Subsurface structure, physical properties, fault-zone characteristics and stress state in scientific drill holes of Taiwan Chelungpu Fault Drilling Project
Introduction
The 1999 Chi-Chi earthquake (Mw 7.6) produced over 90 km-long surface rupture zone along the north–south trending, west-vergent Chelungpu fault (MOEACGS, 2000; Fig. 1). The most striking feature of the coseismic displacement field in the hanging-wall is that areas of large surface displacement lie above the footwall ramp of the thrust and at the northern termination; by contrast, relatively smaller, but significant (1–3 m) displacements are recorded at the footwall detachment (Yu et al., 2001, Dominguez et al., 2003, Lee et al., 2003). Along strike, both horizontal and vertical components of surface displacements increase from south to north and reach up to 12 m at the northern end near the Shihgang area. Surface ruptures indicate that the Chelungpu thrust runs parallel in map-view to the hanging-wall of the Pliocene Chinshui Shale, which indicates that the fault plane is a detachment in the Chinshui Shale.
In map-view (CPC, 1982), south of Wufeng village, the Chelungpu fault merged with the Sanyi fault to the west into single fault (Chang, 1971; called Chelungpu-Sanyi fault hereafter) and emplaced the Pliocene Chinshui Shale on top of the Pleistocene Tokoushan Formation and Holocene deposits. The subsurface Chelungpu–Sanyi fault plane, imaged by both shallow and deep seismic profiling (Wang et al., 2002, Wang et al., 2004), shows a ramp-flat geometry from the base of the Chinshui Shale (Hung and Suppe, 2002, Yue et al., 2005).
Towards north, the Chelungpu–Sanyi fault branches into two segments (Fig. 1): a) the underlying fault, called Sanyi thrust, which steps up from a deeper Pliocene and late-Miocene detachment in the Kueichulin and Tungkeng Formations and is probably not active, and b) the North Chelungpu detachment (called Chelungpu fault hereafter) which is a relatively young, nearly bedding-parallel fault in the Chinshui Shale and is the focus of some largest displacements in the Chi-Chi earthquake. Subsurface investigations of northern fault segments through a number of shallow seismic (Wang et al., 2002), deep petroleum seismic profiles (Hung and Wiltschko, 1993) and shallow drilling (Tanaka et al., 2002, Huang et al., 2002) confirm that the near-surface segment of the Chelungpu thrust is parallel to both bedding and the underlying Sanyi fault to a depth of 3 km. The TCDP deep boreholes are drilled through this high-slip portion of the Chelungpu fault and through the Sanyi ramp in the footwall.
An important question that needs to be addressed is what physical properties or dynamic processes within the fault zone cause large coseismic displacement in the northern segment. Hypotheses have been proposed include: 1) change of the fault-plane geometry; 2) static (long-term) physical properties such as intrinsic low coefficient of friction, high pore-pressure and solution-transport chemical processes, and 3) dynamic change of physical properties during slip. A detailed three-dimensional subsurface structure of the Chelungpu fault shows that surface coseismic displacements mimic the growth of active structures and geomorphology (Hung and Suppe, 2002). Areas of large displacements in the north of the Chelungpu fault could be associated with a reduction of overburden as a result of elevated Chelungpu ramp and flat (Yue et al., 2005).
The level of dynamic friction and stress during earthquake rupture is a key unknown, and are earthquake faults “low” or “high” strength? (Zoback et al., 1987, Hickman, 1991). If the coefficient of friction, μ, follows Byerlee's Law and ranges from 0.6 to 0.9 (Byerlee, 1978), frictionally generated heat should be observed near the Chelungpu fault zone (Mori et al., 2002). On the other hand, the presence of clay-rich fault gouges (Vrolijk and Van der Pluijm, 1999) and/or permanently high pore pressure (Hubbert and Rubey, 1959, Suppe and Wittke, 1977) within the fault zone can effectively reduce fault strength and the coefficient of friction. Other solution-transport mechanisms such as pressure solution, fluid-assisted mineral reactions may be important to determine the rheology of fault zones and the time scale of interseismic strength recovery (Blanpied et al., 1998, Rutter et al., 2001). The role of these mechanisms in determining the fault strength and earthquake instability mechanisms are unknown because of uncertainties regarding the mineralogy, microstructures and physical properties of fault-zone materials and nature and distribution of fluids at focal depths. Moreover, the “smooth” (low-level of high-frequency radiation) and “rapid” (high velocity) motion and large displacement in the north as opposed to larger accelerations and smaller displacements in the southern segment (Ma et al., 2000) may be attributable to the heterogeneity of fault rocks. The fault plane lies at contact of shale and conglomerate in the south but within the Chinshui shale in the region of large displacements as revealed by cores from two shallow holes drilled into the Chelungpu fault in the area near Nantou and Fengyuan, respectively (Huang et al., 2002). In this regard, the physical properties of the fault-zone material and width and roughness of the fault zone probably vary considerably along the fault, and this heterogeneity may play an important role for the above mechanisms to operate during the rupture propagation (Tanaka et al., 2002, Heermance et al., 2003).
