博碩士論文 104622018 完整後設資料紀錄

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DC.creatorYu-Tse Linen_US
DC.descriptionNational Central Universityen_US
dc.description.abstract背景噪訊技術有許多優點,因為噪訊無時無刻都在發生且分佈廣泛,也不受大地震事件或人工震源的限制,因此,可以從任意測站對資料經過交對比後得到站間的格林函數來獲得所需要資訊。目前台灣已經有許多的陸上測站用此方法估算淺層的速度構造,相對地,海底地震儀 (Ocean Bottom Seismometer;OBS) 除了施測成本較高外,記錄時間也相當有限,因此此項技術很少被應用在台灣海域微地動資料的分析上。然而,隨著海底資源探勘及海域災害潛勢評估需求增加,如何取得海底沉積物剪力波速度以評估海底滑坡的剪力強度成為相關災害的重要關鍵。   本研究收集台灣東北及西南各兩組海底地震儀臨時地震網資料:東北的兩組觀測網落在琉球隱沒系統附近;西南的兩組則分別靠近高屏陸坡及手掌海脊區域。透過交對比函數得知微震的來源方向,並進一步藉由相速度頻散估算當地淺部沉積物的剪力波速度。分析結果在東北海域顯示多變的噪訊訊號來源,比較靠近台灣的部份可能是海浪拍打海岸回彈後產生,而離岸較遠及海槽內側則可能源自於海浪與崎嶇海床間的交互作用。而在西南海域,噪訊源有較一致的來源方向,從南方向北方傳遞,推測應是由大洋中的海浪拍打到淺水域的大陸斜坡回彈所產生的噪訊訊號。   剪力波速度可決定相速度頻散曲線,東北海域的部分頻散曲線速度略高於西南海域,可能與東北海域淺層有較多火成岩體有關。這結果與南沖繩海槽火山活動地區速度普遍較周遭沉積層快相符。把高屏陸坡的ㄧ維剪力波速度模型比對先前的震測剖面,高速區能對應到較為堅硬的海脊和泥貫入體,而低速區則對應到較鬆散的崩塌區域;整個手掌海脊地區的剪力波速度分佈相當一致並普遍偏低,可能是因為此區遍佈崩塌構造所造成。本研究所得到的淺層沉積物剪力波速度約在0.38和0.62 km/s之間,和西南外海研究區域附近由海底地震儀折射剖面資料所逆推出的速度構造非常一致,證明使用背景噪訊技術能夠以較簡單的方式獲得當地的剪力波參數。未來如果能增加測站的密度與記錄時間,相信對於海床底下的能源開採或是探討海底邊坡穩定性都會是有利的貢獻。zh_TW
dc.description.abstract Seismic ambient noise technology has many advantages over the traditional two-station method, the most important reason is that noise is happening all the time, so the signal can be widely and evenly distributed. Also, data resources will not be limited by earthquake events or artificial source. Since Green’s Function of any two stations can be obtained easily through cross-correlation method, related studies become more and more popular in the recent ten years. There are many studies using this concept to obtain the underground velocity structures based on onshore seismic stations. Only a few studies were reported for the marine area due to the relatively shorter recording time of ocean bottom seismometers (OBS) deployment and the high cost of the marine experiment. However, the understanding about the shear-wave velocity (Vs) of the marine sediments is very crucial for the hazard assessment related to submarine landslides, particularly with the growing of submarine resources exploration. In this study, we applied the ambient noise technique to the four OBS seismic networks located offshore Taiwan in the aim of getting more information about the noise sources and having the preliminary estimation for the Vs of the marine sediments. Two of the seismic networks were deployed in the NE part of Taiwan, near the Ryukyu subduction system, whereas the others were in the SW area, near the Kao-Ping Canyon and Palm Ridge areas. Generally, ambient seismic noise could be associated with wind, ocean waves, rock fracturing and anthropogenic activity. In the southwestern Taiwan, the cross-correlation function obtained from two seismic networks indicate similar direction, suggestion that the source from the south of the network could be the origin of the noise. However, the two networks in the northeastern Taiwan show various source directions, which could be caused by the abrupt change of bathymetry. The Vs determined from the dispersion curve shows a relatively higher value for the networks in the Okinawa Trough (OT) off NE Taiwan than that in the continental margin offshore SW Taiwan. This observation could be linked to the presence of numerous volcanic outcrops in the shallow marine sediments is the OT area. By comparing the 1-D velocity shear-wave profile with the previous studies, we found that the low Vs area could be associated with a sedimentary layer filled with gas in the OT and the creeping or slumping area along the continental margin. Relatively high shear wave velocity structures near the Kao-Ping Canyon could be linked to the presence of mud diapir or submarine ridge. The whole area of Palm Ridge is characterized by a relatively lower shear wave velocity, which may be due to the entire area is covered by a thick distribution of submarine material. The Vs range between 0.38 to 0.62 km/s was estimated from our study also shows a good agreement with the velocity profile obtained based on the OBS seismic refraction experiment, suggesting that this method could be a more economical and effective way for the acquisition of the Vs parameters.en_US
DC.subjectambient seismic noiseen_US
DC.subjectoffshore Taiwanen_US
DC.subjectdispersion curvesen_US
DC.subjectshear wave velocityen_US
DC.titleShear-wave velocity of shallow marine sediments offshore Taiwan using ambient seismic noiseen_US
DC.publisherNational Central Universityen_US

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