中臺灣海峽軟質沉積物變形構造分析

Abstract

臺灣海峽中部曾被報導好發流體移棲與土壤液化，但過去之研究較少針對沉積物如何受到流體影響而產生流體移棲及沉積物變形特徵，若能進一步辨識與探討中臺灣海峽的軟質沉積物變形，將有助於更好地瞭解沉積物的變形過程、發育環境、流體移棲機制及對周遭地層可能產生之影響。本研究利用臺灣海峽收集之底質剖面與反射震測資料，辨識不同尺度下的軟質沉積物變形。並透過湧浪效應修正與複反射消除等進階的底質與震測剖面資料處理技術，有效提高本研究所使用的高解析震測成像成果，更提升解釋工作效率與對沉積物特徵辨識之可信度。另藉由穿透深度較深之反射震測剖面，觀察較深部地層有無可能導致變形發生的地質因子。本研究藉由不同的震測相特徵於底質剖面中辨識出荷重型、流體型F1和流體型F2三種軟質沉積物變形型態。流體型F1與荷重型僅於底質剖面資料於淺部地層中辨識，而流體型F2則可於底質剖面及反射震測資料中同時出現，且從震測剖面中可進一步於地層中觀察到大量裂隙出現，故判斷流體型F2可由深部發育並貫穿上部地層，為大尺度之軟質沉積物變形。透過軟質沉積物變形之形貌特徵與分布，結合過往研究之觸發指標，本研究提出中臺灣海峽軟質沉積物變形應有不同觸發機制，推測為地震、快速沉積或波浪等因素。結合軟質沉積物變形尺度與震測相特徵分析，也推論出三種軟質沉積物變形與沉積物液化過程：(1)荷重型為淺層之受壓地層受觸發導致變形，孔隙水與沉積物向上逃逸至海床釋放壓力，沉積物重新排列形成下凹碗狀之軟質沉積物變形；(2)流體型F1為淺層超壓地層受觸發而造成變形，使孔隙水與沉積物逃逸至上覆沉積層，變形結束後於沉積層內部形成不規則狀之軟質沉積物變形；(3)流體型F2為地層深部受壓地層被觸發變形，而孔隙水壓貫穿上覆地層以釋放壓力，最終於地層內形成垂直管狀之軟質沉積物變形。

The central Taiwan Strait is a region prone to fluid migration and soil liquefaction, few studies have investigated these processes from the perspective of soft-sediment deformation structures (SSDS) yet, leaving their formation mechanisms unclear. This study identifies and interprets SSDS using sub-bottom profiler (SBP) and multi-channel seismic (MCS) data. MCS profiles have also been used to examine deeper geological factors potentially responsible for triggering deformation. Advanced processing techniques, including swell filter and demultiple, were applied to enhance data resolution and improve interpretation reliability. These methods significantly enhanced the data quality, boosting both interpretation efficiency and the reliability of identifying SSDS. Based on seismic facies and morphology, three SSDS types have been identified: Load, F1 (irregular-shaped), and F2 (vertical pipe-like) types. Load and F1 types were observed only in SBP profiles, while F2 type appeare in both SBP and MCS data. Significantly, MCS data revealed abundant faults and fractures within the strata, suggesting that the F2 type deforms from depth and penetrates the overlying strata, indicative of large-scale SSDS. This study proposes that earthquakes, rapid sedimentation, and wave activity are likely triggers of SSDS in this region. Correspondingly, three liquefaction processes are inferred: (1) Load type deformations when shallow over-pressured strata release pore fluid to the seafloor, resulting in bowl-shaped features; (2) F1 type deformations when pore fluids escape into overlying strata, producing irregular shapes; and (3) F2 type deformations when deep pore fluids migrate upward, forming vertical pipe-like structures.