利用全球電離層圖(Global Ionosphere Map, GIM)之全電子含量(Total Electron Content, TEC)，探討2014年8月24日M6.0美國加州南納帕地震電離層異常前兆。首先統計研究2000年1月1日至2014年8月23日(近15年期間)，美國加州地區52筆規模M≧5.0地震前後30天GIM TEC異常變化。Z檢定統計結果呈現這些地震前2-4天02:00-04:00UT期間，GIM TEC會顯著異常減少(負異常)。時序分析結果顯示GIM TEC於2014年8月22日(南納帕地震前2天)02:00UT時段出現負異常，此一事件之時間異常前兆與統計結果相吻合。空間分析異常之全球分佈，結果發現此一負異常持續於2014年8月22日02:00-12:00UT(地震前2天) 且唯獨出現在震央附近。比對GIM TEC空間梯度之全球異常分佈，發現東向梯度於8月19日16:00-24:00UT出現異常極大，僅發生於震央附近。此兩空間異常說明南納帕地震前2-5天，震央附近之GIM TEC出現異常變化。進一步以物理模式重製此一電離層負異常現象，顯示地震孕震期間所產生之西向電場可能扮演重要角色。;This study examines seismo-ionospheric precursors (SIPs) in the global ionosphere map (GIM) of the total electron content (TEC) associated with the 24 August 2014 M6.0 South Napa earthquake. We separate analysis of GIM to three parts including statistical, temporal and spatial analysis. Extract 52 M≧5.0 earthquake during 1 January 2000 to 23 August 2014 to do z-test. Z-test result indicates that there are significantly TEC decreasing (negative anomaly) 2-4 days before earthquake during 02:00-04:00UT. The temporal SIP in the GIM TEC around the epicenter significantly decreasing at 02:00UT on 22 August. Spatial SIP in the GIM TEC indicate that negative anomaly only happen around the epicenter during 02:00-12:00UT on 22 August. Check spatial gradient of GIM TEC and find that there are negative anomaly of eastward gradient during 16:00-24:00UT on 19 August. Both of spatial analysis show the result that SIP was triggered 2-5 days before the South Napa earthquake. Model simulation is further carried out to reproduce the SIP in the GIM before the earthquake. Results indicate that the westward electric field generated over the epicenter area during the earthquake preparation period might be essential.
