## 1 Introduction  
The mountains of Taiwan, with the highest peaks rising to almost 4000 m a.s.l, are characterized by fractured rock formations, high relief and steep stream gradients. These mountains, particularly the central mountain range (CMR), influence the tracks and intensity of typhoon events [1]. During summer and autumn, Taiwan is regularly affected by typhoons (tropical cyclones), three to four per year on average, which bring heavy rainfall [2]. Some studies indicate that the number of typhoons hitting Taiwan increased after the year 2000, also resulting in heavier precipitation in recent years [3; 4]. Heavy rainfall is the main landslide triggering factor in Taiwan [5]. The rainfall events are usually associated with typhoons, which account for almost 50% of the island's total rainfall [6; 7]. Studies have found relationships between long-duration and moderate intensity rainfall events and large and deep-seated landslides [8], sediment yield and debris flows [5; 9]. Large, rapid landslides and debris flows frequently lead to fatalities. Major endeavors are necessary to protect people and settlements in areas at risk, and to implement prevention and early warning measures [10; 11; 12; 13]. In general, as well as in Taiwan, such catastrophic landslides also cause severe damages to infrastructure, and efforts both in time and money are needed to recover and maintain the transportation infrastructure such as roads and bridges [14; 15; 16; 17]. Beyond the direct landslide hazard, large landslides can initiate natural hazard cascades by damming rivers and inducing catastrophic flash floods and debris flows [18; 19; 20]. The large amount of mobilized debris that originates from landslides significantly affects the drainage system, for example, resulting in an increase in erosion and sediment discharge in rivers, and changes in channel size and shape [5; 21; 22]. According to [PERSON] et al. [9] about 384 Mt y\({}^{-1}\) of sediment is transported into the ocean in Taiwan, whereby the high proportion of large landslides significantly contributes to this high annual sediment yield. In August 2009, typhoon Morakot caused a record-breaking cumulative rainfall (more than 2000 mm in three days), which led to debris flows and mudflows, flooding in coastal areas, and massive landslides [23; 24]. The rainfall also triggered one of the most famous and fatal landslides in Taiwan, the Xiaolin landslide [25; 26; 27; 28]. The Butangbunasi landslide [29; 30; 31] is another example of a large rainfall-triggered landslide in Taiwan. Reactivation and extension of this landslide have been resulting in repeated sediment delivery to the Laonong River, especially during torrential rainfall. The river course has been frequently affected, leading to the formation of a landslide-dammed lake several times during the past three decades [29; 32; 33; 34; 35; 36]. The magnitude of the Butangbunasi landslide is even significantly larger than of the disastrous Xiaolin landslide [29]. A deeper knowledge of the evolution of landslides and their triggering factors is crucial for hazard mitigation [37; 38]. Therefore, mapping and analyzing the evolution of such large landslides over time helps to better understand their reactivation rates and their impact on downstream areas. Remote sensing plays a key role in studying landslides and provides an adequate and cost-effective source to derive information about landslide distribution and types [39; 40; 41; 42]. The use of remote sensing data also helps to investigate the potential impacts of landslides such as the damming of rivers, particularly in difficult to access and remote mountain regions [43]. The value of remote sensing for landslide studies becomes more and more evident with the increasing amount of freely available Earth observation (EO) data, which provides remarkable opportunities to map and monitor landslides over time [44; 45; 46].