区域选取对图像信息法可预测性的影响

Effect on the predictability of pattern informatics method related to selection of studied regions

图像信息(PI)法是一种基于统计物理学的地震预测方法,因其对中长期地震预测有较好的效果已在国内外广泛应用。PI方法在计算过程中对选取区域的所有网格参量进行了归一化,因此不同的区域选取会产生PI热点结果的变化。本文基于中国地震台网中心自1970年以来的全国M_(S)≥3.0地震目录,采用5年尺度的“异常学习时段”和“预测时间窗”以及1年尺度的滑动时间步长,以南北地震带2016年以来发生的M_(S)≥6.0地震的回溯性预测检验为例,研究了不同空间范围的选取对PI方法地震预测效能的影响。地震预测效能检验采用R值评分法和受试者工作特征(ROC)检验方法。结果显示:①在其它计算参数相同的情况下,不同的区域选取对PI预测结果有较大影响;②利用R值评分和ROC检验方法对不同研究区的预测效能进行评估时,区域内部地震活动性差异小的区域预测效果较好,而对于地震活动性存在较大差异的区域,地震活动性高的区域内发生的地震更容易被预测,推测导致这一结果的可能原因是地震活动性较强的区域出现的异常更显著,而算法里的归一化过程会抑制地震活动性较低区域出现的异常,从而造成漏报;③对于具体的目标地震,震中附近的PI热点图像会经历演化,因此利用PI方法向前预测时要结合多个时间窗口进行综合预测;④不同于其它天然构造地震的热点演化趋势,2019年四川长宁M_(S)6.0地震和2021年四川泸县M_(S)6.0地震震中附近热点反复出现、消失,可能与人工活动有关;⑤滇西南地区、海原断裂中东部附近、小江断裂中部地区、龙门山断裂南部和小江断裂东北部地区存在持续出现的PI热点,这些区域为值得关注的M_(S)≥6.0地震发震区域。

The Pattern Informatics(PI)method is a approach for earthquake forecasting based on statistical physics,and has been widely applied both at home and abroad due to its good performance in medium to long term earthquake forecasting.The algorithm of PI method includes the process of normalization of all grid parameters in the selected region,so the distribution of PI hotspots might be different with the different selected studied regions theoretically.However,the predictability of PI due to the selection of studied regions has not been systematically studied so far.We performed the retrospective forecasting for seven earthquakes above M_(S)6.0 in the North-South Seismic Zone since 2016 under different size regions.The earthquake catalogue since 1970 is taken from the China Earthquake Networks Center.Both the anomaly learning period and forecast interval are fixed as five years and the moving step is taken as one year in this study.The forecasting efficiency of PI is tested by R score and ROC(receiver operating characteristic)test.The results showed that different region selection might lead to different forecasting results with the same calculation parameters.The R score and ROC tests results for the selected regions with lower seismicity-difference are better than those with higher seismicity-difference.In the selected regions with higher seismicity-difference,target earthquakes in the areas with higher seismicity tend to be predicted more easily than those in the areas with lower seismicity,which is supposed to be caused by the fact that PI hotspots are more obvious in the areas with higher seismicity and they will suppress the anomalous signal detected by PI algorithm in the areas with lower seismicity,resulting in the missing prediction for the target earthquakes in the areas with lower seismicity.For a specific target earthquake,the imagine of PI hotspot around the epicenter will evolve,so the combination of multiple forecasting windows should be considered when the forward events are predicted using PI method.Different from the hotspots evolution trend of other natural tectonic earthquakes,the hotspots of 2019 M_(S)6.0 Changning and 2021 M_(S)6.0 Luxian earthquakes in Sichuan appeared and disappeared repeatedly near their epicenters,which may be related to human activities.There are continuous PI hotspots at the boundary of southwestern Yunnan,middle and eastern section of Haiyuan fault,the middle Xiaojiang fault,the southern Longmenshan fault and the northeastern Xiaojiang fault,suggesting that there will be seismic potentials with M_(S)6.0 or above in these regions.