| 摘要: | 台灣西南海域有許多獨立盆地，為研究事件層理想場域。本研究分析從獨立盆地取得之三站沉積物岩芯，包含MD18-3547（35.27公尺長）、MD18-3548（20.08公尺長）和 MD18-3552（45.98公尺長），以研究極端事件之沉積物。對這些岩芯進行了高解析度粒徑分析（1公分採樣間隔）、加速器質譜 (AMS) 放射性碳十四定年 (14C) 和電腦斷層(X- CT)掃描，詳細描繪岩相並計算事件層的沉積年代。
本研究確定了四種不同岩相：半遠洋沉積物、粉砂質濁流岩－均質岩、濁流岩和薄層粉砂質沉積層。這是在台灣西南海域首次發現均質岩，與先前研究在其他地區之均質岩有顯著差異。我們將這新岩相命名為：粉砂質濁流岩－均質岩。所有粉砂質濁流岩－均質岩之下方有一層薄層（通常厚度小於10厘米）顆粒向上變粗再向上變細的層序（粉砂質濁流岩），頂部是一層厚層、無構造泥層，且無生物擾動（均質岩）。
本研究從浮游性有孔蟲中獲取47個AMS放射性碳（14C）定年。事件層沉積年代為根據半遠洋沉積速率進行內插而得，最近一次事件可追溯自～70 cal BP，最古老事件可追溯自～23.5 cal kyr BP。所有放射性碳十四定年結果以 MARINE20 曲線進行校準，對於年齡小於 5,500 年之沉積物，應用局部儲集層校正ΔR = -154 ± 59。均質岩平均厚度約為100公分，最厚約為225公分，最薄約為40公分。濁流岩的平均厚度約為 4－5公分，最厚記錄約為 16 公分，最薄記錄約為 1 公分。
大地震被認為是均質岩沉積之主要驅動因素，本研究區域在 2006 年恆春地震 (Mw 7.0) 後，並沒有發現均質岩沉積現象。在台灣西南海域，大型破壞性地震可能與分歧斷層與板塊交界斷層錯動層有關。
在分析之岩芯共識別 71 個事件層，包括 21 個粉砂質濁流岩－均質岩、24 個濁流岩和 26 個薄層粉砂質沉積層。為了評估重現期，事件層被分為兩類：第1類，包括所有事件層；第2類，僅考慮粉砂質濁流岩－均質岩來估計較大規模地震的重現期。這兩類別都顯示三次沉積間歇期，期持續時間約為1,100至1,400年。
在第1類中，識別出三個主要的重現期與兩個離群值。排除離群值後，粉砂質濁流岩－均質岩的重現區間約為180至1,400年，而濁流岩與薄層粉砂質沉積層的重現區間則約為110至500年。在三支岩芯中，所有事件層平均重現期約為534年。
在第2類中，識別出三個重現期群組：第1組平均重現期約為1,000至1,300年，第2組約為180至1,300年，第3組約為900至1,400年。粉砂質濁流岩－均質岩之間的時間間隔主要被濁流岩及薄層粉砂質沉積層所填充，這些層位可能對應較小規模地震事件。在三支岩芯中，粉砂質濁流岩－均質岩的平均重現期約為864年。
透過針對所有事件層（包含粉砂質濁流岩－均質岩、濁流岩與薄層粉砂質沉積層）以及專門針對粉砂質濁流岩－均質岩（代表較大規模地震）分別進行重現期分析，本研究提供對地震重現模式更為全面之理解。此一方法有助於提升未來地震災害潛勢之評估能力。;Offshore southwestern Taiwan, with its numerous perched basins, provides an ideal setting for the study of event beds. Three giant piston cores—MD18-3547 (35.27 m), MD18-3548 (20.08 m), and MD18-3552 (45.98 m)—were retrieved from perched basins in this region to investigate extreme event deposits. High-resolution grain-size analysis (1 cm sampling interval), accelerator mass spectrometry (AMS) radiocarbon dating (14C), and X-CT (Computed Tomography) scanning were conducted on these cores, allowing for detailed lithofacies delineation and calculation of emplacement ages for the event beds.
Four distinct lithofacies were identified: hemipelagic sediments, silty turbidite-homogenites, turbidites, and thin silty layers. Homogenites were documented for the first time in offshore southwestern Taiwan, exhibiting significant differences from previously reported homogenites in other regions. These distinctions led to the introduction of a new facies designation: the silty turbidite-homogenite unit. All silty turbidite-homogenite units are underlain by a thin (typically <10 cm thick) coarsening-upward followed by fining-upward sequence (silty turbidite unit) and capped by a thick, structureless mud layer largely devoid of bioturbation (homogenite unit).
A total of forty-seven AMS radiocarbon (14C) dates were obtained from planktonic foraminifera. The depositional ages of the event beds were interpolated based on hemipelagic sedimentation rates, with the most recent event dated to ~70 cal BP and the oldest to ~23.5 cal kyr BP. All radiocarbon dates were calibrated using the MARINE20 curve, applying a local reservoir correction of ΔR = -154 ± 59 for sediments younger than 5,500 years. The average thickness of homogenite units is ~100 cm, with the thickest recorded at ~225 cm and the thinnest at ~40 cm. Conversely, turbidites exhibit an average thickness of approximately 4-5 cm, with the thickest recorded at ~16 cm and the thinnest at ~1 cm.
Large earthquakes are considered the primary drivers of homogenite deposition, as evidenced by the absence of homogenites following the 2006 Hengchun doublet earthquake (Mw 7.0) at our study site. In offshore southwestern Taiwan, large destructive earthquakes are likely associated with either splay faults or out-of-sequence thrust faults.
A total of seventy-one event beds were identified within the analyzed cores, comprising twenty-one silty turbidite-homogenites, twenty-four turbidites, and twenty-six thin silty layers. To assess recurrence intervals, the event beds were classified into two categories: Category 1, which includes all event beds, and Category 2, which considers only silty turbidite-homogenites to estimate recurrence intervals of larger earthquakes. Both categories exhibit three hiatuses, with durations ranging from approximately 1,100 to 1,400 years.
Within Category 1, three major recurrence periods and two outliers were identified. Excluding the outliers, the recurrence interval for silty turbidite-homogenites ranges from ~180 to 1,400 years, while turbidites and thin silty layers have recurrence intervals of ~110 to 500 years. Across all three cores, the average recurrence interval for all event beds is approximately 534 years.
For Category 2, three recurrence clusters were identified: Cluster 1 with an average interval of ~1,000–1,300 years, Cluster 2 with ~180–1,300 years, and Cluster 3 with ~900–1,400 years. The intervals between successive silty turbidite-homogenites are predominantly occupied by turbidites and thin silty layers, which likely correspond to smaller-magnitude events. Across all three cores, the average recurrence intervals for silty turbidite-homogenites within the cluster is approximately 864 years.
By delineating recurrence intervals for all event beds (silty turbidite-homogenites, turbidites, and thin silty layers) and separately for silty turbidite-homogenites (larger earthquakes), this study provides a more comprehensive understanding of earthquake recurrence patterns. Such an approach may facilitate improved assessments of future seismic hazard potential. |
