Landslides Triggered by the Chi-Chi Earthquake
Intensity of ground shaking
We compare landslides and earthquake intensity map. We found that landslides usually occurred at the area of PGA greater than 250 gals. Areas of PGA less than 250 gals contain fewer landslides. This result is similar with some other researches.
Characteristic of the Geologic formation
Geologic conditions have great influences on landslides. This study used the 1:250000 geologic map of Taiwan from the Central Geological Survey as base map and also 1:100000 geologic maps of Maioli, Taichung and Chiai from the Chinese Petroleum Corporation for detail examinations. We found that the Toukoshan Formation, Chinsui Shale and the Tachien Sandstone were more easily influenced by the earthquake shaking, especially at Huoyenshan Facies. Because these stratums are weak, the resistances against ground shaking are relatively low.
Distance to rivers
The erosion of the rivers will make the slopes become unstable. Drainages from 1:25000 topographic maps were extracted to a GIS layer. By using buffer analysis inside the MapInfo GIS, we found that a large number of landslides happened in the distance of 300 meters from the drainage channels.
Distance to Roads
The appearances of road represent human activities that accelerate the erosion rate. This study also extracted roadways from 1:25000 topographic maps into GIS. By applying the same process we found landslides at the distance less than 80 meters from the road are more than the landslides at the distance between 80 meters and 200 meters. But much more landslides were located outside the vicinities of roadways.
Slope Angle
Slope angle is the most important factor controls the landslide occurrence. Slope angle map was calculated from the 40 meters resolution digital terrain model (DTM). We found that the slopes inclination larger than 100% involve much more landslides.
Slope Orientation.
Earthquake shaking may have preferred orientation. Therefore, landslides may occur more frequently at slopes facing to certain direction. This study extracted slope direction from DTM. We found that at the hangingwall, most S and SE facing slopes collapsed. Meanwhile, slopes at the footwall collapsed mostly at S, SE and SW direction. Slopes facing to other directions were not so seriously damaged. It shows that the movement direction of the Chelungpu fault was from SE to NW.
Conclusion
There were 9272 larger landslides (area greater than 625 square meters or 4 pixels on a SPOT image) occurred during earthquake shaking. The total area of landslides is 127.8 square kilometers. There were 8843 landslides located within the area of PGA value 250 gal and above. These landslides were distributed in an ellipse-shaped region with the major axis striking NNE, coinciding with the trend of regional faults.
Statistics shows the following results: (1) most landslides locate within the area of PGA greater than 250 gal, especially within the area of PGA greater than 300 gal. (2) The majority of the landslides occurred within a distance of 20 kilometers from the fault rupture plane. The greatest distance from the fault rupture plane is about 60 to 70 km. (3) The Toukoshan Formation, the Chinsui Shale and the Tachien Sandstone were more easily influenced by the ground shaking, especially at Huoyenshan Facies of the Toukoshan Formation. (4) Slopes with gradient larger than 100% involved with more landslides. (5) At the hangingwall, most landslides occurred at S and SE facing slopes. At the footwall, most landslides concentrated at S, SE and SW facing slopes, whereas slopes facing to other directions were not so seriously damaged. It shows that the movement of the faulting during the Chi-Chi earthquake was from SE to NW.
Reference
- Keefer, D. K. (1984) Landslides caused by earthquakes: Geol. Soc. Amer. Bull., 95, 406-421.
- Cheng, C.T., Lee, C.T., and Tsai, Y.B. (2000) Fault rupture plane and attenuation model associated with the 1999 Chi-Chi earthquake. Proceedings of the 2000 Annual Meeting of the Geological Society of China, 21-23. (in Chinese)
