In early August 2009, Typhoon Morakot attacked Taiwan, and caused significant loss of lives and property in the Lawnon River basin [1]. About 1218 landslides occurred and the cumulative landslide area exceeded 133.7 km², which accounted for 6.6% of the entire basin area and led to various types of damage to the infrastructure and transportation systems. However, before typhoon Morakot, large scale landslides were rarely observed in this area. The potential and triggering factors of the landslides in the Lawnon River basin have seldom been studied and emphasized. Therefore, this paper clarifies the effects of rainfall and earthquake on landslide occurrence. Based on the analysis results, the following conclusions were made:

1. From 2005 to 2009, the landslide events and scales increased year by year and reached the peak value at the storm event of typhoon Morakot 2009, which drew the public's attention to the landslide behaviour in this area. Undoubtedly, rainfall is one of the primary triggering factors that caused the slides. However, in comparison of the rainfall data for the events except for typhoon Morakot, typhoon Haitang in 2005 had a higher accumulative rainfall (1882 mm in average) than the other storm events, but its landslide area was smaller than those of most storm events. The patterns of landslide scale variation and rainfall record seemed to be identical after 2008, which implies the existence of other factor(s) to trigger the transition of the landslide sensitivity.
2. Earthquakes are another primary triggering mechanism to induce landslides. The landslide behaviour observed in central Taiwan during 1999 Chi-Chi earthquake (M_L=7.3) was different. Co-seismic landslides are not frequently found in the Lawnon River basin during the quakes from 2005 to 2009, which is possibly due to the limited seismic intensities. Nevertheless, it is well accepted that after an earthquake, the stability of slopes would be influenced for a long time. In this study, the 4 March 4 2008 earthquake with a peak ground acceleration close to 140 gals was inferred to disturb the slope stability so as to result in the apparent increment of landslide ratios in the four rainfall events since July 2008. Note that the critical seismic intensity leading to the transition of slope stability is not determined in this study.
3. The lithological effect on landslide sensitivity is not obvious before the 4 March 2008 earthquake. After the quake, the sedimentary formations (Cc and Tn) were more prone to the increment of landslide ratios compared to metamorphic rock areas (Co and Ep).
4. It is observed that the existing landslide areas are more sensitive to landslide enlargement or development. The failure ratios of enlarged landslides is larger than those of new landslides, which indicates as a landslide occurs, it will be more easily to initiate subsequent failure nearby. As a result, the soil and water conservation works should be taken up soon to prevent further development of failure slopes.

1

T.C. Chen, C.C. Wu, M.C. Weng, K.H. Hsieh, C.C. Wang, "Slope failure of Lawnon basin induced by Typhoon Morakot", Sino-Geotechnics, 122, 13-20, 2009. (in Chinese)

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