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This research uses eigenvalue characteristics of nonlinear principal component analysis (NLPCA) and principal component analysis (PCA) to investigate total electron content (TEC) anomalies associated with Taiwan’s Chi-Chi earthquake of 21 September 1999 (LT) (Mw=7.6).The transforms are used for ionospheric TEC from 01 August to 20 September 1999 (local time) using data from 13 GPS receivers.The data were collected at 22°N-26°N Lat.and 120°E-122°E Long..Applying the NLPCA to the multi-channel total electron content records of GPS receivers,the earthquake-associated TEC anomalies were represented by large principal eigenvalues of NLPCA (>0.5 in a normalized set) on 14 August and 17,18,and 20 September,with allowance given for the Dst index,which was quiet for the study period.Comparisons were then made with other researchers who also found TEC anomalies on September 17,18,and 19 associated with the Chi-Chi earthquake,which cannot be detected by PCA.Consideration is also given for reported ground level geomagnetic field activity that occurred between mid-August and late October,leading up to and including the Chi-Chi and Chia-Yi earthquakes,which are associated with the same series of faults.It is possible that Aug.14 is representative of an earthquake-associated TEC anomaly.This is an interesting result given how much earlier than the earthquake it occurred.