A Creeping-type Landslide Observed on Aerial Photographs Following the Ji-Ji Earthquake of Taiwan on 21 September 1999
Creeping-Type Landslide
Landslides with a small amount of surface creeping would give little clues on images and photographs, as shown in Figure 3. The place indicated by capital letter A shows no clear evidence of landslide. However, the field pictures (Figure 4) showed a disturbance of ground surface materials of the area. A close-look with scrutiny for the image with high resolution reveals a tensional linear cracks extending 1300m from point C to D. This indication gives a clue for further inspection of the photographs under stereoscope. The linear zone of the extensional crack shows clearly in the enlargement of the image. As depicted in Figure 5, roads along the crack zone were destroyed, this corresponding to a deformation zone in the field with more than 5 meters in width.
Figure 3 In a synoptic view of aerial photograph taken on 26 September 1999, landslides with exposed scars are conspicuous. However little clues can be found for a creeping type of landslide indicated at A.
Figure 4 Field pictures show that the lands surface are disturbed by a creeping process in the area indicated at A in Figure 3. Though this is hardly can be clearly identified by aerial photo-interpretation.
Fugure 5 An enlargement of the crack zone, the area is also indicted in the rectangle of Figure 3.
For a visualization of the change of this landslide, stereo-pairs both prior to and aft the event were also registered, rectified, and processed to obtain ortho-photos, contours, and DTMs (Figure 6). The sequence of the event is clearly exposed and the extent of the disturbed area of ground surface materials can be interpreted from the 3d images. The creeping-type landslide with an area of more than 100 hectares were thus observed. And, the existence of this creeping-type landslide was further proved by field observations (Figure 4).
Discussion and Conclusion
Generally speaking, earthquake-triggered landslides are easily enumerated by using satellite images or aerial photographs as the scars of land surface are depicted conspicuously on the places of the landslides, where there are no vegetation and fresh rocks are exposed. It is also possible to detect and identify creeping-type landslide by using aerial photographs especially when multi-temporal and high-resolution images are used for a close study on the characteristics of landform change.
Figure 6 A sequence of the landform change prior to and aft the Ji-Ji Earthquake as simulated by merging orthophotos and DTMs of the Hon-Tsai-Ping study area.
References
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ERL (1999) Final Report on the investigation of landslides triggered by 921 Ji-Ji Earthquake and a general mitigation proposal. By Energy and Resources Laboratories, ITRI. 31 March 2000.
