摘要 本篇論文利用全球定位系統(Global Positioning System, GPS)研究台灣上空全電子含量(Total Electron Content, TEC)在1994-2003年327筆M≧5.0地震前後濃度的變化，結果發現M≧5.0地震前第5天到地震後第2天1000 - 2000 LT會有明顯的TEC減少。除去季節效應與日變化的影響，TEC減少的發生時間與前述相同，且地震規模越大，TEC減少越明顯。另外之前15天之下四分位數建立參考探討TEC異常減少機會之發生時間，結果顯示地震前第5天到後第2天0700 - 2000 LT。同時考慮TEC強度與其減少異常機會，發現地震規模越大，異常天越靠近地震天，且與地震規模成正比。統計研究結果顯示，地震規模越大其異常天之勝算越大。 Abstract In this paper, we utilize ground based GPS receivers measured total electron content (TEC) variations of three hundred and twenty seven earthquakes in Taiwan during 1994-2003. The TEC values are further divided by their median values in order to remove the ionospheric seasonal and diurnal effects. Results show that the TEC decreases significantly and often concurrently registers pronounced decrease anomalies during 1000-2000 LT from five days prior to the earthquake day to two days after. It is also found that for M≧5.6 earthquakes, the magnitude is proportion to the TEC decrease. From the odds of the TEC decrease day prior to the earthquake day, we conclude that the anomalous day of decreasing TEC is closer to the earthquake day if the earthquake is larger. The statistics result also confirms that the larger earthquakes yield the greater odds values.