Paleo-earthquakes revealed by rock magnetic evidence from the Anxian-Guanxian Fault, Sichuan Province, China

To determine the rock magnetic characteristics of this co-seismic rupture generated by the 2008 Wenchuan earthquake in Sichuan Province, China, we excavated a trench on the Anxian-Guanxian fault. High-resolution magnetic susceptibility and other rock magnetic measurements were conducted on the field section and on samples from the trench. We divided the fault rocks into 8 units according to lithology and measurements of magnetic susceptibility were made on fault rocks within different units, both in the field and in the laboratory. The results demonstrate that the highest magnetic susceptibility occurs in the red gouge (64.08 x 10(-6) SI for surface magnetic susceptibility and 30.61 x 10(-8) m(3)/kg for mass magnetic susceptibility), while relatively high magnetic susceptibility occurs in the black gouge (14.31 x 10(-6) SI for surface magnetic susceptibility and 7.15x 10(-8 )m(3)/kg for mass magnetic susceptibility) compared to the protolith. More detailed magnetic measurements were used to determine the magnetic mineralogy of each unit. The principal mechanism responsible for the high magnetic susceptibility values of the red gouge is the neoformation of magnetite from siderite or pyrrhotite caused by thermal pressurization during the latest earthquakes, which reveals the location of the latest seismic slip zone. By contrast, the magnetic susceptibility values of the black gouge have been reduced by the formation of goethite during weathering. We conclude that although the occurrence of goethite reveals paleo-earthquake, factors such as the oxidation or dissolution of fluids that effect magnetic minerals need further investigation.