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, foF(2), observed by the Chung-Li ionoso nde (25.0 degreesN,121.2 degreesE) reveals three clear precursors at 1, 3, and 4 days prior to the earthquake. This paper examines the ionospheric tot al electron content (TEC) observed by a network of the global positioning s ystem (GPS) receivers in Taiwan area. It is found that variations in foF(2) and overhead TEC recorded at Chung-Li have a similar tendency. Combining t he data of the network of 13 GPS receivers, time, and spatial variations of TEC prior to the Chi-Chi earthquake are examined. Results show that the eq uatorial anomaly crest moves equatorward and its TEC value significantly de creases 1, 3, and 4 days before the earthquake. A comparison between the di sturbed and reference (previous 15-day median) days confirms that TEC decre ases significantly around the epicenter in the afternoons of these days. Fi nally, possible mechanisms are proposed and discussed.