台灣西南部麓山帶位於弧陸碰撞造山帶最前緣，孕震潛勢高且構造複雜。2025年1月20日該區發生ML 6.4大埔地震序列，其破裂與深部隱伏斷層系統有關，傳統地震走時層析成像受近地表射線多近乎垂直入射、缺乏水平交會覆蓋之限制，於淺層解析度嚴重不足。本研究採用結合地震走時與布蓋重力異常之順序反演流程：以中央氣象署2012 – 2020年背景地震目錄進行雙差分層析成像（TomoDD），透過速度—密度轉換與三維重力反演交互迭代，建立兼具深部穩定性與淺層高解析度之三維基準模型；以EQTransformer進行波相拾取、GaMMA進行波相關聯，自動化建置高完整度之餘震目錄，反演震後速度模型以解析主震前後之速度變化。在淺層構造方面，重力資料有效補足水平射線覆蓋之不足，將原本均勻之區塊狀構造重新解析為高、低速相間構造，與活躍褶皺逆衝帶之地表構造分布相符，並抑制淺層 VP⁄VS 之數值假性異常；速度與密度兩組獨立資料達成良好之物理一致性。構造剖面顯示，木屐寮—六甲斷層對應清楚之高低速邊界，標示褶皺逆衝帶與海岸平原之構造界面；潮州斷層對應位置處於低速區塊內，未見明顯速度邊界，反映該斷層帶周圍變形較為彌散。在孕震構造方面，重新定位之餘震集中於8 – 18 km處，深度超出西部麓山帶典型薄皮構造滑脫面之作用範圍；沿A–A’與B–B’剖面均可識別出東傾與西傾兩組地震分布，構成具空間連續性之共軛幾何，餘震主要位於 VS 高低速梯度邊界處，反映破裂受剛—弱物性邊界控制之深部隱伏共軛破裂系統。在震後速度時變方面，差值剖面於餘震分布區內識別出局部 VP 不變或些許上升、 VS 些許下降、 VP⁄VS 上升之異常區塊，指示破裂帶內局部流體滲入之飽和裂隙區域，其空間分布之不均質特徵可能反映震後流體遷移受破裂帶滲透率異質性所控制，顯示同震破裂、孔隙壓變化與震後流體再分布之耦合作用。本研究證實地震—重力順序反演能顯著提升孕震區淺層解析度，並為大埔地震之深部共軛破裂幾何與震後裂隙／流體作用提供具物理一致性之觀測約束。;The Western Foothills of southwestern Taiwan， situated at the leading edge of the arc–continent collision orogen， constitute a structurally complex and highly seismogenic environment. On 20 January 2025， the ML 6.4 Dapu earthquake sequence ruptured a deep blind-fault system in this region. Conventional seismic travel-time tomography resolves shallow structures poorly because near-vertical ray incidence and the scarcity of crossing rays leave the shallow crust under-constrained. This study applies a sequential inversion workflow integrating seismic travel times with Bouguer gravity anomalies. Long-term background seismicity recorded by the Central Weather Administration (2012 – 2020) drives double-difference tomography (TomoDD)， which is iteratively coupled with velocity–density conversion and three-dimensional gravity inversion to build a baseline model that is stable at depth and sharp at shallow levels. A high-completeness aftershock catalog， constructed automatically through phase picking by EQTransformer and phase association by GaMMA， is then inverted to obtain a post-mainshock model for resolving temporal velocity changes. Regarding shallow velocity structure， the gravity data effectively compensate for the absent shallow crossing rays， resolving formerly uniform blocky patterns into along-strike alternating high- and low-velocity bands consistent with the surface structural distribution of the active fold-and-thrust belt， while suppressing spurious VP⁄VS anomalies; the physical consistency between the independently derived velocity and density models enhances the credibility of shallow structural interpretation. The velocity cross-section reveals a clear high-to-low velocity boundary at the Muchiliao–Liuchia Fault， marking the structural interface between the fold-and-thrust belt and the coastal plain， whereas the Chaochou Fault corresponds to a low-velocity zone without a distinct velocity boundary， consistent with diffuse deformation and its low current activity. Regarding the seismogenic structure， relocated aftershocks concentrate at depths of 8 – 18 km in the upper-to-middle crust， exceeding the depth range typical of thin-skin detachments in the Western Foothills; both cross-sections reveal east- and west-dipping linearly distributed seismicity forming a spatially continuous conjugate pattern， with aftershocks predominantly located along VS high-to-low velocity gradient boundaries， indicating a deep， basement-involved blind conjugate rupture system controlled by competent–weak material boundaries. Regarding post-seismic velocity changes， difference cross-sections within the aftershock zone reveal localized anomalies characterized by unchanged or slightly increasing VP， slightly decreasing VS， and positive ΔVP⁄VS ， interpreted as fluid-saturated fracture patches resulting from post-seismic fluid infiltration; the spatially heterogeneous distribution of these anomalies may reflect post-seismic fluid migration controlled by permeability heterogeneity within the rupture zone， indicating coupled co-seismic fracturing， pore pressure change， and post-seismic fluid redistribution. This study demonstrates that seismic–gravity sequential inversion markedly improves shallow resolution in seismogenic zones and provides physically consistent observational evidence for the deep conjugate rupture geometry and post-seismic crack/fluid processes of the Dapu earthquake.
