High magnetic susceptibility produced in high-velocity frictional tests on core samples from the Chelungpu fault in Taiwan

We carried out high-velocity frictional tests on crushed fault gouge from core samples from Hole B of the Taiwan Chelungpu-fault Drilling Project to investigate the cause of high magnetic susceptibilities in the fault core. Black ultracataclasite resembling that observed in Hole B formed during the experiments, even under low axial stress of 0.5 to 1.5 MPa. The bulk magnetic susceptibility of the tested samples was proportional to the frictional work applied and increased as slip increased. Thermomagnetic analysis of the samples before frictional testing revealed that magnetization increased at temperatures above 400 degrees C, probably because of thermal decomposition of paramagnetic minerals. Both the thermally and mechanically induced formation of ferrimagnetic minerals by high-velocity friction might have caused a magnetic susceptibility anomaly. Our experimental results support the assumption that heat generation of short duration, even if it is below the melting point, can increase magnetic susceptibility.