Non-catastrophic landslides induced by the Mw 7.6 Chi-Chi earthquake in central Taiwan as revealed by PIV analysis

The Chi-Chi earthquake occurred in the western foothills of central Taiwan, which triggered coseismic and catastrophic Chiufengershan dip-slope landslides, Tsaoling rockslides and surface stripping of the Jiujiufeng area. After a few days of the Chi-Chi event, the Hongtsaiping area in central Taiwan was reported to have several meters of slide in a small scale area. The reported landslides are dissimilar to the catastrophic events. To better understand the characteristics of such non-catastrophic and unobvious landslide, we try to characterize in detail the sliding directions, their magnitudes and region of the landslide. We analyzed three orthorectified aerial photographs of the Hongtsaiping area, which were taken in 1998/06/13, 1999/11/18 and 2002/10/26, using the Particle Image Velocimetry (PIV) technique. Through the method we correlated the image pixels of the two aerial photographs in order to derive the displacement vector field. The results of the PIV analysis have helped us not only find the areas where sliding had happened, but also evaluate the azimuth and magnitudes of the sliding event. The PIV analysis in the Hongtsaiping area covers a dimension of about 3228 × 2448 pixels in an area of about 1.2 km2. Our results show that the maximum horizontal displacement of the landslide is about 24 m towards NW in the study area. This unobvious but wide-coverage landslide may need further investigation in mechanisms combining with available or new geological data.