns of stress changes in the vicinity of faults and fractures in TCDP hole B: (i) the stress orientation (breakout azimuth) rotates abruptly (discontinuously) in the vicinity of the faults or fractures; (ii) the orientation rotates gradually; (iii) breakouts are suppressed at faults, fractures, or lithologic boundaries; or (iv) the orientation does not change across faults or fractures. We recognized stress fluctuations, that is, heterogeneous mesoscale (N10 cm) stress distributions with regard to both stress orientation and magnitude. We also found that stress state changes were common in the vicinity of faults, fractures, and lithologic boundaries. As further research, the stress change in the vicinity of faults and fractures should be quantified and interpreted in detail to better understand the relationships between stress changes and faults or fractures. Acknowledgments We greatly appreciate Mark Tingay, the guest editor and two reviewers (Birgit Müller and the other anonymous reviewer) for their valuable and constructive comments and suggestions which helped us to improve our manuscript much. We gratefully acknowledge H. Ito, W. Soh, M. Kinoshita, and S. Saito of the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) for useful discussions, C.-Y. Wang, K.-F. Ma, S.-R. Song, and Y.-B. Tsai, who are the principal investigators of TCDP, Taiwan, and Y. Kawamura, Y. Sanada, and T. Moe of the Center for Deep Earth Exploration at JAMSTEC for helpful support during the FMI logging in TCDP hole B. W. Lin thanks the Japan Society for the Promotion of Science (JSPS) for financial support (Grant-in-Aid for Scientific Research C: ). References Barton, C.A., Zoback, M.D., Stress perturbations associated with active faults penetrated by boreholes: possible evidence for near-complete stress drop and a new technique for stress magnitude measurement. J. Geophys. Res. 99, Barton, C.A., Zoback, M.D., Burns, K.L., In situ stress orientation and magnitudes at the Fenton geothermal site, New Mexico, determined from wellbore breakouts. Geophys. Res. Lett. 15, Blenkinsop, T.G., Kinematic and dynamic fault slip analyses: implications from the surface rupture of the 1999 Chi-Chi, Taiwan, earthquake. J. Struct. Geol. 28, Byerlee, J.D., Friction of rocks. Pure Appl. Geophys. 116, Chen, C.-W., The Study of the Mechanical Characteristics from the Host Rock of the Chelungpu Fault, M.S., Thesis, 169 pp, National Taiwan University, Taipei. (In Chinese with English abstract). Haimson, B., Micromechanisms of borehole instability leading to breakouts in rocks. Int. J. Rock Mech. Min. Sci. 44, Haimson, B.C., Chang, C., True triaxial strength of the KTB amphibolite under borehole wall conditions and its use to estimate the maximum horizontal in situ stresses. J. Geophys. Res. 107, doi: /2001jb Haimson, B., Lin, W., Oku, H., Song, S.-R., Integrating borehole breakout dimensions, strength criterion, and hydraulic fracturing to constrain the state of stress across the Chelungpu Fault, Taiwan. Tectonophysics. doi: /j.tecto Hardebeck, J.L., Stress triggering and earthquake probability estimates. J. Geophys. Res. 109, B doi: /2003jb Hickman, S., Zoback, M., Stress orientations and magnitudes in the SAFOD pilot hole. Geophys. Res. Lett. 31, L15S12. doi: /2004gl Hirono, T., Lin, W., Yeh, E.