Rock magnetic study of fault gouge zones within the Taiwan Chelungpu Fault System

During an earthquake, physical and chemical processes could lead the alteration and formation of magnetic minerals within the gouge layer in a mature fault zone. In this study, we apply the rock magnetic methods to study the Chelungpu fault gouge obtained from the Taiwan Chelungpu-fault Drilling Project hole-B core samples. Three major zones, labeled FZB1136 (fault zone at depth of 1,136 m in hole B, hosts the principal slip zone of the Chi-Chi earthquake), FZB1194, and FZB1243, were recognized in the Chelungpu fault system. Magnetite and goethite are found ubiquitously in all fault zones. However, the magnetic mineral assemblages of those three fault zones are different. A shift between the peak of remanence and susceptibility is observed, which may reflect varying concentrations of magnetic minerals in the gouge. Two remarkable observations from FZB1194 and FZB1243 point out a fundamental difference with FZB1136: 1) the preservation of magnetic nano-grains in FZB1194 and FZB1243, 2) the absence of pyrrhotite, which could be an indicator of high temperature transformation. These results reveal that different seismic physical/chemical alteration processes occurred among the three fault zones. Thus, magnetic properties of fault zone gouges could provide us valuable information for understanding the mechanism about the faulting processes syn- and post- the severe earthquakes.