利用反射震測探討梅山斷層東側之特性

Abstract

梅山斷層位於台灣西南部嘉義地區，1906年發生的梅山地震過去普遍被認為由該斷層所引發。然而，近期研究指出，此地震可能是由兩個以上的斷層系統錯動所導致的，除原有的梅山斷層外，可能還包含一條東北-西南走向的逆衝斷層，即嘉義盲斷層。針對梅山斷層已有多項地球物理探勘研究，包括地電阻、重力與震測剖面調查。然而其東側的反射震測研究多集中於斷層周邊，且測線長度普遍較短，限制了理論可視深度，使得難以釐清梅山斷層與嘉義盲斷層間的構造關係。此外，具嘉義盲斷層錯動跡象的震測剖面多分布於梅山斷層以南，北側則缺乏明確證據，致使該斷層北段之構造特性及其與梅山斷層的關係仍不明確。 為釐清梅山斷層與嘉義盲斷層之間的交互關係，本研究於嘉義大林鎮崎頭地區，針對梅山斷層東側及前人推測之嘉義盲斷層位置，施測一條長約4.2公里的反射震測測線，以震盪震源車(MiniVib)作為震源，並使用240台接收器(smartsolo)收集訊號，接收器間隔為5公尺，炸點間隔為15公尺，資料重合數(Fold)為40。資料處理後可達3秒之時深，解析深度約為2.1秒（約為3公里）。除了原始震測剖面外，本研究還使用震測屬性分析協助層位與潛在構造之辨識，並整合鄰近測線資料、鑽井紀錄、地層描述與前人研究成果，進行地層對比與構造解釋，探討梅山斷層東側之構造特性，以及其與嘉義盲斷層之間的構造關係。 根據震測資料解釋結果，辨識出自二重溪層至南莊層間的六個主要地層界面，其反射訊號與前人震測剖面及鑽井資料對應良好。六雙層至二重溪層間的反射訊號具良好連續性，澐水溪層以下則因訊號品質降低與資料空白帶出現，導致反射訊號連續性明顯下降。測線東南側、對應梅山斷層分支區辨識出兩處花狀構造，顯示梅山斷層於淺部可能具有分支延伸，推估破裂帶寬度約1.5公里，深部則收斂為單一主破裂帶。雖本剖面未直接觀察到梅山斷層東側與嘉義盲斷層北段的交互截切現象，推測可能與嘉義盲斷層北段位於深部、向北尖滅、地層錯動量減小，以及剖面深部解析度有限有關。綜合南北側震測剖面結果，推論小梅背斜可能為斷層擴展褶皺（Fault-propagation fold）所致。

The 1906 Meishan earthquake was traditionally attributed to the Meishan Fault in the Chiayi region of southwestern Taiwan. However, recent studies suggest that this earthquake may have involved the simultaneous movement of multiple fault systems. In addition to the well-known Meishan Fault, a northeast–southwest trending thrust fault, identified as the Chiayi Blind Fault, may also have been involved. Multiple geophysical investigations have been conducted on the Meishan Fault, including electrical resistivity, gravity, and seismic reflection surveys. However, seismic reflection studies on the eastern segment have mostly focused near the fault zone with relatively short survey lines, limiting the theoretical imaging depth and thus hindering a clear understanding of the structural relationship between the Meishan Fault and the Chiayi Blind Fault. Furthermore, seismic profiles showing evidence of displacement along the Chiayi Blind Fault are predominantly distributed south of the Meishan Fault, while clear evidence in the northern section is lacking. As a result, the structural characteristics of the northern segment of the Chiayi Blind Fault and its relationship with the Meishan Fault remain unclear. To clarify the interaction between the Meishan Fault and the Chiayi Blind Fault, a 4.2-kilometer-long reflection seismic survey was conducted in the Qitou area of Dalin Township, Chiayi. The survey targeted the eastern segment of the Meishan Fault and the previously inferred location of the Chiayi Blind Fault. A MiniVib seismic vibrator was used as the source, and signals were recorded using 240 SmartSolo receivers with a spacing of 5 meters between receivers and 15 meters between shot points, resulting in a data fold of 40. After processing, the seismic data reached a maximum two-way travel time of 3 seconds, with an effective resolution depth of approximately 2.1 seconds (about 3 kilometers). In addition to the original seismic profiles, this study utilized seismic attribute analysis to aid in the identification of stratigraphic boundaries and potential structural features. The interpretation was further supported by integrating data from nearby seismic lines, borehole records, stratigraphic descriptions, and previous studies, enabling stratigraphic correlation and structural interpretation aimed at understanding the structural characteristics east of the Meishan Fault and its relationship with the Chiayi Blind Fault. The interpretation results identified six major formation boundaries from the Erhchungchi formation to the Nanchuang formation, showing good correlation with earlier seismic profiles and borehole data. The seismic reflectors from the Liushuang to Erhchungchi formations exhibit high continuity, whereas reflectors below the Yunshuichi formation display degraded signal quality and data gaps, resulting in decreased reflector continuity. Two flower structures were identified in the southeastern part of the profile, near the branching zone of the Meishan Fault, suggesting that the fault may exhibit shallow-level branching. The rupture zone is estimated to be up to 1.5 km wide near the surface and converges into a single main rupture at depth. Although no direct evidence of interaction or cross-cutting between the eastern segment of the Meishan Fault and the northern segment of the Chiayi Blind Fault was observed in this profile, it is inferred that this may be due to the greater depth of the Chiayi Blind Fault's northern segment, its northward tapering, reduced stratigraphic displacement, and the limited resolution of the seismic data at depth. Based on the integrated interpretation of seismic profiles to the north and south, the Hsiaomei anticline is inferred to be a fault-propagation fold.