A method for calculating the serious degree of geological disaster in high vegetation covered region using remote sensing: a case of Minjiang basin from Moutuo to Xiao Shawan in Maoxian County

Abstract

In order to calculate the serious degree of geological disaster caused by earthquake taken place in Wenchuan county, Shichuan province, China with magnitude 8.0 on May 12th 2008, a method for extracting the changed vegetation using remote sensing taken the example of Minjiang basin from Moutuo to Xiao Shawan in Maoxian county, Shichuan province, China is proposed here. Because the study area has the characteristic of "high vegetation cover" (ratio of vegetation cover is 88.3% before earthquake takes place), changed vegetation can reflect the serious degree of geological disaster damage. Flow chart of this method is as following: Firstly, different spatial resolution between two images acquired before and after earthquake is uniformed and the two images are registered with accuracy less than one pixel; Then, vegetation changed map is made by extracting the difference area covered by vegetation in two images using Normalized Difference Vegetation Index (NDVI); Finally, statistics and analysis are performed according to the vegetation changed map. Results from vegetation changed map show that in the total area of 201.7 km2, vegetation changed area is 24.4 km2 in which 99.6% is caused by geological disaster with area of 24.3 km2, and the 0.4% left is caused by the change of farm operations. Therefore, changed vegetation in study area is mainly caused by geological disaster. According to geological map and high resolution images acquired by airborne remote sensing on May 23rd 2008, for "high fragility and hardness" of limestone, phyllite and quartzose sandstone in study area, the type of geological disaster is debris flow caused by slacktip.