02/03/2017
專題討論
時間:106年3月3日(五) 下午兩點
地點:理化大樓 格致廳小講堂
主持人:陳燕華 老師
主講人:(1)蔡敏嘉同學 (2)曾柏鈞同學
演講題目:
(1)全新世中期以來台灣地區高山土壤剖面及其環境變遷評估
(2)Coseismic and Postseismic Velocity Changes caused by the 2016 Mw 6.5 Meinong, Taiwan Earthquake using Ambient Seismic Noise
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摘要(1):
環境變遷議題近年受到高度關注,而高山土壤未受到人為活動干擾, 為研究區域尺度環境變遷的重要工具,土壤的化育受到時間、母岩、地形、氣候和生物等因素的影響,是為各種環境因子下的產物,長時間尺度下,氣候則是成土過程最有影響力的因素。氣候系統和陸源風化作用構成一個相互影響的回饋機制。陸源風化作用與二氧化碳量的連動關係,將擾動氣候狀態,而氣候狀態變動也將影響陸源風化的程度。與大氣相比,有更多的碳儲存在土壤中,作為碳循環的重要角色,並且可能因氣候變遷導致改變,對於氣候變遷影響碳儲存的變化意見分歧。在本研究中採集台灣中部高山全新世中期以來四個土壤剖面,分別為合歡山兩個土壤剖面、八通關一個土壤剖面、南華天池一個土壤剖面,依照美國農部土壤分類法四者皆為典型腐植冷凍弱育土(Typic Humicryepts),特別的是台灣高山土壤剖面普遍有明顯碳層存在,過去研究中對於土壤來源及風化反應了解甚少,本研究預計使用土壤剖面的主要元素、微量元素、硼同位素並與過去岩性、鍶同位素研究綜合比較,試圖了解此區域的風化來源及水岩反應的過程。鍶同位素為良好地質示蹤計,與主要元素和硼同位素搭配能夠有效辨別來自不同母岩的風化來源,硼元素易溶於流體、易揮發、對pH值反應靈敏、維管束植物生長必須,其元素穩定核種相對質量差異大、自然界固相/液相分化明顯等特色,對於水岩反應、化學風化的良好指標性,為指示出水岩反應與環境變遷的良好工具。
本研究參考Pi (2014)的酸溶解萃取方法,有效降低化學消化過程對硼的潛在污染以及氣化損失,並以感應耦合電漿原子發射光譜儀(ICP-OES)對樣品進行濃度量測,初步數據顯示土壤剖面的硼濃度範圍位於53.4ug/g~83.2ug/g與總體密度呈現正相關,與土壤水、有機碳、無定形鐵鋁呈現負相關,應受控於母岩成份、土壤質地與粒徑、有機質、土壤酸鹼度,但詳細控制因子仍需近一步驗證。未來將使用目前已發展成熟的微昇華純化技術提取硼同位素,萃取硼同位素進一步分析,再以多接收器感應耦合電漿質譜儀(MC-ICP-MS, Thermo-Fisher, Neptune)進行量測,探討硼同位素在各分層間的變化,分析其中硼同位素及主要元素討論風化反應的能力,希望釐清土壤中主要元素、微量元素、硼同位素的垂直變化與風化程度強弱的關係,藉以剖析長時間尺度的環境變遷。
摘要(2):
The 6 February 2016 MW 6.5 Meinong earthquake with a focal depth of 14.6 km produced widespread strong shaking in the 30-km-away Tainan city and caused about 10 buildings collapsed and 117 death. We collected seismic waveforms from 11 broadband stations within 40 km epicentral distances and reconstruct the Green’s functions from cross-correlation function of ambient seismic noise between two stations. We first analyzed seismic data for six different frequency ranges from 0.01 to 2 Hz, which yielded time series for different station pairs from January 2014 to August 2016. Then we used an exponential model to fit the time series of velocity variation consisting of a coseismic velocity drop followed by seasonal changes and postseismic recovery. We found coseismic velocity drops of about 0.20% mostly in 0.5 to 1 Hz at the Hsinhua fault area and the region 20 km SW of the epicenter, however postseismic velocity variation differs between these two regions. The time series of velocity change presented a non-recovery trend in the Hsinhua fault area, however the SW region is indicated by a recovering trend three months after the Meinong earthquake. For the surface wave tomography results in southwestern Taiwan, the regional geological structures are recognizable in the estimated phase-velocity dispersion maps. The dispersion map in the 7.0s Rayleigh wave displays low velocity in the alluvial plain, but indicates high velocity in the north of the Hsinhua fault. The anisotropy direction changes from SSW in the south to SW in the north, which followed the strike of regional geological structure. During the three months of the postseismic period, based on the GPS observations in the Hsinhua fault area, the block south of the fault continuously moved 15 - 20 mm along the southwest direction while the north of the fault remained stationary. The Hsinhua fault is located near the boundary between the Tainan basin and the muddy continental shelf, and where the block south of the fault is on the continental slope. We suggest that the coseismic velocity drop was caused by the earthquake shaking in the soft deposited materials with increase of porosity. The shorter recovery time and thus velocity increase in the block south of the Hsinhua fault was resulted from the afterslip of the Meinong earthquake, which resulted in the stress increases and the closure of the micro-fracture. On the contrary, the Hsinhua fault acts as a barrier blocked the afterslip south of the fault.