Contrary to high rock strength from both laboratory experiments and in-situ stress measurement, many dynamic weakening mechanisms (Mori et al., 2002) include slip weakening (Scholz, 1998, Scholz, 2002), thermal pressurization (Noda and Shimamoto, 2004, Wibberley and Shimamoto, 2005), mechanical lubrication (Ma et al., 2003) and melting (Hirono et al., 2006). To differentiate these broad ranges of mechanisms will require the knowledge of physical properties of fault rocks, heat flow and stress measurements before and after the earthquake to quantify the energy release in seismogenic zones (Wang, 2006). For example, physical examination of fault-zone rocks could make it possible to infer deformation mechanisms of fault zone and dynamic frictional characteristics.
To address the above questions two holes (A and B) were drilled for Taiwan Chelungpu Fault Drilling Project (TCDP) during 2004–2005 at Dakeng, west-central Taiwan, where large surface slip (~ 10 m, station M324 shown in Fig. 2) was observed. Continuously coring and geophysical down-hole logging in two holes 40 m apart were completed from a depth of 500 to 2003 m (hole-A) and 950 to 1350 m (hole-B). Down-hole long-term monitoring such as temperature and seismicity were deployed after the drilling. In this paper we integrate results from cores and wire-line down-hole geophysical logs to characterize subsurface structure, physical properties of formations and fault zones, focusing on the Chi-Chi rupture zone. In-situ stress measurements, high-resolution microresistivity images (FMI and FMS, both marks of Schlumberger) of the borehole wall and shear-wave velocity anisotropy provide stress state post-Chi-Chi earthquake around the drill site.
Section snippets
Geology at drill site and fault-zone characteristics
Surface geology (CPC, 1982) near the drill site shows that surface ruptures of the Chi-Chi earthquake are confined within but cuts up and down horizons of the Chinshui Shale (Fig. 1). Formations encountered in hole-A are mainly composed of clastic sedimentary rocks from Upper Miocene Kueichulin Formation to Pliocene Cholan formation. Precise locations of formation boundaries were made by, 1) correlating wire-line logs among hole-A and other nearby petroleum wells and 2) comparing stratigraphic
Rock physical properties from down-hole logging
A comprehensive suit of geophysical logs was collected in hole-A from a depth of 500 m to 1750 m. P- and S-wave sonic velocity, gamma ray, electrical resistivity, density and temperature were recorded at a 15 cm sampling interval. Electrical image logs (FMS/FMI) are also acquired in both hole-A and hole-B to facilitate the analysis of fractures and faults intersected by the borehole as well as stress-induced wellbore failures (borehole breakout). The hole-deviation is less than 4° above depth
Hydraulic fracturing
A standard commercial procedure of open-hole, extended leak-off tests (see Engelder, 1993) were conducted in hole-B at depths between 940 and 1350 m to determine in situ magnitudes of maximum (SHmax) and minimum (Shmin) horizontal stresses. Dual straddle packers connected by tubing pipes were used to isolate an interval of the wellbore, and fluid was pumped into the open-hole section between the upper and lower packers. We apply fresh water to pressurize the intervals with several
Discussion
We have characterized the subsurface structure, and physical properties of formation and fault zones, in-situ stress in the TCDP boreholes through core studies, down-hole geophysical measurements and hydrofracturing experiments. Essential features of individual category have been illustrated in the previous sections. In this section we provide interpretation on the origin of subsurface structure and the causes that could affect the anisotropy of shear-wave velocity. An further extension study
Conclusions
The major scientific goal of the TCDP boreholes is to understand the physical mechanism involved in the large displacement during the Chi-Chi earthquake. We have attempted to answer this and relevant questions through studies of subsurface structure, fault-zone fabrics, formation physical properties and in-situ stress state. Consistent correlations among fault-zone fabrics, physical properties and clay mineralogy enable us to identify the bedding-parallel fault zone at depth 1111 m in hole-A
Acknowledgements
We are grateful to all the participants of TCDP including project principal investigators, international collaborators from USGS, Stanford University, JAMSTEC, and universities and research institutes in Japan, and field assistants and students from NCU and NTU. Funding of the TCDP is primarily from National Science Council (NSC), R.O.C. We thank the International Continental Scientific Drilling Program (ICDP) for providing partial fund and technical consult. Comments and suggestions from David
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Now at the Department of Geological Sciences, National Taiwan University, Taipei, Taiwan.