菲律賓民都洛島西北地震地體和上部地函速度構造;Seismotectonics and Upper Mantle Structures of Northwest Mindoro, Philippines

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题名:	菲律賓民都洛島西北地震地體和上部地函速度構造;Seismotectonics and Upper Mantle Structures of Northwest Mindoro, Philippines
作者:	李奎模;Li,Kuei-Mo
贡献者:	地球科學學系
关键词:	民都洛;隱沒板塊;殘餘到時差;Mindoro;subducting slab;differential residual time
日期:	2016-08-31
上传时间:	2016-10-13 13:23:47 (UTC+8)
出版者:	國立中央大學
摘要:	馬尼拉海溝位在西太平洋聚合板塊邊界上，分布範圍從台灣西南外海向南延伸至菲律賓民都洛島西北外海。馬尼拉海溝南北兩端的終止，都是由於不同時期弧陸碰撞所造成。北端是由呂宋島弧於中新世至上新世時期碰撞上中國大陸邊緣，南端則是中新世早期菲律賓移動帶和巴拉望陸塊擠壓導致。本研究為探討北端及南端不同時期的弧陸碰撞，分析兩端區域隱沒板塊的形貌與上部地函的速度構造。首先分析馬尼拉海溝兩端弧陸碰撞處的隱沒板塊幾何形貌，選定海溝北端北緯20度到22度，南端北緯12度到15度，作為搜尋EHB地震目錄的研究範圍，並使用試誤法(trial and error)找尋最佳描述班尼奧夫帶的走向和傾角，合併ETOPO1的水深投影六條剖面，討論班尼奧夫帶的空間變化，並由水深的最低處決定參考海溝的位置，來量測參考海溝和隱沒板塊在200公里深的水平距離的改變。得到北端的海底地形有兩處相對低區，南端只有單一一個低區，推測北端形成兩處低區的原因是受不同時期弧陸碰撞的結果。從剖面來看隱沒板塊形貌變化，南端較陡峭且剖面傾角變化較多，推測南端和地震觀測到的down-dip extension相關。而兩端參考海溝與隱沒板塊最深處的水平距離皆為縮短。其次為研究上部地函速度構造，北端近台灣地區之速度構造，已由TAIGER計劃(Taiwan Integrated Geodynamic Research)深入研究。而南端近菲律賓民都洛島缺乏地震觀測資料且速度構造研究成果有限，因此本研究以民都洛島西北為主要範圍建置寬頻地震站，利用殘餘到時差(DRTs, Differential Residual Times)分析從2013年至2016年2月遠震事件初達P波，DRTs分析方法選擇BATS之南沙站當作參考站，民都洛島上五個寬頻地震站當作觀測站。使用下半球投影表示DRTs值，分析不同後方位角來的遠震事件，能得出陡峭傾斜的隱沒板塊存在西北和中部民都洛島底下，隱沒板塊特徵漸漸模糊於民都洛島南部地區。 ;Manila trench, a convergent plate boundary in the west Pacific Ocean, distributes from offshore SW Taiwan to the north to offshore NW Mindoro to the south. Both ends of the Manila trench terminate as a result of arc-continental collision in different periods. To the north, the Luzon arc collided with the Chinese Continental Margin during Miocene to Pliocene. To the south, the Palawan Continental Block indented into the Philippine Mobile Belt during the Miocene. In this study, we focus on time evolution of arc-continent collision by comparing the slab geometries and upper mantle velocity structures of the two ends. Using EHB catalog, we sort out earthquakes from 20°N to 22°N and from 12°N to 15°N mapping geometries of subducting slab for the north and south ends of the Manila trench, respectively. A trial-and-error approach is adopted to determine the optimal strike-normal direction to project the earthquakes and ETOPO1 bathymetry into six profiles ranging from subduction to collision. The horizontal distances between reference trench and slab at 200 km depths are also measured. Results show that while there are two bathymetry lows in the north, there is only one in the south, probably due to incipient collision of continents for the former. In terms of geometries of subducting slabs, the southern one is more various than the northern one, with steeping of dip angles initiated at depths and progressively propagating upwards from subduction to collision. The steeping of dip angle can be attributed to the negative buoyancy of slab as attested by the down-dip extension stresses exhibited by intermediate-depth earthquakes. While the upper mantle velocity structures of the north end near Taiwan are well analyzed by TAIGER project (Taiwan Integrated Geodynamic Research), those of the south end near Mindoro are less well known due to limitation of observations. Therefore, we built broadband stations around NW Mindoro for the investigation. We used DRTs (Differential Residual Times) methods to analyze different first arrival times of teleseismic events, with those of Nasha station as a reference station. Having projected DRTs by lower hemispherical projection, we conclude that there exists steeply dipping aseismic slab beneath central Mindoro, with signatures getting blurred southwards.
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