1999集集地震(Mw 7.6)滑動帶礦物學與磁學性質之研究與應用

Abstract

During an earthquake, the physical and chemical transformations along a slip zone lead to alteration and formation of minerals within the gouge layer of a mature fault zone. The gouge contains ferromagnetic minerals, which could be formed under the combined action of friction heat and fluid. Thus, gouge has the capacity to behave as a magnetic recorder during an earthquake. This may constitute an efficient way to identify earthquakes slip zones. Besides, altered and neoformed magnetic minerals can be used as tracers of some earthquake processes. In this study, we investigate the rock magnetism and paleomagnetism of the Chelungpu Fault gouge that hosts the principal slip zone of the Chi-Chi earthquake (Mw 7.6, 1999, Taiwan) using Taiwan Chelungpu-Fault Drilling Project (TCDP) hole-B core samples. We also took a Chelungpu fault outcrop sample for identification of nanoparticle, which associated with fracture energy estimation in fault gouge. In the first part of this thesis, we studied the rock magnetism and paleomagnetism of the 0.16 m thick gouge at 1,136 m depth (labeled FZB1136). The rock magnetic investigation pinpoints precisely the location of the Chi-Chi mm-thick principal slip zone. A modern magnetic dipole of Earth magnetic field is recovered throughout this gouge but not in the wall rocks nor in the two other adjacent fault zones. This magnetic record resides essentially in two magnetic minerals; magnetite in the principal slip zone, and neoformed goethite elsewhere in the gouge. We propose a model where the magnetic record: 1) is preserved during inter-seismic time, 2) is erased during co-seismic time and 3) is imprinted during post-seismic time when fluids cooled down. We suggest that the identification of a stable magnetic record carried by neoformed goethite may be a signature of friction-heating processes in the seismic slip zone. In the second part of the thesis, we investigate pyrite and magnetic minerals within the host Chinshui siltstone and the FZB1136 gouge. In the Chinshui siltstone, pyrite framboids of various sizes and euhedral pyrite are observed. The magnetic mineral assemblage comprises stoichiometric magnetite, greigite, and fine-grained pyrrhotite. The pyrite content is generally lower in the gouge compared to the wall rock. The magnetic mineral assemblage in the gouge consists of goethite, pyrrhotite, and partially oxidized magnetite. The pyrrhotite, goethite and some magnetite are neoformed. Pyrrhotite likely formed from high temperature decomposition of pyrite (>500°C) generated during co-seismic slip of repeated earthquakes. Goethite is inferred to have formed from hot aqueous co-seismic fluid (>350°C) in association with the 1999 Chi-Chi seismic event. Elevated fluid temperatures can also explain the partial alteration of magnetite and the retrograde alteration of some pyrrhotite to pyrite. We suggest that characterization of neoformed magnetic minerals can provide important information for studying earthquake slip zones in sediment-derived fault gouge. In the third part of the thesis, we aimed to model the observed 40 mm shift between the maximum of magnetic susceptibility and the maximum of magnetic remanence. The result of the model suggests that the maximum of the concentration of magnetite and goethite correspond to the maximum of magnetic susceptibility and magnetic remanence, respectively. By modeling the concentrations of these two magnetic minerals, we explain satisfactorily the profiles of magnetic susceptibility and remanence. This modeling indicates that ~300 ppmv of magnetite formed in the principal slip zone and its main contact area. Similarly, ~1% of goethite is formed in the center of the gouge, where the fluids are more enriched in iron. We propose that the magnetite and goethite are formed and altered during successive seismic cycles. In the fourth part of the thesis, we determined the ultrafine nano-scale grains of the Chelungpu fault gouge. The particle size range was analyzed using the synchrotron X-ray diffraction and observed through transmission electron microscopy. The minerals of gouge are predominantly composed of quartz, plagioclase, smectite, illite, chlorite, and kaolinite. The mineral association of <100 nm particles are quartz, smectite, and illite. However, there are only smectite and illite without quartz in the 1 to 25 nm fractions. We propose that quartz is the index mineral associated with co-seismic fracture and the minimum grain size is 25 nm. The smectite and illite nano-particles may be associated with weathering process of gouge at shallow or surface conditions. In the fifth part of this thesis, we show the preliminary results of magnetic analysis of FZB1194 and FZB1243. These two fault zones have a very dark centimeter black material disk (BMD) that is not present in the FZB1136. In both fault zone, the paleomagnetic record indicates the presence of stable components with normal and reverse polarities. However, these components appear to result from an overlap of several contributions, which the analysis did not separate properly. The identification of opposite polarity, could have serveral origins: 1) an age of Chelungpu fault greater than 780 ka, 2) earthquake occuring during paleomagnetic excursion, 3) self-reversal processes of magnetic mineral. There are similarities between these two fault zones and FZB1136. A shift between the peak of remanence and susceptibility is observed, which may reflect varying concentrations of magnetic minerals in the gouge. Magnetite and goethite are found ubiquitously in both fault zones. However, three observations mark a fundamental difference with FZB1136: 1) the absence of a homogeneous single component paleomagnetic throughout the gouge, 2) the preservation of magnetic nano-grains in FZB1194 and FZB1243, 3) the absence of pyrrhotite, which could be an indicator of high temperature transformation. We suggest that seismic events recorded in these two fault zones had a magnitude lower than that recorded in the FZB1136 (Chi-Chi, Mw 7.6).