2004年12月26號，在印尼蘇門達臘發生規模9.3的大地震。此地震是近一百年來發生規模第二大的地震，在印度洋板塊和緬甸板塊的互相碰撞下，釋放出巨大的能量，因而造成海底結構的巨大變化。此地震造成約一千兩百公里長的破裂帶和十五公尺的錯動量；造成海水面的劇烈變動，在印度洋沿岸許多地區，甚至遠至非洲近岸都產生數十公尺高的海嘯，估計造成二十多萬人的傷亡。 在這巨大的海底破裂與錯動情況下，使海水面劇烈變動；造成此地區大地水準面（geoid）的變化，經由雷達測高衛星(Jason-1、Topex/Poseidon、Envisat、GFO、ERS-2等衛星)所測量到的海平面高度資料，利用EOF(Empirical Orthogonal Function)的方法來計算震前與震後的海平面變化，因為海水為流體物質，所以平均海平面會依大地水準面的分佈而達到平衡狀態。因此，推算平均海平面的變化，就等同於大地水準面的變化。 另外，將此計算結果與GRACE(Gravity Recovery and Climate Experiment)衛星計畫所觀測到的蘇門達臘地區震前與震後重力值得變化情形作比較。 The magnitude MW = 9.3 earthquake in Sumatra-Andaman on December 26, 2004, was the second largest earthquake in the past century, rupturing along a 1200-km long reverse fault delineated by aftershocks. In this study we use the sea level change as a proxy for the regional geoid change (because the mean sea surface conforms to the geoid), focusing on the co-seismic and post-seismic behavior. For the sea level, we use the radar altimetry data form several satellites (Topex/Poseidon, Jason-1, GFO, Envisat, ERS-2) available from AVISO, from 2003 through 2006 (four years), with gridded resolution of 0.3° and time interval of seven days. We select the region of 15°N ~ 10°S and 90°E ~ 115°E. We calculate the EOF (Empirical Orthogonal Function) to analyze the space-time variation of the sea level signal before and after the Sumatra-Andaman earthquake. Besides the strongest but climate-induced mode which is highly correlated with the ENSO index, we find that the second strongest mode signifies the geoid change caused by the earthquake. This mode also shows a post-seismic recovery of the geoid on a timescale of about two years. We compare this result with recent findings from the GRACE satellite data of time-variable gravity, and confirm that both results of geoid change from pre-seismic to post-seismic are consistent.
