Potential reasons for ionospheric anomalies immediately prior to Chinaʹs Wenchuan earthquake on 12 May 2008 detected by nonlinear principal component analysis

Nonlinear principal component analysis (NLPCA) is used to detect total electron content (TEC) anomalies for Chinaʹs Wenchuan earthquake on 12 May 2008 (UT) (Mw = 7.9). NLPCA is applied to global ionospheric maps (GIMs) at height ranging from 150 to 200 km with transforms conducted for the time period 00:00–06:00 UT on 12 May 2008. The GIMs are analyzed using NLPCA whereby the GIMs are separated into 100 smaller maps of 36° in longitude and 18° in latitude. These smaller maps are constructed at 71 × 71 pixels forming the transform matrix of the NLPCA. The transform allows for a principal eigenvalue to be assigned for each of the smaller maps. The results of the transforms provide 100 principal eigenvalues covering the region and the epicenter of the Wenchuan earthquake. The possibility of TEC anomalies being caused by X-ray flux and geomagnetic activity is eliminated by reviewing X-ray flux data and the Kp index. The eigenvalues of NLPCA are compared with the eigenvalues of principal component analysis (PCA), and TEC anomalies are clearly detected using NLPCA. Large principal eigenvalues representative of earthquake-related TEC anomalies were found nearby the epicenter for the time period 00:00–0600 UT using NLPCA. The earthquake occurred at 06:28 UT. A potential cause of the clear TEC anomaly almost directly over the epicenter in the time period 0200–0400 would be very stark p-type semiconductor effects caused by rocks under extreme stress. The stress may have abated during other time periods reducing p-type semiconductor effects and associated TEC anomalies.