Potential reasons for ionospheric anomalies detected by nonlinear principal component analysis just before the China Wenchuan earthquake, and their relationship to source conditions

Nonlinear principal component analysis (NLPCA) was performed to examine the total electron content (TEC) anomalies for the China Wenchuan earthquake of May 12, 2008 (= 7.9). This was applied to global ionospheric maps (GIMs) at heights from 150 km to 450 km, with the transforms conducted for the time period 00:00 to 06:00 UT on May 12, 2008. The earthquake occurred at 06:28 UT. The GIMs were analyzed by PCA and NLPCA, whereby they were separated into 100 smaller maps of 36° in longitude and 18° in latitude. These smaller maps are constructed at 71 × 71 pixels, which forms the transform matrix for NLPCA. The transform allows a principal eigenvalue to be assigned for each of the smaller maps. The results of the transforms provide 100 principal eigenvalues that cover the region, which includes the epicenter of the Wenchuan earthquake. The possibility of TEC anomalies caused by X-ray fluxes and geomagnetic activity is eliminated by reviewing X-ray flux data and the Kp index. The eigenvalues of NLPCA are compared with those of PCA. TEC anomalies were clearly detected using NLPCA, with large principal eigenvalues that represent earthquake-related TEC anomalies near the epicenter for the time period 00:00-0600 UT. The results of this study are discussed in terms of the potential causes of these TEC anomalies, especially for the time period 02:00-04:00 UT, when rocks around the epicenter were potentially subjected to a rapid increase in stress, which might have induced very pronounced p-type semiconductor effects. These effects will probably have been present for the other time periods, but might not have been so pronounced