New evidence of repeated earthquakes alongWenchuan earthquake fault zone

High resolution magnetic susceptibility and detailed rock magnetism studies of the Yingxiu-Beichuan fault zone were conduced on the field section at Bajiaomiao, Hongkou Town. The systematic analysis reveals the existence of quite a few high magnetic susceptibility fault rock zones. A brown layer fault rock has the following features:(1) the highest magnetic susceptibility, (2) significant characteristics of magnetite new formation, and (3) similar demagnetization behavior of natural remanent magnetization and anhysteretic remanent magnetization. The principal mechanism responsible for the high magnetic susceptibility of the brown fault rock was most likely caused by the production of new magnetites from iron-bearing paramagnetic minerals. These new magnetites might be formed by frictional heating on slip planes along a seismic fault. Combined with the analytical result of the core from the first drill hole of the Wenchuan Earthquake Fault Scientific Drilling Project (WFSD-1), the authors hold that the presence of high magnetic susceptibility fault gouges in the same country rock can be considered to be an indicator of earthquakes or seismic signatures. Quite a few layers of high magnetic susceptibility fault rocks indicate that strong earthquake repeatedly occurred alongWenchuan earthquake fault zone.