The Finest Particle for Calculating Fracture Energy of Taiwan Chi-Chi Earthquake

Analyzing the particle size, shape and orientation on the faulting material has become a routine but important work in the fault zone. These data provide characteristics of fragmentation and comminuting process, and go further for information of fracture energy. In order to calculate the fracture energy, we need to know the particle size distributions of fault gouge which determine total fracture surface area. However, the finest particle of gouge or threshold of lower cut-off particle size is the most important parameter to dominate the amount of surface area. For example, there were different lower cut-off particle sizes in the Punchbowl and Chelungpu faults, with 1.6 nm and 50 nm respectively. Criteria for determining the threshold of lower cut-off size in the Chelungpu fault is the main purpose in this study.
For comparing the mineral assemblage and the finest particles, we collected the samples from wall rock, fault damage zone, and fault core. Then, using the wet sieving, sedimentation, ultracentrifugation and automated ultrafiltration device (AUD) we separated the sizes of particle. Furthermore, particles in different size ranges (whole, 450-2000nm, 100-450nm, 50-100nm, < 50nm) and mineralogy were analyzed by the SEM, TEM and Synchrotron XRD.
The wall rock from Chelungpu fault zone consists predominantly of quartz, chlorite, feldspar, kaolinite and 2M1-illite with the minimum particle size ranging from 50 to 100 nm. The results infer that the finest particle in natural sedimentary grinding is around 50 nm in diameter. In the other words, the fault core has the same mineral assemblage as wall rock, but the particle is finer than 50 nm, which comminuted by faulting. Based on the images of TEM, diameter of finest particle can be measured as, 29nm in average. Accordingly, this study calculates the fracture energy of 1999 Taiwan Chi-Chi earthquake as 6.04 MJm-2. Furthermore, the radiation efficiency is 0.83 that shows Chelungpu fault is a mature fault and the energy will be released with the radiation energy rather than the fracture energy during faulting.