由海底地震儀資料探討宜蘭外海琉球 隱沒帶之地震地體構造;Seismotectonic structures offshore Northeast Taiwan from OBS data

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题名:	由海底地震儀資料探討宜蘭外海琉球隱沒帶之地震地體構造;Seismotectonic structures offshore Northeast Taiwan from OBS data
作者:	蘇怡璇;Su,Yi-hsuan
贡献者:	地球科學學系
关键词:	海底地震儀;琉球海溝;沖繩海槽;地震構造;宜蘭平原;OBS;Ryukyu Trench;Okinawa Trough;seismotectonic structure;Ilan plain
日期:	2014-08-14
上传时间:	2014-10-15 14:43:54 (UTC+8)
出版者:	國立中央大學
摘要:	台灣位於菲律賓海板塊與歐亞大陸板塊之聚合帶上，其中菲律賓海板塊以8.2 cm/yr.的速度朝 ~312˚N方向移動並沿著琉球海溝隱沒，在台灣東部海域產生一系列的隱沒帶構造，島上亦有兩板塊直接碰撞造山運動。而東北部區域因同時受到碰撞作用、隱沒作用與沖繩海槽弧後擴張作用的影響下，一直都是個構造複雜且地震活動頻繁的地區。因受限於此區域之構造皆處在海底並覆蓋豐厚的沉積物，本研究藉由臨時海底地震儀觀測網資料來研究並了解海域地體構造。為解決此問題，ATSEE 2011 計畫(Across Taiwan-Strait Explosion Experiment)於2011年8月15日至9月7日於台灣東部外海放置了16個海底地震儀，觀測範圍自台灣宜蘭外海可延伸至與那國島，包含沖繩海槽西側與弧前盆地。加上台灣東北鄰近之陸上測站資料，藉由蒐集天然地震資料藉以了解宜蘭外海區域、和平與南澳海盆等琉球隱沒區域之地震活動特性，並探討其地震形成之原因。經Antelope程式初步定位後，總計有1058個地震事件被成功定位出來，深度範圍介於0~125 km，規模Md介在1.66~5.22。進一步使用HypoDD軟體重新地震定位後，在23天記錄中有443個地震事件被重新定位。定位後結果顯示地震多集中於三個區域，第一群地震位在宜蘭平原東南側蘇澳一帶，地震深度介在10到60 km，因同時位於碰撞、隱沒與海槽擴張之作用帶上，地震活動多且成因複雜。在此區域我們發現兩向南傾之地震分布，淺層呈現左移走向滑移機制，但於20~35 km深則為側向擠壓地震帶(LCSZ)，應與造山高峰時期之逆衝斷層於後造山時期轉變成正斷層或張裂型平移斷層作用有關。另有一地震分布呈向西傾的分布型態，顯示此區已不受碰撞作用之影響而向西隱沒。第二個地震群位在南澳弧前盆地內，深度介於5到40 km，地震多屬板間地震帶(ISZ)之低角度逆衝斷層，亦有因板塊界面上局部突起隨板塊隱沒所形成之正斷層機制。盆地之北側則有一形成原因仍需探討之地震群位於高速帶中。最後一地震群位於沖繩海槽，此區域多為弧後張裂作用所造成的一系列正斷層及火山活動地震。火山活動相關地震則主要發生在橫貫弧後火山帶 (Cross Backarc Volcanic Trail, CBVT)。由以上特性可知台灣東北宜蘭外海屬地震頻繁區域，對此區域之地震觀測改善為必要的。而本研究佈置之海底地震觀測網對於沖繩海槽與弧前盆地區域之紀錄，皆比中央氣象局和全球地震中心同期間所記錄到的地震數目多，因此對於此區的構造環境有更好之了解。;The NW moving Philippine Sea plate (PSP) collides with the Eurasian plate (EUP) near Taiwan. Simultaneously, it subducts toward the north along the Ryukyu trench at a rate of about 8.2 cm/yr in the direction of ~312˚N and produces a series of major subducting structures. The western termination of the Ryukyu subduction zones hit against eastern Taiwan. In such a complex tectonic environment, a large number of seismic activities is expected. However, restricted by the seismic station distribution, the seismotectonic setting of this area is still not well understood. To obtain more information about the submarine tectonic structures offshore eastern Taiwan, we combined the data from 15 OBSs as well as 11 CWB (Central Weather Bureau) onland stations for more detail earthquakes analysis. The 15 OBSs were deployed during the ATSEE2011 (Across Taiwan-Strait Explosion Experiment) project, from August 15 to September 7, 2011, in the eastern offshore Ilan plain, including the area of the forearc basin, Ryukyu arc and Okinawa trough. Seismic events were detected manually with the Antelope software and the global velocity model iasp91was used for the initial localization. In total, 1058 events with a depth range of 0~125 km and magnitude range (Md) of 1.66~5.22 were located. However, a 1-D seismic velocity model is not sufficiently accurate for a precise hypocenter determination. Thus, the HypoDD software was used for the earthquake relocation. Finally, 443 events were relocated during a recording period of about 23 days. Results show that the relocated earthquakes were concentrated mainly in three clusters. The first cluster is located to the south of Ilan plain (about 24.5N, 121.9E), at the boundary of Taiwan and the PSP. The earthquakes located in this cluster possess a relatively larger depth (~10 to 60 km) compared to the other two clusters. Located at the transition zone of the collision, subduction and backarc extension processes, this area has high and complex seismicity activity. In this region, two south-dipping seismic cluster were observed. The earthquakes is characterized by sinistral strike-slip faultings at shallow depth but becomes to be dominated by lateral compression mechanism at depth between 20~35 km. Another seismic cluster shows a westward dipping distribution which reveals the westward subduction of the PSP. The second cluster occurs in the forearc basin area between 5 and 40 km in depth. According to the former studies, this area is affected simultaneously by both trench-parallel extension and trench-perpendicular compression. Most earthquakes in this region occur along the plate interface, characterized by low-angle thrusting effect. However some normal faulting type earthquakes are generated by local asperity subducting effect. The last earthquake cluster is located at the Okinawa trough, with a depth range of 5 to 30 km. Earthquakes in this region are generally caused by the backarc volcanism and series of normal faults in consequence of extension of the back-arc basin opening. Some earthquakes located at the area of Cross Backarc Volcanic Trail (CBVT) are characterized by low-frequency waveforms, suggesting their volcanic origin. Much more events were recorded by our OBS network in comparison with existing seismic networks, which help to get more understandings about the tectonic environment in the northeastern offshore Taiwan.
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