Rock magnetic properties of fault rocks from the rupture of the 2008 Wenchuan earthquake, China and their implications: Preliminary results from the Zhaojiagou outcrop, Beichuan County (Sichuan)

Rock magnetic measurements were conducted on fault rocks from the Zhaojiagou outcrop at Leigu Town, Beichuan County, Sichuan Province (China), located on the Yingxiu-Beichuan Fault, the main fault ruptured in the 2008 Wenchuan Ms 8.0 earthquake. Hysteresis measurements indicate that fault rocks contain very low concentrations of ferrimagnetic minerals, and their magnetic behavior is dominated by para-/diamagnetic components. Temperature-dependent magnetic susceptibility increased significantly above 400°C in the heating run. Magnetic behavior reveals the presence of siderite, lepidocrocite, chlorite and smectite as the possible major Fe-bearing minerals in fault rocks. Their thermal decomposition to ferrimagnetic minerals (e.g., magnetite and/or maghemite) could be responsible for the high magnetic susceptibility during the thermal treatment. A kinetic model analysis indicates that frictional heating could increase the temperature within the fault zone up to 320-908°C at depth of 590. m, and 335-952°C at depth of 618.3. m, respectively, during the seismic slip of the Wenchuan earthquake, depending on shear stress. Such high temperatures are enough to drive thermal decomposition of Fe-bearing minerals in fault rocks (in particularly, in fault gouge), to form ferrimagnetic magnetite and/or maghemite. This is the most likely mechanism for the reported high magnetic susceptibility anomalies within the fault zone in the pilot hole of Wenchuan Earthquake Fault Scientific Drilling Project (WFSD-1).