Variations of ionospheric total electron content during the Chi-Chi earthquake

On 20 September 1999 UT (21 September in local time, LT) a large earthquake Mw=7.7 struck central Taiwan near the small town of Chi-Chi. The greatest plasma frequency in the ionosphere, foF2, observed by the ChungLi ionosonde (25.0N, 121.2E) reveals three clear precursors at 1, 3, and 4 days prior to the earthquake. This paper examines the ionospheric total electron content (TEC) observed by a network of the global positioning system (GPS) receivers in Taiwan area. It is found that variations in foF2 and overhead TEC recorded at Chung-Li have a similar tendency. Combining the data of the network of 13 GPS receivers, time, and spatial variations of TEC prior to the ChiChi earthquake are examined. Results show that the equatorial anomaly crest moves equatorward and its TEC value significantly decreases 1, 3, and 4 days before the earthquake. A comparison between the disturbed and reference (previous 15-day median) days confirms that TEC decreases significantly around the epicenter in the afternoons of these days. Finally, possible mechanisms are proposed and discussed.