The long-term evolution of landslide activity near the epicentral area of the 2008 Wenchuan earthquake in China
Introduction
Disastrous earthquakes in mountainous areas not only induce thousands of landslides but also cause severe casualties and economic losses. It is believed that mountainous earthquakes lead to the increasing instability of slopes and to impressive chains of geohazards in subsequent years (Huang and Li, 2014). In recent years, some geological hazard chains have been triggered by earthquakes, such as the 1999 Chi-Chi earthquake in Taiwan, the 2010 Haiti earthquake, the 2013 Lushan earthquake and the 2017 Jiuzhaigou earthquake (Chigira et al., 2003; Gorum et al., 2013; Chang et al., 2016; Fan et al., 2018b). Apart from slope instability, earthquakes can cause severe destruction of vegetation, surface deformation and cracks, geomorphological changes and a large amount of loose materials, which will have a significant impact on geological hazard activities in the future (Lin et al., 2009; Wang et al., 2014; Xu et al., 2016). The risk of earthquake disasters varies in time and space and can persist long after coseismic activities have ceased. Focused research efforts must be made to detect the occurrence and density of landslides over a long time, to analyze the evolution characteristics of landslide activity and to forecast the duration period of increased landslide activity after an earthquake (Lin et al., 2006a).
On May 12, 2008, the MW 7.9 Wenchuan earthquake triggered numerous coseismic landslides (Gorum et al., 2011; Parker et al., 2011; Cui et al., 2012). After the Wenchuan earthquake, large amounts of loose materials were deposited on hillslopes, which provided sufficient source materials for debris flows triggered by rainstorms. Several catastrophic debris flows occurred frequently and resulted in a significant challenge for reconstruction works in the years following the Wenchuan earthquake, e.g., in Beichuan in September 2009, in Yingxiu in August 2010, and in Wenchuan in July 2013 (Tang and Liang, 2009; Tang et al., 2011a; Hu and Huang, 2017). During the reconstruction stage of the Wenchuan earthquake, human settlements threatened by coseismic hazards are at risk of the continuous reactivation of landslides and debris flows. Most of the studies since the Wenchuan earthquake focused on the fundamental issues of disaster distribution characteristics, formation mechanisms, movement processes, rainfall thresholds, risk assessment, prediction and early warning (Keefer, 2000; Gorum et al., 2011; Tang et al., 2012; Ni et al., 2014; Chang et al., 2017). Recently, more researchers have attempted to evaluate the long-term impacts of the Wenchuan earthquake on geological hazards. They devoted themselves to research on the occurrence characteristics, activity trends, and durations of geological disasters caused by rainstorms following a severe earthquake (Huang and Li, 2014; Zhang and Zhang, 2017). Huang et al. (2016) reported that the frequency of geohazards increased dramatically after the earthquake, and the activity of post-earthquake geohazards was 2 to 5 times higher than that before the Wenchuan earthquake. Owing to the progressive depletion of the coseismic debris, grain coarsening, densification, and revegetation, the frequencies of geohazards gradually decreased to the pre-earthquake level (Tang et al., 2016; Hu et al., 2018). The total area of active landslides has decreased in the first five years following the Wenchuan earthquake (Li et al., 2015; Tang et al., 2016; Yang et al., 2017), but the duration period of increased landslide activity remains poorly understood.
Among studies in other parts of the world, we also observe that the frequencies of landslides show a transient increase after a major shock. Lin et al. (2008) and Hovius et al. (2011) reported that high landslide activity was observed for several years after the 1999 Chi-Chi earthquake and 2005 Kashmir earthquake. Saba et al. (2010) also found that landslide activity was very high during the first two years after the 2005 earthquake in Pakistan and then the frequency decreased. Shou et al. (2011) analyzed the spatial and temporal characteristics of landslides in the central parts of Taiwan after the Chi-Chi earthquake and proposed a model of landslide activity. It was found that landslide activity decreased to approximately 50% in three years and to approximately 10% in ten years after the Chi-Chi earthquake. Marc et al. (2015) studied the evolution of landslide frequency in the epicentral region of four major earthquakes (MW 6.6–7.6) around the world and revealed that the landslide frequency peaked after the earthquakes and decayed to the pre-earthquake level within 1–4 years, and the decay period may be proportional to the earthquake magnitude.
Although some efforts have been made on the decay pattern of geohazards after the Wenchuan earthquake, a quantitative prediction approach has yet to be developed. To fill this substantial knowledge gap, this paper aims to quantitatively assess the long-term evolution of landslides in the epicentral area after the Wenchuan earthquake. We compiled multi-temporal landslide inventories to analyze the landslide activities in the epicenter region of the earthquake in the past decade. The detailed objectives are as follows: (1) develop a quantitative methodology to identify the spatial and temporal evolution of landslide activity; (2) examine the evolution of landslide activity under the influence of different environmental triggering factors; and (3) assess vegetation recovery and its influence on the activity of post-seismic landslides. In this paper, the term ‘landslide’ refers to a slope failure, i.e., masses of rock, earth or debris moving down a slope, and the term ‘landslide active area’ refers to the area of landslide remobilizations and newly occurring landslides.
Section snippets
Study area
The Wenchuan earthquake is the most devastating earthquake ever recorded in China, leading to 69,197 fatalities, 18,341 people missing, 374,176 people injured, 6.5 million houses destroyed and 5 million people left homeless. The earthquake directly induced more than 60,000 coseismic landslides covering a mountain region of approximately 35,000 km2 (Gorum et al., 2011). The study area is situated in the epicentral area around Yingxiu town, Wenchuan County, Sichuan Province, China (Fig. 1), and
Data and methods
The epicentral area around Yingxiu town was severely affected by the Wenchuan earthquake (Fig. 2), and the frequency of subsequent geohazard activity was very high. This work involves three types of data: six remote sensing images, a series of Landsat data, and a digital elevation model (DEM). Table 2 shows detailed information about the data sources. In this section, we also present the preparation of our inventory and the analysis method that was used to determine the research contents.
Analysis of the changes in landslide active area
Our inventory (Fig. 4) provides a polygon-based delineation of coseismic landslides, post-seismic remobilizations and new landslides. Here, we proposed this term of the activity rate (P) to represent the intensity of the post-earthquake landslides. The ratio of the active area (Sa) to the coseismic landslide area (Sc) defines the activity rate (P) in different periods. We set the activity rate of landslides in 2008 to be 100% as a baseline for studying the changes in enhanced landslide
Discussion
How did new landslides develop? How fast did vegetation recover? How long will it take landslide activity to recover to the pre-earthquake level? Answers to these questions are important for understanding the evolving characteristics of post-earthquake landslides. We found that very few new landslides were initiated, while most of the landslide activities occurred in coseismic deposits. The 2011 image only reveals some new landslides with a total area of 18 × 104 m2, while new landslides are
Conclusions
Research on the evolution of post-earthquake landslides is of great significance for the prediction and mitigation of future geological disasters. Continuous monitoring of landslide activities is helpful for understanding how long landslides and debris flow activities can last in an earthquake-stricken region. In this study, we quantified the spatio-temporal evolution of post-earthquake landslides based on the multi-temporal inventories of active landslides from 2008 to 2018. We analyzed the
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the National Key Research and Development Program of China (No. 2017YFC1501004) and the National Natural Science Foundation of China(No. 41672299). The authors wish to express their sincere thanks to Professor Van Asch T.W.J. for the suggestions on earlier versions of the manuscript. The authors thank the anonymous reviewers for their helpful suggestions to improve the paper.
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