X光螢光掃描結合多變量統計之化學地層對比在中部日本海溝之古地震學應用

Abstract

諸如2011年東日本大地震等巨型隱沒帶地震對人類社會具有深遠的影響，這類地震通常具有較長的再現週期，短時間尺度的儀器及歷史記錄常無法一窺其再現模型之全貌。為了建立可信的長時間古地震記錄，國際海洋發現計畫 (IODP)第386航次針對日本海溝沿線之11個盆地採集了超過800公尺的沉積物岩芯，其中的事件層記錄對於重建該區古地震的時空分布至關重要。因此，本研究利用X光螢光掃描結合多變量統計之主成分和群集分析，在中部日本海溝建立了精確且詳盡的化學地層對比。其中，在同個盆地的岩芯M0083D和M0089D共有八個事件層可被對比，這些事件層可被分為兩類，分別為擁有較高的鈣和鍶且其變化趨勢為向上漸少的事件層，以及擁有較高的矽、銣、鉀且無向上減少趨勢的事件層。這兩類不同的化學特徵指示著事件層成分和沉積過程的差異。而在岩芯M0089D與跨盆地岩芯M0090D的化學地層對比結果顯示，儘管兩根岩芯的八個事件層可被對比，但其元素趨勢和內部結構有所差異，這代表事件層的來源在空間上相似，但其侵蝕和搬運的過程有所不同。此外，不同時間沉積的事件層擁有不同的化學特徵，這不僅暗示了其時間上的來源差異，且進一步指示觸發這些濁流的機制可能是表層沉積物再搬運，而非海底山崩。因此，透過化學地層對比可以識別來自同一事件的沉積物，這證明了化學地層對比在確定事件地層來源上的實用性。在此基礎之上，岩芯中事件層的厚度與空間分布可用以揭示濁流的流動路徑，並有助於確定古地震的可能震央。例如，有高含量矽、銣、鉀的事件層向南變薄，暗示濁流可能源於北方；而有較高含量鈣和鍶的事件層在南方顯著增厚，表明震央可能位於南方。本研究採用X光螢光掃描和多變量統計方法進行精確的化學地層對比，不僅揭示了中部日本海溝事件層的沉積過程和來源，且深化了我們對該地區古地震時空分佈的理解。此外，本研究也為全球其他類似地質環境的古地震學研究提供了重要參考。

Megathrust earthquakes in subduction zones, such as the AD 2011 Mw 9.1 Tohoku-oki earthquake, have a significant impact on human society. Such earthquakes usually occur cyclically with long recurrence intervals, which makes it challenging to reconstruct recurrence models by short instrumental and historical records. Holding the mission to extend the long-term giant paleo-earthquake record over wide spatial extents, the International Ocean Discovery Program (IODP) Expedition 386, Japan Trench Paleoseismology, retrieved over 800 meters of sediment cores containing event deposits from 11 trench-fill basins along the Japan Trench. To establish reliable paleo-earthquake records, the spatiotemporal distributions of event deposits through precise correlation are pivotal. This study examines chemostratigraphic correlations in the central Japan Trench, focusing on cores M0083D and M0089D in the northern basin and M0090D in the southern basin. Employing X-ray Fluorescence Core Scanning (XRF-CS) along with Principal Component Analysis (PCA) and Cluster Analysis (CA), efficient and high-resolution chemostratigraphic correlations were achieved. Within the same basin, eight thick event deposits in M0083D are chemically correlated with those in M0089D. These event deposits can be characterized by higher Ca and Sr with decreasing trends upward, or higher Si, Rb, and K without decreasing trends upward, indicating different compositions and depositional processes of the turbidity current. Across basins, eight event deposits in M0090D showed the same cluster sequence but different elemental trends and internal structures compared to their correlated ones in M0089D, suggesting that while the source of the deposits is spatially similar, the erosion and transport processes may be different. On the other hand, the distinct chemical characteristics of event deposits from different times suggest temporal source differences along stratigraphic sequences. These temporal differences may indicate surficial sediment remobilization rather than landslides as the primary triggering mechanism of turbidity currents. This concept may further confirm the reliability of chemostratigraphic correlation for event-stratigraphic correlation especially in the context of paleoseismology. The relationship between event deposit thickness and spatial distribution across the cores can be further used to indicate the paths of turbidity currents and, thus, potential rupture areaof paleo-earthquakes. Southward thinning of higher Si, Rb, and K event deposits suggests a northern source, while thicker Ca and Sr deposits in the southern core imply southern rupture area. This study therefore establishes a robust chemostratigraphic correlation that enhances our understanding of the spatiotemporal distribution of paleo-earthquakes in the central Japan Trench and provides valuable insights for paleoseismology research in similar geological settings globally.