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|Title: ||利用海底地震儀分析智利馬烏萊 2010 年M=8.8 隱沒地震的可能孕震機制|
Using OBS aftershock data to analyze the possible seismogenic processes of the 2010 Maule Chile mega thrust
|Authors: ||Shih-Jie Wang|
|Contributors: ||NTOU:Institute of Applied Geosciences|
aseismic zone;seismogenic zone;Maule Chile
|Issue Date: ||2013-10-07T03:03:44Z
|Abstract: ||西元2010年2月27日智利中部馬烏萊(Maule)地區發生規模8.8的地震，在海域裡產生了四百多公里長的破裂，而引發海嘯，造成沿海地區嚴重的人員傷亡及房舍的破壞。而在西元1960年，智利也發生了規模9.5的巨大地震，它是從人類對地震觀測有記錄以來最大規模的地震。本實驗室在2010年主震發生後所產生的餘震區域佈放了A、B兩個陣列 (A陣列7月15日-8月7日; B陣列8月14日-9月6日)，共33顆的海底地震儀 (Ocean Bottom Seismometer, OBS)，目的是為了收集大地震發生之後所產生的餘震，並且利用餘震的分佈、特性、型態來瞭解地震破裂後的地體活動，以推測主震孕震及其發震的可能過程。本研究使用B陣列，共17顆OBS，資料連續記錄23天。本研究使用Antelope挑選P波和S波到時，並用HypoDD來重新定出餘震發生的位置。初步的結果得知，大部分的地震都沿著隱沒海溝的軸心分佈。在前緣增積岩體(frontal accretionary prism)因為含水量較多的關係而呈現幾乎無震區(aseismic zone)的現象。而在古老增積岩體 (paleo-accretionary prism)以東一直到陸緣的部分則明顯呈現出此區間是屬於主要孕震帶(seismogenic zone)孕育深度介於50-100公里之間。比較主震前與本研究定出來的地震分佈，顯示餘震分佈大多分佈在破裂面邊緣，但在破裂面區域亦有餘震的發生，另外在海溝以西與主震以南的地震事件都有明顯增多的趨勢。因此我們由地震形態觀察出，由於納斯卡板塊隱沒活動使得主震發生後所產生的逆衝型斷層破裂，釋放了部分隱沒所累積的能量，但應力並未釋放完成。在海溝後側的地震群聚走向從南緯33.3度呈北北東-南南西，而至南緯34.3度之後改變成西北-東南走向，地震分佈延伸至南緯34.5度，西經71.5度的區域，而參考同震滑移模型圖可以看到在該區域有一線性的邊界，這可能是由海岸山脈進入中央窪地的交界處。而在主震發生之後，在皮智勒姆 (Pichilemu)地區附近發生了一系列的正斷層機制的地震，這可能是受到馬烏萊主震所產生的破裂，造成該區域在隱沒應力上的改變，未來需要更詳細的研究。臺灣東部的琉球隱沒帶（菲律賓言板塊隱沒進入歐亞板塊）和智利的隱沒帶，在地體構造上非常類似。我們希望藉由智利的餘震研究中，來探討臺灣大地震以及海嘯的可能產生原因，以為未來防災之應用。|
In the early morning of February 27, 2010, a mega-earthquake now known as the “Maule Earthquake” (M=8.8) took place in the Maule region in central Chile. In May 1960, Chile was hit by the largest earthquake ever recorded with a magnitude of 9.5. In general, the west coast of Chile is a convergent boundary between the Nazca and South American Plates, with the Nazca Plate subducting beneath the South American Plate in a NE direction. With a convergence rate of 6-7 cm per year, stress accumulates in the lower part of the oceanic plate to a certain extent resulting in huge destructive earthquakes. In 2010, our team deployed two Ocean Bottom Seismometer (OBS) arrays (the A and B arrays), with a total of 33 deployments to record the aftershocks along the rupture area. We collected data for a total of 46 days (July 15 to August 7 for the A array and August 14 to September 6 for the B array). The aim of our study was to analyze the distribution and characteristics of the aftershocks to get a better understanding of the tectonic activity after the main event, and conjecture on the seismogenic processes that occurred during the rupture. Using the Antelope software on the B array data we picked the P- and S-wave arrivals and located the events. To obtain more accurate earthquake epicenter locations we also applied the HypoDD software. We recognized a total of 1,972 events in 23 days of monitoring with many of them distributed along both sides of the trench. Immediately behind the trench axis, along the frontal accretionary prism, there is an aseismic zone, possibly due to the high content of water in the sedimentary strata. On the other hand, the paleo-accretionary prism on the landward side of the trench accumulated most of the earthquakes. These events focus at depths of 50-100 km in the subduction zone. This is called the seismogenic zone. The comparison of events before the main shock and the HypoDD results of this study show that most of the events cluster along the edge of the northern portion of the rupture zone. In addition, the events apparently increase in west of the trench and south of the main shock. We suggest that the subduction activity of the Nazca Plate released more energy in ruptures after the main shock. However, the stress is probably not totally released yet. The events cluster landward of the trench with a trend NNE-SSW at about 33.3°S, and change to the NW-SE direction at 34.3°S. Events extended to the area located at 34.5°S and 71.5°W. This is a new linear boundary in this area after the main shock. The boundary is probably located at the intersection between the Andean Cordillera and the central depression (Central Valley). There is a series of thrust and normal type faulting near Pichilemu. This was probably triggered by the main shock of the Maule rupture which caused a change on the subducted stress in this region. Further detailed study will be needed. The tectonic structure of Taiwan and Chile are similar. By studying the aftershock and crustal structure in Chile we hope to improve our understanding of the seismogenic zone, which may cause mega-earthquakes and tsunamis, in the Taiwan region and other subduction zones in the world.
|Appears in Collections:||[應用地球科學研究所] 博碩士論文|
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