One sliding PCA method to detect ionospheric anomalies before strong Earthquakes: Cases study of Qinghai, Honshu, Hotan and Nepal earthquakes

A sliding principal component analysis (PCA) method is proposed to detect pre-earthquake ionospheric anomalies. We analyzed the precision of this new method with different length of time window in detecting the reference background total electron content (TEC). The results showed that the most suitable window length is 27 days which is consistent with the solar rotation period. We compared the precision of this new method with the sliding inter quartile range (IQR) method and the sliding average method in detecting the background TECs, and found that the precision of sliding PCA is better than those of the traditional methods in the middle and low latitudes because the detected background TEC residual errors by the sliding PCA method are less than that by the traditional methods. We adopted a more reasonable method to calculate the background value's upper and lower bounds and then take four strong earthquakes (Qinghai earthquake, Honshu earthquake, Hotan earthquake and Nepal earthquake) as examples to prove the effectiveness of the sliding PCA method. From the detection results of the sliding PCA we found that obvious ionospheric anomalies appeared on April 1, 2010, March 8, 2011, February 2, 2014, April 11, 23, year 2015. After the further analysis of the solar-terrestrial environment and the distribution of TEC anomaly area, it can be considered that the pre-earthquake anomalies on these days before the earthquake may have a large correlation with the following earthquake. In addition, the TEC anomaly area spans largely in the longitude direction along the bound of equator anomalous zone, and the anomalous morphology has the conjugate structure. This regular can provide a valuable reference for the short-impending earthquake prediction in the future.