-C., Soh, W., Hashimoto, Y., Sone, H., Matsubayashi, O., Aoike, K., Ito, H., Kinoshita, M., Murayama, M., Song, S.-R., Ma, K.-F., Hung, J.-H., Wang, C.-Y., Tsai, Y.-B., High magnetic susceptibility of fault gouge within Taiwan Chelungpu fault: nondestructive continuous measurements of physical and chemical properties in fault rocks recovered from hole B, TCDP. Geophys. Res. Lett. 33, L doi: /2006gl Hirono, T., Yeh, E.-C., Lin, W., Sone, H., Soh, W., Hashimoto, Y., Matsubayashi, O., Aoike, K., Ito, H., Kinoshita, M., Murayama, M., Song, S.-R., Ma, K.-F., Hung, J.-H., Wang, C.-Y., Tsai, Y.-B., Kondo, T., Nishimura, M., Moriya, S., Tanaka, T., Fujiki, T., Maeda, R., Muraki, H., Kuramoto, T., Sugiyama, K., Sugawara, T., Nondestructive continuous physical property measurements of core samples recovered from hole B, Taiwan Chelungpufault Drilling Project. J. Geophys. Res. 112, B doi: /2006jb Hung, J.-H., Ma, K.-F., Wu, Y.-H., Wu, H.-Y., Ito, H., Lin, W., Yeh, E.-C., Structure geology, physical property, fault zone characteristics and stress state in scientific drill holes of Taiwan Chelungpu Fault Drilling Project. Tectonophysics 466, doi: /j.tecto Ji, C., Helmberger, D.V., Wald, D.J., Ma, K.-F., Slip history and dynamic implications of the 1999 Chi-Chi, Taiwan, earthquake. J. Geophys. Res.108. doi: /2002jb Kao, H., Chen, W.-P., The Chi-Chi earthquake sequence: active out-of-sequence thrust faulting in Taiwan. Science 288, Kao, H., Angelier, J., Stress tensor inversion for the Chi-Chi earthquake sequence and its implications on regional collision. Bull. Seismol. Soc. Am. 91, Lee, J.-C., Chu, H.-T., Angelier, J., Chan, Y.-C., Hu, J.-C., Lu, C.-Y., Rau, R.-J., Geometry and structure of northern surface ruptures of the 1999 Mw=7.6 Chi-Chi Taiwan earthquake: influence from inherited fold belt structures. J. Struct. Geol. 24, Lin, W., Yeh, E.-C., Ito, H., Hung, J.-H., Hirono, T., Soh, W., Ma, K.-F., Kinoshita, M., Wang, C.-Y., Song, S.-R., 2007a. Current stress state and principal stress rotations in the vicinity of the Chelungpu fault induced by the 1999 Chi-Chi, Taiwan, earthquake. Geophys. Res. Lett. 34, L doi: /2007gl Lin, W., Yeh, E.-C., Ito, H., Hirono, T., Soh, W., Wang, C.-Y., Ma, K.-F., Hung, J.-H., Song, S.-R., 2007b. Preliminary results of stress measurement by using drill cores of TCDP hole-a: an application of anelastic strain recovery method to three-dimensional in-situ stress determination. Terr. Atmos. Ocean. Sci. 18. doi: /tao (tcdp). Lin, A.T., Wang, S.M., Hung, J.H., Wu, M.S., Liu, C.S., 2007c. Lithostratigraphy of the Taiwan Chelungpu-fault Drilling Project-A borehole and its neighboring region, central Taiwan. Terr. Atmos. Ocean. Sci. 18. doi: /tao.tcdp0706s03. Lin, W., Matsubayashi, O., Yeh, E.-C., Hirono, T., Tanikawa, W., Soh, W., Wang, C.-Y., Song, S.-R., Murayama, M., Profiles of volumetric water content in fault zones retrieved from hole B of the Taiwan Chelungpu-fault Drilling Project (TCDP). Geophys. Res. Lett. 34, L doi: /2007gl Ma, K.-F., Song, T.-R., Lee, S.-J., Wu, H.-I., Spatial slip distribution of the September 20, 1999, Chi-Chi, Taiwan earthquake (Mw 7.6) inverted from teleseismic data. Geophys. Res. Lett. 27, Ma, K.-F., Chan, C.-H., Stein, R.S., Response of seismicity to Coulomb stress triggers and shadows of the 1999 Mw=7.6 Chi-Chi, Taiwan, earthquake. J. Geophys. Res. 110, B05S19. doi: /2004jb Ma, K.