论文部分内容阅读
利用归一化植被指数(NDVI)数据,探测了汶川Ms8.0级地震前植被变化情况。对震前9年的NDVI数据先做整体变化分析,没有发现明显的植被退化现象;再对时间序列数据进行逐年比对,发现两个比较明显的植被退化现象。其一,2006年与2005年相比,龙门山断裂东部的四川盆地大部分地区6—9月NDVI均值明显减小,幅度为0～20%。推测原因为震前四川盆地受挤压变形,产生热弹效应放热致使地表升温,进一步与大气耦合导致长期干旱少雨,破坏植被正常生长条件,使植被退化。因此,2006年四川盆地大范围植被退化现象可视为震前异常。其二,2008年5月上旬与2007年相比,龙门山断裂带上发现明显的植被退化现象,成条带状分布,NDVI下降幅度超过20%。这可能与临震前龙门山断裂带上发生某些物理化学变化有关,即临震前龙门山断裂带地壳岩石的破裂发育、断裂之间以及岩石之间的摩擦滑移,导致地下热物质上涌和摩擦生热释放,造成短期(近)地表高温,破坏了植被的正常生长条件,使其退化。因此,可以看作震前短期异常。此外,沿着时间尺度和空间尺度,找到一些震前中期和短期异常佐证,以增加植被异常判断的可信度。
Using the normalized vegetation index (NDVI) data, the change of vegetation before Wenchuan Ms8.0 earthquake was detected. NDVI data before the earthquake for the first time to do the overall changes in the analysis, did not find any obvious phenomenon of vegetation degradation; and then compare the time series data year by year and found two obvious vegetation degradation phenomenon. First, compared with 2005, the average NDVI of June-September in most parts of the Sichuan Basin east of the Longmenshan fault decreased obviously, ranging from 0 to 20%. It is presumed that the reason is that before the earthquake, the Sichuan Basin was squeezed and deformed, resulting in the thermo-elastic exotherm resulting in the surface warming. Further coupling with the atmosphere resulted in prolonged drought and less rainfall, destroying the vegetation normal growth conditions and degenerating the vegetation. Therefore, the large-scale vegetation degradation in the Sichuan Basin in 2006 can be regarded as a pre-earthquake anomaly. Second, in early May 2008, compared with 2007, obvious degeneration of vegetation was found on the Longmenshan fault belt, which distributed into a banded distribution with the NDVI decreasing by more than 20%. This may be related to some physical and chemical changes occurring in the Longmen Shan fault zone before the quake: the rupture of the crustal rocks in the Longmen Shan fault zone before the quake, the frictional slippage between the fractures and the rock, resulting in the formation of underground thermal substances Chung and friction heat release, resulting in short-term (near) surface temperature, undermining the normal growth of vegetation conditions, so that degradation. Therefore, it can be regarded as short-term anomalies before the earthquake. In addition, along the time scale and the spatial scale, some anomalous evidence of medium- and short-term anomalies before the earthquake are found to increase the credibility of the abnormal vegetation judgment.