Increase in basin sediment yield from landslides in storms following major seismic disturbance

The landslide area along the Tachia River catchment of central Taiwan was investigated using the remote sensing images of various typhoon and earthquake events taken from 1996 to 2004 and the sediment discharge measured at hydrometric stations. Our findings indicate that 88% of the coseismic landslides triggered by the 1999 Chi-Chi earthquake were first-time occurrences. After the Chi-Chi earthquake, 59% of the landslide area was reactivated during typhoon Toraji and 66% during typhoon Mindulle. The landslides prone to reactivation were on the formations with closely spaced discontinuities. It is suggested that further rock-mass defects in the formations were added or opened by the earthquake, and that the landslide areas continued to extend until the end of 2004. Post-1999 rainstorms delivered large amounts of colluvial sediment into the main channel, leading to a 2-fold increase in post-seismic sediment discharge while the precipitation was only half that of the pre-earthquake rate.