-F., Tanaka, H., Song, S.-R., Wang, C.-Y., Hung, J.-H., Tsai, Y.-B., Mori, J., Song, Y.-F., Yeh, E.-C., Soh, W., Sone, H., Kuo, L.-W., Wu, H.-Y., Slip zone and energetics of a large earthquake from the Taiwan Chelungpu-fault Drilling Project. Nature 444, doi: /nature Mizoguchi, K., Takahashi, M., Tanikawa, W., Masuda, K., Song, S.-R., Soh, W., Frictional strength of fault gouge in Taiwan Chelungpu fault obtained from TCDP hole B. Tectonophysics 460, doi: /j.tecto Moos, D., Zoback, M.D., Utilization of observations of well bore failure to constrain the orientation and magnitude of crustal stresses: application to continental, deep sea drilling project, and ocean drilling program boreholes. J. Geophys. Res. 95, Seeber, L., Armbruster, J.G., Earthquakes as beacons of stress change. Nature 407, Shamir, G., Zoback, M.D., Stress orientation profile to 3.5 km depth near the San Andreas Fault at Cajon Pass, California. J. Geophys. Res. 97, Shin, T.-C., Teng, T.-L., An overview of the 1999 Chi-Chi, Taiwan, Earthquake. Bull. Seismol. Soc. Am. 91, Stein, R.S.,1999. The role of stress transfer in earthquake occurrence. Nature 402, Vernik, L., Zoback, M.D., Estimation of maximum horizontal principal stress magnitude of from stress-induced well bore breakouts in the Cajon Pass Scientific Research Borehole. J. Geophys. Res. 97, Wang, C.-Y., Li, C.-L., Yeh, H.-Y., Mapping the northern portion of the Chelungpu fault, Taiwan by shallow reflection seismics. Geophys. Res. Lett. 29. doi: / 2001GL Wang, W.-H., Chen, C.-H., Static stress transferred by the 1999 Chi-Chi, Taiwan, earthquake: effects on the stability of the surrounding fault systems and aftershock triggering with a 3D fault-slip model. Bull. Seismol. Soc. Am. 91, Wu, H.-Y., Ma, K.-F., Zoback, M., Boness, N., Ito, H., Hung, J.-H., Hickman, S., Stress orientations of Taiwan Chelungpu-fault Drilling Project (TCDO) hole-a as observed from geophysical logs. Geophys. Res. Lett. 34, L doi: /2006gl Yabe, Y., Song, S.-R., Wang, C.-Y., In-situ stress at the northern portion of the Chelungpu fault, Taiwan, estimated on boring cores recovered from a 2-km-deep hole of TCDP. Earth Planets Space 60, Yamashita, F., Fukuyama, E., Omura, K., Estimation of fault strength: reconstruction of stress before the 1995 Kobe earthquake. Science 306, Yeh, Y.-H., Barrier, E., Lin, C.-H., Angelier, J., Stress tensor analysis in the Taiwan area from focal mechanisms of earthquakes. Tectonophysics 200, Yeh, E.-C., Sone, H., Nakaya, T., Ian, K.-H., Song, S.-R., Hung, J.-H., Lin, W., Hirono, T., Wang, C.-Y., Ma, K.-F., Soh, W., Kinoshita, M., Core description and characteristics of fault zones from hole-a of the Taiwan Chelungpu-fault Drilling Project. Terr. Atmos. Ocean. Sci. 18, doi: /tao (tcdp). Yue, L.-F., Suppe, J., Hung, J.-H., Structural geology of a classic thrust belt earthquake: the 1999 Chi-Chi earthquake Taiwan (Mw=7.6). J. Struct. Geol. 27, Zoback, M.D., Moos, D., Mastin, L., Andersson, R.N., Well bore breakouts and insitu stress. J. Geophys. Res. 90, Zoback, M.D., Barton, C.A, Brudy, M., Castillo, D.A., Finkbeiner, T., Grollimund, B.R., Moos, D.B., Peska, P., Ward, C.D., Wiprut, D.J., Determination of stress orientation and magnitude in deep wells. Int. J. Rock Mech. Min. Sci. 40,