| 摘要: | 近年來，大量研究關注於大地震前的電磁異常和相關現象，而在這些研究之中，全球定位系統全電子含量（TEC）之地震電離層前兆為最重要的一環。本論文利用全球全電子含量圖（GIM）之TEC進行時間與空間分析，搜尋可能之地震電離層前兆。對某一固定點的TEC時間序列進行檢驗，以偵測地震前兆（單點TEC變化監測）。每一時間點，以前15日之資料計算移動中位數和上、下四分數作為參考值，探測於觀測日（第16日）的異常訊號。當觀測之TEC高於上界或低於下界，則視為探測到正異常或負異常。為了解探測到的異常是否為可能之地震電離層時間前兆，應用Z檢定統計相同地區大量地震之TEC異常，以找出此地區時間前兆的特性，包括正負極性、出現的當地時間和領先日期等。在地磁寧靜期，探測到的異常與統計的特性吻合時，表示此異常為可能之時間前兆。沿監測點經線上的TEC（即緯度-時間圖監測）也被用來探測並確認地震電離層前兆。此外，電離層易受到太陽輻射、磁暴等效應所擾亂，而出現正和負異常。為了區分區域效應（如：地震）和全球效應（如：太陽擾動、磁暴），則進行TEC異常和其梯度異常之全球分布檢驗（即緯度-經度圖監測），若異常頻繁或持續出現在探測到時間前兆的地區，則視為探測到空間前兆。
本論文研究分析2010年1月12日規模7.0海地地震、1999年10月16日規模7.1美國赫克托麥地震、2004年12月26日規模9.1印尼蘇門答臘地震和2011年3月11日規模9.0日本東北地震之GIM TEC地震電離層時空前兆。結果顯示其時間前兆為：海地地震前1日和東北地震前3日出現正異常，而蘇門答臘地震前5日出現負異常。赫克托麥地震的時間前兆較為不明確，在震前7–5日出現兩種極性之異常。緯度-時間圖則呈現，海地地震前1日在震央緯度和共軛點緯度的TEC顯著增加，東北地震前3日於赤道異常區的TEC顯著增加，而蘇門答臘地震前5日其北赤道異常區的TEC明顯減小。緯度-經度圖進一步呈現蘇門答臘地震前5日和東北地震前4–3日，分別有TEC 30日極大負和正異常頻繁、持續和固定地出現在蘇門答臘地震震央東南方1000–1600 km和東北地震震央東北方800 km。這兩個地震之梯度異常並沒有十分明顯。另一方面，規模7.0海地地震前1日，TEC 30日極大正異常持續和固定地出現在震央西北方1000–2700 km，而東向梯度負異常則持續出現於震央東北方900 km。此外，規模7.1赫克托麥地震前6–5日，北向梯度負異常持續出現在震央西方1300 km。梯度異常似乎對此兩個規模相對較小的地震更為靈敏。儘管四個地震案例的相關異常與震央之距離約數千公里，然而與地震準備帶的尺度相比，蘇門答臘和東北地震的TEC異常位置非常靠近震央。經本論文的邏輯實證判斷，四個地震案例皆偵測到地震電離層空間前兆。最後，電離層異常經常同時出現在震央和共軛點上，說明地震伴生電場的重要性。;Recently, seismo-electromagnetic anomalies and related phenomena before large earthquakes have intensively studied. Among these existing studies, the seismo-ionospheric precursors (SIPs) of the GPS total electron content (TEC) is the most promising one. In this dissertation, to search SIPs, the global ionosphere map (GIM) of the TEC is employed, while both temporal and spatial analyses are conducted. In a certain region, to see if there is a forthcoming earthquake, a time series of the TEC at a fixed location (TEC variation, a point, zero-dimensional, monitoring) of the region is examined. At each time point, the previous 15-day running data are used to compute the median and upper/lower quartile constructing a reference for detecting anomalies on the observation (16th) day. When the observed TEC exceeds its upper or lower bounds, it can declare that a positive or negative anomaly is detected. To see if the detected anomaly is a temporal SIP, TEC anomalies associated with a large amount of earthquakes in the same region are statistically investigated by z test to find the characteristics of the temporal SIP, which are the polarity of positive/negative, the appearing local time period, the leading day, etc. When the detected anomaly meets the associated characteristics during the magnetic quiet, it can be considered to be a temporal precursor. Here, the TEC along the monitoring point longitude (Latitude-Time-TEC (LTT), a line, one-dimensional, monitoring) is also employed to confirm the SIP detection. On the other hand, the ionosphere can be easily disturbed by solar radiations, magnetic storms, etc. and reveals both positive and negative anomalies. To discriminate effects of the local (such as, earthquakes) from those of the global (such as, solar disturbances, magnetic storms), the global distributions of anomalies in the GIM TEC (i.e. Latitude-Longitude-TEC (LLT), two-dimensional, monitoring) and the associated gradient are examined. If anomalies frequently and persistently appear near the monitoring region, where the temporal SIP is detected, it can declare the spatial precursors being detected.
The temporal and spatial SIPs of the GIM TEC associated with the 12 January 2010 Mw7.0 Haiti Earthquake, the 16 October 1999 Mw7.1 Hector Mine Earthquake, the 26 December 2004 Mw9.1 Sumatra Earthquake, and the 11 March 2011 Mw9.0 Tohoku Earthquake are investigated. Results show that the temporal SIPs are the positive anomalies appearing 1 day before the Haiti Earthquake, and 3 days before the Tohoku Earthquake, but the negative anomalies occurring 5 days before the Sumatra Earthquake. The temporal SIP of the Hector Mine Earthquake is unclear, which seems having dual polarities of positive and negative appearing 7-5 days before the earthquake. Likewise, the LTT plots reveal that the TEC significantly enhances at the epicenter- and conjugate-latitude 1 day before the Haiti Earthquake, the TECs at the equatorial ionization anomaly (EIA) region considerably increase 3 days before the Tohoku Earthquake, while the TEC of the northern EIA extensively reduces 5 days before the Sumatra Earthquake. The LLT map further show that the TEC anomalies of 30-day extreme minimum and maximum frequently, persistently, and specifically appear at 1000-1600km southeast from the epicenter, 5 days before the Sumatra Earthquake, and at 800 km northeast from the epicenter, 4-3 days before the Tohoku Earthquake, respectively. There is no obvious gradient anomaly can be found of these two devastating earthquakes. On the other hand, for the Mw7.0 Haiti Earthquake, the TEC anomalies of 30-day extreme maximum persistently and specifically appear at 1000-2700km northwest from the epicenter, 1 day before, while anomalies of the associated westward gradient persistently appear at 900km northeast from the epicenter. For the Mw7.1 Hector Mine Earthquake, anomalies of the southward gradient continuously appear at 1300km west from the epicenter, 6-5 days before. It seems that the gradient anomalies are more sensitive to these two relatively smaller earthquakes. In general, the distances of the associated anomaly from the epicenter of the four earthquakes are approximate thousands km. However, comparing the earthquake preparation area, the anomalies are in fact very close to the epicenter of the Sumatra Earthquake and the Tohoku Earthquake. Finally, it can be seen that the anomalies often appear concurrently over the epicenter and the conjugate points, which indicates that the seismo-generated electric field are essential. |
