## 3 Data and Methods
### Method
#### 3.2.3 Detection of Geologic Hazards  
The mountainous parts of the study area had high vegetation cover prior to the earthquake, spatially covering up to 80% of the entire region [43]. Earthquake-induced geologic hazards such as landslides and debris flows lead to large-scale vegetation interruption and some feature changes in the landscape, including the color, texture, and shape. These distinctive features help with identification of failure areas in remote sensing images. Figure 2 presents the airborne remote sensing images of landslides and debris flows in Qingping Town in 2008 and 2011. After geometric registration and post-processing, the airborne optical images were used to construct maps in true color and were analyzed for hazard damage monitoring. Using an interactive interpretation method between pre-earthquake Landsat TM images and post-earthquake optical \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**No.**} & \multirow{2}{*}{**Location**} & \multicolumn{3}{c}{**Image Date**} & \multicolumn{2}{c}{**Number of**} & **Total Area in** \\ \cline{3-6}  & & **2008** & **2009** & **2011** & **Landslides** & **2008 (ha)** \\ \hline
1 & Wenjiagou, Qingping Town, Mianzhu City & 23 May & 03 June & 07 June & 1 & 260.33 \\
2 & Chenjiaba, Beichuan County & 28 May & 18 May & 31 May & 23 & 631.63 \\
3 & East Yingxiu Town, Wenchuan County & 23 May & 16 May & 07 June & 8 & 121.68 \\
4 & Hanwang Town, Mianzhu City & 19 May & 16 May & 25 May & 11 & 113.30 \\
5 & Tianchi Town, Mianzhu City & 23 May & 16 May & 25 May & 12 & 148.18 \\
6 & Shawan, Nanba Town, Pingwu County & 28 May & 18 May & 31 May & 3 & 48.49 \\
7 & Yingxiu Town, Wenchuan County & 23 May & 03 June & 28 May & 14 & 664.73 \\
8 & Jinhelinkuang, Mianzhu City & 23 May & 03 June & 07 June & 12 & 616.85 \\
9 & Jiuding Mountain, Bailongchi, Mianzhu City & 23 May & 16 May & 25 May & 2 & 110.33 \\
10 & Tangjiashan, Beichuan County & 27 May & 18 May & 31 May & 27 & 487.14 \\
11 & Guanzhipu, Beichuan County & 28 May & 18 May & 31 May & 13 & 155.32 \\
12 & Jinchi Town, Wenchuan County & 24 May & 21 May & 28 May & 5 & 191.11 \\
13 & Gaochuan County, Jinyang City & 23 May & 16 May & 25 May & 6 & 373.82 \\
14 * & Changheba, Mianzhu City & 23 May & miss & 25 May & 2 & 141.90 \\
15 * & Daguangbao Group, An County & miss & 16 May & 25 May & 4 & 959.59 \\ \hline \hline \end{tabular} Note: “*” used here in the No. 14 and 15 sites indicates that the missing data are infilled with data from 28 April 2010 and 27 April 2010, respectively.  
\end{table} 
Table 2: Statistics for the 15 geologic hazard sites. airborne images, the 15 selected geologic hazard areas from 2008, 2009, and 2011 were carefully interpreted in the ArcGIS platform through polygons drawn based on the specific features and differences described in Section 3.2.1. Based on the interpretation results, the slope failure and vegetation recovery areas were obtained by performing an overlay analysis with the previous year's failure areas. After eliminating newly developed landslide mass areas and areas with registration errors on the overlay analysis results through artificial screening, every site's newly developed debris flow areas (the newly emerged hazard area caused by debris flows) were obtained. Then, the vegetation recovery areas based on the previous failure areas were calculated with Equation (1). The rates of vegetation recovery, debris flow development, and geologic hazard variation were calculated with Equation (2). \[na=\omega a+p-er \tag{1}\]  
\[rate=\frac{\mathrm{var}}{\omega a*years} \tag{2}\]  
In Equation (1), \(na\) refers to the bare failure area after the vegetation recovered; \(oa\) is the bare failure area prior to vegetation recovery; \(p\) is the newly developed debris flow area; and \(er\) is the vegetation recovery area based on the previous failure region. In Equation (2), \(rate\) refers to the variation rate of the three objects, \(years\) refers to the years apart of analyzed variables, and var refers to the three analysis objects.