Subsurface structure, physical properties, fault-zone characteristics and stress state in scientific drill holes of Taiwan Chelungpu Fault Drilling Project

Abstract

Continuous cores and a suit of geophysical measurements were collected in two scientific drill holes to understand physical mechanisms involved in the large displacements during the 1999 Chi-Chi earthquake. Physical properties obtained from wire-line logs including P- and S-wave sonic velocity, gamma ray, electrical resistivity, density and temperature, are primarily dependent on parameters such as lithology, depth and fault zones. The average dip of bedding, identified from both cores and FMI (or FMS) logs, is about 30° towards SE. Nevertheless, local azimuthal variations and increasing or decreasing bedding dips appear across fault zones. A prominent increase of structural dip to 60°–80° below 1856 m could be due to deformation associated with propagation of the Sanyi fault.

A total of 12 fault zones identified in hole-A are located in the Plio-Pleistocene Cholan Formation, Pliocene Chinshui Shale and Miocene Kueichulin Formation. The shallowest fault zone occurs at 1111 m depth (FZ1111). It is a 1 m gouge zone including 12 cm of thick indurate black material. We interpreted this zone as the slip zone during Chi-Chi earthquake. FZ1111 is characterized by: 1) bedding-parallel thrust fault with 30-degree dip; 2) the lowest resistivity; 3) low density, V_p and V_s, 4) high V_p/V_s ratio and Poisson's ratio; 5) low energy and velocity anisotropy, and low permeability within the homogeneous 1 m gouge zone; 6) increasing gas (CO₂ and CH₄) emissions, and 7) appearance of smectite within the primary slip zone.

In situ stresses at the drill site were inferred from leak-off tests, borehole breakouts and drilling-induced tensile fractures from borehole FMS/FMI logs, and shear seismic wave anisotropy from DSI logs. The dominant fast shear-wave polarization direction is in good agreement with regional maximum horizontal stress axis, particularly within the strongly anisotropic Kueichulin Formation. A conjugate set of secondary directions are parallel to microcrack orientations. A drastic change of orientation of fast shear-wave polarization across the Sanyi thrust fault at the depth of 1712 m reflects the change of stratigraphy, physical properties and structural geometry.

Introduction

The 1999 Chi-Chi earthquake (M_w 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.