Fault Structure Control on Fault Slip and Ground Motion during the 1999 Rupture of the Chelungpu Fault, Taiwan

Abstract

The Chelungpu fault, Taiwan, ruptured in a M w 7.6 earthquake on 21September 1999, producing a 90-km-long surface rupture. Analysis of core from two holes drilled through the fault zone, combined with geologic mapping and detailed investigation from three outcrops, define the fault geometry and physical properties of the Chelungpu fault in its northern and southern regions. In the northern region the fault dips 45Њ-60Њ east, parallel to bedding in both the hanging wall and footwall, and consists of a narrow (1-20 cm) core of dark gray, sheared clay gouge. The gouge is located at the base of a 30-to 50-m zone of increased fracture density confined asymmetrically to the hanging wall. Microstructural analysis of the fault gouge indicates the presence of extremely narrow clay zones (50-300 lm thick) that are interpreted as the fault rupture surfaces. Few shear indicators are observed outside of the fault gouge, implying that slip was localized within the gouge zone. Slip localization along a bed-parallel surface resulted in a narrow gouge zone that produced less high-frequency ground motion and larger displacements (average 8 m) during the earthquake than in the southern region. Displacement in the southern region averaged only 2 m, but ground shaking consisted of large amounts of highfrequency ground motion. The fault in the southern region dips 20Њ-30Њ at the surface and consists of a wide (20-70 m thick) zone of sheared, foliated shale with numerous gouge zones. These data demonstrate a potential correlation between fault structure (i.e., gouge width, geometry) and earthquake characteristics such as displacement and ground motion (i.e., acceleration).

How thick is a fault? Fault displacement-thickness scaling revisited

“…In fact, Schultz and Evans [1998, their figure 16] showed that the width of a single fault's DZ can vary by an order of magnitude depending on which deformation elements are used to define the DZ. Furthermore, the DZ is often asymmetric around the FC and PSZ [e.g., Shipton and Cowie, 2001;Heermance et al, 2003;Dor et al, 2005]. The dominant deformation elements within each part of the fault zone may also change over time, for instance due to varying stress conditions [Knipe and Lloyd, 1994] or rock rheology [Johansen et al, 2005].…”

Thermo- and hydro-mechanical processes along faults during rapid slip

“…Field observations of maturely slipped faults show a generally broad zone of damage by cracking and granulation (Chester et al, 1993), but nevertheless suggest that shear in individual earthquakes takes place with extreme localization to a long-persistent slip zone, <1-5 mm wide, within or directly bordering a finely granulated, ultracataclastic fault core (Chester and Chester, 1998;Chester at al., 2003, 2004. Heermance et al, 2003Wibberley and Shimamoto, 2003).…”

Subsurface Structure, Physical Properties, and Fault Zone Characteristics in the Scientific Drill Holes of Taiwan Chelungpu-Fault Drilling Project

“…The fault plane lies at the contact point of shale and conglomerate in the south but within the Chinshui Shale in the region of large displacements (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 Heermance et al 2003).…”