| 摘要: | 摘要
為了重新檢視隱沒帶的孕育構造與機制，本研究使用全球地震目錄(GCMT)1976年1月1日到2011年12月31日所記錄到的資料，並以Christensen and Ruff (1983)所提出的方式，利用隱沒帶板塊外部隆起的地震特性，判斷耦合程度的方法來檢視隱沒帶的孕震特性。另外，我們藉由討論大陸板塊上震前地震所造成重力位能隨時間改變量的變化，對於大地震發生前的應力和地震活動變化能夠有更進一步的解釋與了解。
本研究首先將隱沒帶以Ruff and Kanamori (1980)所提出不同的耦合型態做分類，並且搜索隱沒帶內的地震資料，找出符合板塊外部隆起條件的地震事件。藉由不同隱沒帶內板塊外部隆起地震事件在震前與震後的變化情形，分析大型隱沒帶地震的孕震機制。結果顯示約於外部隆起聚合性地震產生後的4至43年間，在其相對應隱沒帶上會有大型逆衝型地震的發生，顯示此模型的確可以用來評估隱沒帶內地震潛勢的大小。並且本研究在阿拉斯加與阿留申海溝、日本東北、秘魯與智利的區域皆發現有外部隆起聚合性地震，但無相對應的大型隱沒帶逆衝地震，因此這些區域可能具有高地震潛勢。另一方面，弱耦合帶區域主要觀察到的外部隆起地震多為伸張性的地震事件，例如琉球與伊豆小笠原海溝，此現象與Christensen and Ruff (1988)所觀察到的結果相符合。而在強耦合的隱沒帶下，通常在規模大於8的逆衝型地震事件發生之後，立即可以看見伸張性的外部隆起地震。進一步的分析更發現除了智利隱沒帶外，地震後伸張性外部隆起地震事件的持續時間與地震規模之間的呈現正相關性。反之，規模小於8的地震事件，在地震後並無法清楚地看見伸張性的地震的發生。
為了能夠更清楚了解大型逆衝型地震發生前的地震活動變化，本研究藉由Bowman (1988)的方式得到各隱沒帶的地震範圍大小。我們分析地震範圍內由地震所引起的重力位能累積量隨時間的變化。結果顯示在大型逆衝型地震發生前，震源區域內震前由地震所引起的重力位能變化隨時間變化斜率有增加的趨勢。顯示出大型逆衝型地震發生前，震源區可能有忽然升高能量釋放的情形。統計結果顯示，重力位能累積改變量越大，之後產生的地震其規模也越大。
Characterizing the seismogenic zone of major subduction plate boundaries provides us a possibility to reduce large earthquake hazard.
In the past several decades, many scientists have analyzed various geophysical methods and datasets, such as seismic and geodetic ground motion data, historical tsunami deposits, aftershock distributions, and seafloor bathymetry, trying to understand the mechanisms behind great devastating earthquakes, and to estimate the probability of a major earthquake occurrence in the future.
In this study, by using the global earthquake catalog (GCMT) from January 1, 1976 to December 31, 2011, we firstly re-examines the outer-rise earthquake model proposed by the Christensen and Ruff (1988) at the subduction zones suggested to have different coupling levels. The compressive stress cumulated during the subducting processes are often reflected by the occurrence of compressional outer-rise earthquakes. Thus, in the region where the compressional outer-rise earthquakes take place without any corresponding large underthrusting earthquakes, the seismic potential is usually considered to be high. We re-examined the high seismic potential areas determined by this criteria in Christensen and Ruff (1988) and confirm that the large underthrusting earthquakes did really occur in the 30 years following the appearance of compressional outer-rise events, such as in Tonga region in the vicinity of 20?S, a Mw 8.3 large earthquake occurred in 2006. This result represents that the outer-rise earthquake model could be an indicator for the generation of large earthquakes along subduction zones. In addition, to have a more accurate estimation for the seismic potential, we discuss the relationship between the generation of earthquakes and the change of cumulative gravitational potential energy caused by earthquakes (DGPE) over time. Our result shows an acceleration of DGPE before large earthquakes.
Our result also shows that the extensional outer-rise events for strong seismic coupling subduction zone only presented after the occurrence of earthquakes with magnitude larger than 8, for instance, after the 2012 March Mw 9.1 Tohoku, the 2010 February Mw 8.8 Chili and the 2006 November Mw 8.3 Kamchatka earthquakes, which is consistent with the analysis performed by Christensen and Ruff (1988). Based on our analysis, the outer rise earthquakes occur immediately after the main event which does not coincide with the result stating in Christensen and Ruff (1988) that they occur in the 30 years after the earthquake. In addition, the duration of the extensional outer-rise earthquakes occurrence appears to be correlated with its magnitude. Meanwhile, for the earthquakes with magnitude smaller than 8, as well as in the weak coupling areas, this observation is not engaged. |
