估計萬丹泥火山噴發體積與時空特性

Abstract

泥火山是一種特殊的地形現象，由地底含水泥質往上流動至地表噴發，並常伴隨著甲烷產生。泥火山的作用可視為流體與氣體物質從岩石圈流向地表、水圈與氣圈的重要途徑，研究泥火山可以幫助我們了解地球內部的地殼運動及流體循環，並與噴發災害、地質構造、甲烷氣體的收支評估、能源開採與相關活動、地形景觀與遊憩活動等相關議題有關。而這些研究都需要仰賴噴發量、噴發週期等基礎資訊來進行近一步的分析。臺灣西南部海域與陸上區域存在許多泥火山，本研究便以西南部陸上噴發規模最大、且具長年記載的萬丹泥火山為研究泥火山的代表對象，嘗試估計與觀測其噴發的時空分布特性。 由於萬丹泥火山具有不定時、不定點、不定量的噴發特性，使得其噴發的機制至今仍未釐清。為了了解萬丹泥火山噴發的特性，可藉由估計噴發體積與時空分布，建立泥火山噴發流體的基本資料，有助於日後分析噴發活動及影響噴發的因子。本研究主要透過兩個研究途徑；一為蒐集與彙整歷史文獻紀錄，以建立長時間的噴發時空分布；二為透過現地測量，估計其精確的噴發量。歷史文獻的分析顯示，萬丹火山歷史上原好發於鯉魚山，日後漸往其東方小丘移動，近年則多發生於一個西北－東南走向的條帶，平均一年約發生一至兩次。現地測量方面，本研究針對此種難以預測確切觀測位置的條件，利用無人飛機的方便性，設計一個相對應的攝影測量流程，以建立數值地表模型與正射影像來估計噴發體積，並發展一個專屬的平差模型。以2018年5月15日的噴發事件為例，估計的噴發量為13,363.17417m3。本研究最後提出萬丹泥火山噴發規模評估，期望能為後續泥火山應用建立分析基礎。

Mud volcanoes are special geomorphic features, which erupt mud materials on the surface with abundant water and methane contents. The process of mud volcanoes can be viewed as one important approach to bring fluid and gas from the lithosphere to the surface, hydrosphere, and atmosphere. Studying mud volcanoes can help us understand crustal movements and fluid circulation at depth, which are associated with topics of eruptional hazards, structural geometry, methane budget, energy exploration, landscape preservation and recreational activities. There are many mud volcanoes on land and offshore southwestern Taiwan, and this study aims to study the Wandan mud volcano, which has the largest eruptional magnitude in the region and long historic records, by estimating and characterizing the spatiotemporal distribution of its eruption. Because the Wandan mud volcano erupt at varied locations, times, and volumes, the mechanism of its eruption remains unclear. In order to understand the characteristics of its eruption, it requires fundamental information such as eruption volumes and spatiotemporal distributions. This study has two major research approaches: one is compiling historic records to establish its long-term spatiotemporal pattern; the other is estimate its eruption volume by in-situ surveys. The analysis of historic records show that the Wandan volcano occurred at the Liyushan originally and migrates eastwards with time. In recent years, it occurs along a northwest-southeast zone with an average frequency of 1-2 times per year. In terms of in-situ surveys, this study designs a special photogrammetric procedure specifically for such a condition, which is difficult to predict exact location, by utilizing the mobility of UAVs. The study then generates DSMs and orthoimages for precisely estimating eruption volume and develop an adjustment model for the procedure. For the eruption on May 15, 2018, the method estimates the volume of mud is 13,363.17417 m3. This study also estimates eruption magnitude for documented events, providing basis for future studies.