## 1 Introduction  
On 12 May 2008, the 8.0-magnitude Wenchuan earthquake occurred in Sichuan Province, China. Its epicenter was located at Yingxiu Town, Wenchuan County, in the east of the Longmenshan Fault Zone on the eastern fringe of the Tibetan Plateau. The Wenchuan earthquake caused substantial casualties and financial losses. In response to the massive energy this earthquake released, thousands of geologic failures were triggered such as landslides and debris flows, in addition to the formation of barrier lakes [1,2]. These failures caused enormous damage to the local environment and ecosystem [3], and destabilized numerous hillslides and solid masses, increasing the probability of geologic disasters [4]. Particularly in the mountainous areas, the damage caused by earthquake-triggered secondary geologic failures may have been more severe than the immediate damage caused by the earthquake itself [5]. After the earthquake, timely estimating the development of geo-hazards and disaster conditions through field investigation was necessary for hazards assessment and decision making. However, damaged roads, unstable hillslides, increment weather, and frequent aftershocks made it difficult to perform field work and acquire reliable information about the areas. Remote sensing is an effective means for the timely monitoring and quantitative assessment of devastated areas [6]. Remote sensing data can be acquired at broad spatial and temporal scales, and provide useful information for rapid damage assessment at an early stage when field surveys of a large area are difficult [7]. Remote sensing data have been available for approximately one century and have proven useful in natural-hazard investigation and management in response to a number of natural disasters [8, 9, 10, 11, 12, 13, 14]. Today, as remote sensing technology develops toward being multiplatform, multispectral, high-resolution, and multi temporal, it has become more convenient to use in hazard assessment and rapid monitoring [15]. Much research has been conducted on the Wenchuan earthquake using optical remote sensing data [16, 17, 18] and multi-mode radar data [11, 19, 20], focusing on disaster monitoring [21, 22, 23, 24, 25], hazard assessment [26, 27, 28, 29], mitigation [30, 31] and vegetation damage assessment [3, 32]. [PERSON] _et al._[33] discussed the distribution and characteristics of seismically triggered landslides from the Wenchuan earthquake using satellite images. [PERSON] and [PERSON] [34] studied the distribution of geo-hazards triggered by the earthquake, and identified a total of 11,308 geo-hazards in sixteen seriously damaged counties based on field investigations, air photos, and satellite imaging data. [PERSON] _et al._[35] estimated the damage to vegetation caused by the secondary hazards of the Wenchuan earthquake through SPOT and aerial images. The authors found that farmland and grassland were less damaged than forestland because of the steep slopes and that the damage to the vegetation first decreased and then increased with increasing distance from the three main faults of the Longmenshan fault zone. [PERSON] _et al._[36] evaluated the vegetation degradation caused by the earthquake and its recovery two months after the disaster from the analysis of MODIS Gross Primary Productivity (GPP) time series products and other ancillary GIS data. [PERSON] and [PERSON] [37] summarized a meta-synthesis pattern of the post-disaster recovery and reconstruction by comparing studies of 14 earthquakes in the last 100 years. The authors found that remarkable achievements have been made with respect to post-Wenchuan earthquake recovery and reconstruction and that these post-disaster recovery and reconstruction programs can be used as a foundation for future earthquake recovery. Moreover, dynamic monitoring of geologic hazards triggered by the earthquake and aftershocks in successive years using remote sensing data is necessary and important to uncover the mechanism and influence factors in the geological processes.