Leveraging time series analysis of radar coherence and normalized difference vegetation index ratios to characterize pre-failure activity of the Mud Creek landslide, California

Assessing landslide activity at large scales has historically been a challenging problem. Here, we present a different approach on radar coherence and normalized difference vegetation index (NDVI) analyses - metrics that are typically used to map landslides post-failure - and leverage a time series analysis to characterize the pre-failure activity of the Mud Creek landslide in California. Our method computes the ratio of mean interferometric coherence or NDVI on the unstable slope relative to that of the surrounding hillslope. This approach has the advantage that it eliminates the negative impacts of long temporal baselines that can interfere with the analysis of interferometric synthetic aperture (InSAR) data, as well as interferences from atmospheric and environmental factors. We show that the coherence ratio of the Mud Creek landslide dropped by 50% when the slide began to accelerate 5 months prior to its catastrophic failure in 2017. Coincidentally, the NDVI ratio began a near-linear decline. A similar behavior is visible during an earlier acceleration of the landslide in 2016. This suggests that radar coherence and NDVI ratios may be useful for assessing landslide activity. Our study demonstrates that data from the ascending track provide the more reliable coherence ratios, despite being poorly suited to measure the slope's precursory deformation. Combined, these insights suggest that this type of analysis may complement traditional InSAR analysis in useful ways and provide an opportunity to assess landslide activity at regional scales.

大规模评估滑坡活动在历史上一直是一个具有挑战性的问题。在此，我们提出一种关于雷达相干性和归一化植被指数（NDVI）分析的不同方法——这些指标通常用于绘制滑坡发生后的地图——并利用时间序列分析来描述加利福尼亚州马德河滑坡发生前的活动情况。我们的方法计算不稳定斜坡上的平均干涉相干性或NDVI与周围山坡的相应值之比。这种方法的优势在于它消除了可能干扰干涉合成孔径雷达（InSAR）数据分析的长时间基线的负面影响，以及大气和环境因素的干扰。我们发现，在2017年马德河滑坡灾难性崩塌前5个月开始加速时，其相干性比率下降了50%。巧合的是，NDVI比率也开始近乎线性下降。在2016年该滑坡早期加速期间也出现了类似的情况。这表明雷达相干性和NDVI比率可能对评估滑坡活动有用。我们的研究表明，尽管升轨数据不太适合测量斜坡的前兆变形，但它能提供更可靠的相干性比率。综合来看，这些见解表明这种类型的分析可能以有用的方式补充传统的InSAR分析，并为在区域尺度上评估滑坡活动提供机会。