Magnetic properties of the Wenchuan Earthquake Fault Scientific Drilling Project Hole-1 (WFSD-1), Sichuan Province, China

We measured the magnetic susceptibility of the core from the first borehole of the Wenchuan Earthquake (May 12, 2008, Mw7.9) Fault Scientific Drilling Project (WFSD-1) at 1-cm intervals. The correlations between magnetic susceptibility anomalies and fault rock occurrence are shown by a few fault zones in the WFSD-1 core. The values for the mass and ferromagnetic material magnetic susceptibility for the sample at 589.25-m depth are higher than those for the other samples. All the thermomagnetic curves display a rapid increase in slope after 380 degrees C, and a marked peak occurs at about 510 degrees C in the heating curves. The cooling curves are clearly higher than the heating curves. The saturation magnetization (Ms) shows a significant peak at a depth of 589.25 m, as do the mass magnetic susceptibility and the ferrimagnetic magnetic susceptibility. The mechanism principally responsible for the high magnetic susceptibility at a depth of 589.25 m might be the production of new magnetite from iron-bearing silicates (e. g., chlorite) or clays caused by frictional heating during seismic slip. Therefore, we suggest that the presence of high magnetic susceptibility fault gouges in the same country rock can be considered as an indicator of earthquakes or seismic signatures.