Post-seismic surface processes in the Jiufengershan landslide area, 1999 Chi-Chi earthquake epicentral zone, Taiwan

The Jiufengershan rock avalanche was one of the largest and most damaging landslides triggered by the Chi-Chi earthquake (M_L = 7.3, M_W = 7.6) of 21 September 1999. The rock avalanche transported a mass of sedimentary rock 50 m thick and 1.5 km long, located on the western limb of the Taanshan syncline. The surface of rupture coincides with the bedding plane and dips moderately toward the Jiutsaihu valley. This paper is mainly devoted to the study of post-seismic surface processes that affected the sliding surface as well as the debris deposit, from September 1999 to February 2003. Large fractured blocks and a debris layer observed on the surface of rupture were subjected to mass wasting processes and denudation. The quantification of erosion was made using two different approaches. First, the subpixel correlation method was used to determine the horizontal displacement field from aerial photographs taken, respectively, 2 and 3.5 months after the earthquake. Displacements ranging from 1 to 6 m were observed around unstable blocks located at the western flank of the surface of rupture. Second, the co-seismic and post-seismic volume distributions in the sliding zone were determined from three digital elevation models, including a LiDAR image taken in 2002. Post-seismic erosion and deposition from September 1999 to April 2002 were mainly associated with mass wasting and denudation at the surface of rupture, deposition in small basins and lakes located in the debris deposit, and evacuation of materials from the debris deposit along natural and artificial drainage channels. The vertical compaction is 1% of the initial height of the deposit.