Rou-Fei Chen

Empirical Modal Decomposition of Near Field Seismic Signals of Tsaoling Landslide

Giant landslides can achieve high-speed sliding and long run out distances and estimating the kinematic is crucial for the next generation of the hazard mitigation system. Direct measurement data when landslides are in motion are valuable for the purpose. However, because of their scarce occurrences and short flow durations, such measurements are rarely recorded with instruments. In 1999, the Chi–Chi earthquake triggered a major landslide in this area, with a source volume 125×106 m3. Near the scar area, there is a strong ground motion station, CHY080, recorded the seismic signals during the earthquake and the recorded data exhibit some distinctive signatures. We analyze the polarizations of the seismic waves and use Ensemble Empirical Mode Decomposition (EEMD) with an additional clustering analysis to decompose the seismic signals. Series of peculiar wave packets associated with the landslide are identified. Based on these results, a complementary rigid sliding model is deployed to verify the sliding process. The results reveal that with the sliding distance 1,990 m, the maximum velocity reaches 78 m/s, and the mass generates a large collision impact against the riverbed and the steep slope on the other side of the river. The friction angle of the sliding surface could be as low as 6.9o. These results are agreeable with the numerical simulation of the landslide. These findings provide the evidence that the earthquake and the landslide induced ground motion coexist in the seismogram records.

The 1999 Tsao-Ling Rockslide: Source Area, Debris, and Life Cycle of Associated Rockslide-Dammed Lake (Central Taiwan)

The Tsao-Ling rock avalanche was the largest landslide triggered by the 1999 Chi–Chi earthquake in central Taiwan. In addition to detailed and extensive field observations, several Digital Elevation Models (DEMs) generated from sets of aerial photos have been utilized to document and measure the coseismic and post-seismic morphological changes at Tsao-Ling. The estimated volume of the initial rock avalanche is about 125.5 and 138 Mm3 for the depleted and accumulated zones, respectively, indicating an increase in volume due to fragmentation. The average thickness was about 150–170 m, up to 195 m of the slid mass–over 140 m on the Chingshui River channel while between 30 and 90 m debris covered on the preexisting debris deposit hill. The landslide debris blocked the river channel and formed a dammed lake, with a maximum volume of 45 Mm3. In the deposit area, strong river erosion has removed 72 Mm3 of the debris since the earthquake. By August 2004, the rockslide-dammed lake had filled up completely with sediment. The filling materials originated in numerous landslides in the upstream area.

Direct Measurements of Bedrock Incision Rates on the Surface of a Large Dip-slope Landslide by Multi-Period Airborne Laser Scanning DEMs

This study uses three periods of airborne laser scanning (ALS) digital elevation model (DEM) data to analyze the short-term erosional features of the Tsaoling landslide triggered by the 1999 Chi-Chi earthquake in Taiwan. Two methods for calculating the bedrock incision rate, the equal-interval cross section selection method and the continuous swath profiles selection method, were used in the study after nearly ten years of gully incision following the earthquake-triggered dip-slope landslide. Multi-temporal gully incision rates were obtained using the continuous swath profiles selection method, which is considered a practical and convenient approach in terrain change studies. After error estimation and comparison of the multi-period ALS DEMs, the terrain change in different periods can be directly calculated, reducing time-consuming fieldwork such as installation of erosion pins and measurement of topographic cross sections on site. The gully bedrock incision rate calculated by the three periods of ALS DEMs on the surface of the Tsaoling landslide ranged from 0.23 m/year to 3.98 m/year. The local gully incision rate in the lower part of the landslide surface was found to be remarkably faster than that of the other regions, suggesting that the fast incision of the toe area possibly contributes to the occurrence of repeated landslides in the Tsaoling area.