LAUSR

Abstract We used thermogravimetry to determine that the frictionally heated sliding surface reached ∼850 °C at the base of the 1.16 × 10 9  m 3 Daguangbao landslide, triggered during the 2008 Ms8.2 Wenchuan earthquake. Optical and electron microscope images reveal evidence of dolomite thermal decomposition, hot fluid penetrating fine cracks and dynamic recrystallization of minerals. A high-speed rotary shear apparatus simulated the rapid sliding and provided data on very low frictional resistance (friction coefficient, μ ≈ 0.05 ) during shearing above 1 m/s. Electron-probe analysis of samples from the experiments showed lime and magnesia from thermal decomposition of dolomite. Some of the sheared minerals formed a dynamically recrystallized layer ∼0.1 mm thick, and its viscous resistance would have contributed to the low apparent friction during high-speed sliding. A further explanation, however, was very high pore-fluid pressure from hot supercritical CO2 (and superheated steam at a partial pressure of >10 MPa in the field situation with groundwater). These friction-reducing mechanisms are not mutually exclusive and both were present simultaneously at the landslide base.