Recovery assessment for earthquake-induced landslides in Central Taiwan: Changes, patterns, and mechanisms

On September 21, 1999, a devastating earthquake measuring 7.3 on the Richter scale triggered massive deep-seated landslides in the Chiufenershan area of Nantou County in central Taiwan. This study utilized an improved self-organizing maps (SOM), the derived landslide recovery rate and Markov chain model, with its combination of multi-temporal SPOT images and over two decades of survey data, to assess the long-term changes, patterns, and mechanisms at the sites of the landslide. The analyzed results indicate that highly accurate landslide mapping can be extracted using the proposed SOM model. Over two decades after the earthquake, the changes in landslide area gradually recovered from the initial 209.22 ha (September 1999) to 20.54 ha (February 2020), with a vegetation cover of 90.2 %. The deduced landslide recovery patterns have statistically significant differences that can be used to assess the recovery trends of the studied landslides. Recovery of landslides is carried out in three stages: (I) rapid expansion of herbaceous invasion (1999–2004); (II) competition between pioneer plants and woody communities (2004–2014); and (III) towards stabilized secondary forest (2014–2020). Combining the analyzed findings with the field surveys, it can be inferred that the key mechanisms for recovery in Chiufanershan landslide include moisture, topography, native vegetation species, and succession time. The results indicate that nature itself has a strong ability to restore the landslide within a decade, and provide very useful information for government agencies in landslide management.