Triggering and runaway processes of catastrophic Tsaoling landslide induced by the 1999 Taiwan Chi-Chi earthquake, as revealed by high-velocity friction experiments

Pliocene sedimentary rocks of about 130 Mm³ in volume slid along bedding planes dipping by 14°, with an average speed of about 35 m/s, during the Tsaoling landslide. We conducted friction experiments to reproduce the initiation processes of this landslide, by idealizing landslide movements during the earthquake as accelerating/decelerating motion. Experiments were done on shale from the field, at 3 MPa normal stress corresponding to the overburden pressure. Results indicate that the accelerating/decelerating motion causes weakening and strengthening at each oscillation cycle and results in overall slip weakening which can be approximated as an exponential slip weakening. Behaviors during oscillatory slip are fairly similar to those during sliding at constant slip rates. Newmark analysis with measured frictional properties reveals that the landslide can be triggered with wet gouge properties, but the landslide motion stops with parameters for dry shale gouge. Delayed initiation of the landslide is consistent with a survivor's witness.