Abstract

Abstract—This research uses eigenvalue characteristics of the Karhunen–Loève Transform to investigate GPS network ionospheric total electron content (TEC) anomalies associated with Taiwan’s Chi-Chi earthquake of September 21, 1999 (LT) (Mw = 7.6). The transforms are conducted for ionospheric TEC from August 1 to September 20, 1999 (local time), using data from 13 GPS receivers. The data were collected at 22°–26°N Lat. and 120°–122°E Long. Applying the Karhunen–Loève Transform to the GPS receiver data TEC anomalies gave large principal eigenvalues (>0.5 in a normalized set) on August 14 and September 17, 18, and 20, 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. Consideration is also given to 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 August 14 is representative of an earthquake-associated TEC anomaly. This is an interesting result given how much earlier than the earthquake it occurred. Keywords: Karhunen–Loève Transform—GPS network ionospheric total electron content (TEC)—Taiwan’s Chi-Chi Earthquake.