An earthquake slip zone is a magnetic recorder

During an earthquake, the physical and the chemical transformations along a slip zone lead to an intense deformation within the gouge layer of a mature fault zone. Because the gouge contains ferromagnetic minerals, it has the capacity to behave as a magnetic recorder during an earthquake. This constitutes a conceivable way to identify earthquake slip zones. In this paper, we investigate the magnetic record of the Chelungpu fault gouge that hosts the principal slip zone of the Chi-Chi earthquake (M_w 7.6, 1999, Taiwan) using Taiwan Chelungpu-fault Drilling Project core samples. Rock magnetic investigation pinpoints the location of the Chi-Chi millimeter-thick principal slip zone within the 16 cm thick gouge at ~1 km depth. A modern magnetic dipole of Earth's 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 interseismic 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 a frictional heating process in a seismic slip zone.