Dujuan, 2004 typhoon Mindulle and 2005 typhoon Talimweremapped
and rechecked in the field. The landslide ratios triggered by these six
events range between 0.81% and 1.94% (Table 2). The maximum
landslide ratio is triggered by typhoon Mindulle, the first heavy
rainstorm after the 2003 Chengkung earthquake. The second largest
landslide ratio is triggered by typhoon Bilis, the first heavy rainstorm
after the 1999 Chi-Chi earthquake. Additionally, the maximum newlyformed
ratio and reactivated ratio of landslides both are triggered by
typhoon Mindulle (Table 2). This indicates that the reasons why the
typhoon Dujuan triggered a lower landslide ratio than did other
recorded typhoon events are that in 2003 therewere no rainstormswith
cumulative rainfall exceeding 80mm, nor were there any earthquakes
with a magnitude over 4.0.
The ratios of reactivated landslides in the Sinwulu catchment are
over 50% for all of the analyzed typhoon events (Table 2). The
geomaterial with high joint density is the main factor causing the rock
mass in the catchment to be prone to slide continuously along the
weak discontinuities. Earthquakes are known to make stable
hillslopes become unstable (Keefer, 1984), and the newly-formed
landslide ratios triggered by typhoons occurring soon after highmagnitude
earthquakes are always higher than the newly-formed
landslide ratios triggered by typhoons which strike more than one
year after the last earthquake.