## 3 Bibliometric Analysis of Current Trends  
A simple bibliometric analysis was performed in the ISI Web of Knowledge: \"Web of Science Core Collection\" (1900-Present) to capture the main trends in the Special Issue-related topics. The statistics were gathered on 2 November 2021, so only the first ten months of the year 2021 were included in the search. We followed a similar approach to [PERSON] et al. [14] and selected keywords related to the topic of this Special Issue. A number of papers published in every year have been analyzed for the query \"TOPIC\" (Table 1).  
Before 1990, only a small share of the total number of papers had been published. For the topic \"landslides\", only 585 papers (1.65%) were published before 1990 out of 35,440 (till 10/2021); for the \"landslide + monitoring\" topic, only nine papers (0.17%) were published before 1990 out of 5210 (till 10/2021); and for the topic \"landslide + susceptibility, modelling or assessment\", only 13 papers (0.11%) were published before 1990 out of 12,248 (till 10/2021). In the case of the \"landslide + InSAR\" topic, the first publication appeared in 1998, and up to the end of October this year, 1079 papers had been published. Figure 1 shows the yearly growth in the number of papers in the respective topics since 1990. The presented graphs illustrate the fast-growing exponential trend in all the analyzed topics. However, the fastest growth in recent years can be seen in the \"landslide + InSAR\" topic.  
One of the basic variables for assessing the trends in a time-series is the growth rate [15]:  
\[k_{i}=\frac{y_{i}-y_{i-1}}{y_{i-1}}\cdot 100 \tag{1}\]  
where \(k_{i}\) is the growth rate in %, \(y_{i}\) is the value at the time \(i\) and \(y_{i-1}\) is the value at time \(i-1\).  
Positive values mean a growth in percentage terms compared to the previous year while negative values mean a decrease compared to the previous year in percentage terms. As can be seen from Figure 2, rapid growth in topics \"landslides\", \"monitoring\" and \"susceptibility\" in the 1990s has stabilized since 2006; studies on these topics are now increasing at a relatively stable rate of around 10-15% per year. The \"InSAR\" topic is a special case as the first paper appeared in 1998, which consequently resulted in very high growth rates. However, during the last five years, this trend seems to have stabilized, and the topic now seems to follow the trend of the other three topics with a yearly growth rate of between 15 and 20%.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Keyword** & **ISI Web of Science Core Collection (1900-Present) Query for** \\ \multicolumn{2}{c}{**“TOPIC”**} \\ \hline Landslide & (landslide * OR rockslide * OR rockfall * OR rock-fall *) \\ \hline Monitoring & (monitoring * OR surveying *) AND (landslide * OR rockslide * OR rockfall *) \\ \hline Susceptibility or modelling & (susceptibility * OR modelling * OR modeling * OR assessment *) \\  & AND (landslide * OR rockslide * OR rockfall * OR rock-fall *) \\ \hline InSAR & (InSAR* OR DInSAR * OR GBSAR * OR GB-InSAR *) AND (landslide * OR rockslide * OR rockfall * OR rock-fall *) \\ \hline \hline \end{tabular}  
*: Search keywords.  
\end{table}
Table 1: List of queries used in simple bibliometric analysis. Timespan: 1900–Present. Indices: SCI-EXPANDED, SSCI, A&HCI, CPCI-S, CPCI-SSH, BKCI-S, BKCI-SSH, ESCI, CCR-EXPANDED, IC.  
Figure 1: Number of published papers per year on the selected topics since 1990.  
Figure 2: Growth rate of the number of published papers per year on the selected topics since 1990. Lines show the five-year moving average.