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

當地震發生時，斷層泥中之主要滑動面產生物理與化學轉變，導致斷層泥中之礦物發生礦物相改變與新礦物生成。斷層泥中含有磁性礦物，可經由斷層滑動產生之摩擦熱與液體的作用而生成。因此在地震發生的過程中，斷層泥具有獲得磁性紀錄的能力，可用來作為判定地震滑動帶的工具。此外經由分析受地震作用而改變與新生成的磁性礦物，可進一步回推地震時斷層泥中之物理與化學變化過程。 本研究利用台灣車籠埔斷層深鑽計劃中B井所連續取得之岩芯樣品，包含1999年規模7.6集集地震之車籠埔斷層主要滑動帶斷層泥，進行岩石磁學與古地磁學研究分析。此外本研究亦採集車籠埔斷層地表破裂帶之斷層泥進行奈米粒子分析。 本論文第一部份利用古地磁學方法分析指出，深度1,136公尺斷層帶(亦稱FZB1136)之斷層泥之古地磁記錄方向與現今地磁場方向相符，而其他斷層帶與圍岩之記錄不符，判斷FZB1136為集集地震主要滑動帶，此外由岩石磁學結果，可指出集集地震在此斷層泥中主要滑動面之位置。此斷層泥中獲得殘磁記錄之主要磁性礦物為磁鐵礦與針鐵礦，磁鐵礦主要分佈於主要滑動面，而針鐵礦分佈於整個斷層泥之中。本研究提出一斷層泥在地震過程中重新磁化模式：（一）殘磁記錄在地震間隔時期保存（二）地震時殘磁記錄消失（三）在地震之後斷層泥中液體冷卻過程中獲得地磁場記錄。 本論文第二部份，利用顯微觀察與岩石磁學方法，分析FZB1136斷層帶和錦水頁岩中黃鐵礦與磁性礦物之關係。錦水頁岩中可發現許多微球狀黃鐵礦與自形黃鐵礦，磁性礦物以磁鐵礦、硫複鐵礦與細顆粒磁黃鐵礦為主。在斷層泥中黃鐵礦數量較圍岩減少許多，主要磁性礦物為針鐵礦、磁黃鐵礦與部份氧化磁鐵礦。此三種磁性礦物為地震後所新生成，磁硫鐵礦成因可能為黃鐵礦在多次地震過程中，受摩擦熱高溫（溫度高於攝氏五百度）分解形成。針鐵礦成因推論因為集集地震後，造成之熱液（溫度高於攝氏三百五十度）於降溫過程中所形成，此熱液亦可解釋磁鐵礦部份氧化與磁黃鐵礦逆轉化為黃鐵礦之成因。這些新生成之磁性礦物可提供研究沈積岩中斷層帶，於地震過程產生物理化學變化之指標。 本論文第三部份，分析FZB1136斷層帶之磁學參數，可觀察到斷層帶中之磁感率最高值與等溫殘磁最高值，具有約四公分之偏移，磁感率最高值位於主要滑動面，而等溫殘磁最高值則位於斷層帶中央。然而此兩磁性參數皆為磁性礦物豐度指標，斷層帶中主要磁性礦物為針鐵礦與磁鐵礦，此兩礦物之磁學特性不同且分佈範圍不同，可能為造成此兩磁性豐度參數偏移之原因。本研究利用針鐵礦與磁鐵礦分佈模型，利用其磁學特性參數不同，模擬計算斷層帶之磁感率與等溫殘磁分佈曲線，此模式計算出磁鐵礦在主要滑動面的最大豐度值約為300 ppmv，而針鐵礦在斷層帶中央最大豐度約為1%。 本論文第四部份，為判斷地震斷層滑動摩擦造成斷層泥粒子破裂之最小粒徑，用以協助斷層滑動破裂能估算，本研究採樣集集地震車籠埔斷層地表破裂面斷層泥，分離不同顆粒大小粒子至奈米粒徑，使用同步輻射X-光粉末繞射儀與穿透式電子顯微鏡進行分析觀察礦物相與產狀。斷層泥主要礦物成份為石英、長石、澎潤石、伊萊石、綠泥石與高嶺石，而粒徑小於100奈米之粒子礦物組成為石英、澎潤石與伊萊石，然而在粒徑1至25奈米之粒子無石英存在，只含有澎潤石與伊萊石。澎潤石與伊萊石在此露頭環境條件下易風化新生成，石英則為穩定不易改變，因此石英可作為地震摩擦破碎指標，此斷層泥中破裂能估算之最小粒徑可至約25奈米。 本論文第五部份，為FZB1194與FZB1243兩斷層帶之磁學分析初步結果。此兩斷層帶中各含有一層黑色硬質塊，然而在FZB1136中並不含此層。此兩斷層帶之古地磁學記錄，皆具有一磁極正向與反向記錄，然而殘磁是由許多分向量所重疊構成，無法完全的分離每個分向量。反向磁極記錄成因有幾個可能性：（一）車籠埔斷層帶活動年代早於78萬年（二）地震發生在古地磁場漂移期（三）磁性礦物殘磁自反轉；未來需更進一步研究確定成因。由此兩斷層帶磁學參數分析結果中，可觀察到與FZB1136相同之磁感率最高值與等溫殘磁最高值偏移現象，顯示在此兩斷層帶中亦有不同磁性礦物豐度分佈變化。此兩斷層帶中主要磁性礦物種類為磁鐵礦與針鐵礦，然而與FZB1136具有幾點不同：（一）斷層泥中無單一分向量古地磁記錄（二）此兩斷層帶中存有超順磁性磁性顆粒存在（三）此兩斷層帶僅有部份FZB1243含有磁黃鐵礦。由這些現象顯示，此兩斷層帶所記錄之地震規模可能小於FZB1136所記錄之地震規模7.6集集地震。

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).