Holocene Slip Behavior and Shallow Structure of the Milun Fault at the Collision–Subduction Transition, Eastern Taiwan

The Milun Fault forms the northernmost onshore segment of the Longitudinal Valley fault system, a plate‐boundary suture between the Eurasian and Philippine Sea plates in eastern Taiwan, and poses a significant seismic hazard to Hualien City. Despite destructive earthquakes in 1951 and 2018, the shallow structure, long‐term slip behavior, and paleoseismic history of the fault have remained poorly constrained due to limited direct exposure of its principal fault. New paleoseismic trench and borehole data provide direct constraints on the near‐surface geometry of the principal fault and its associated branch faults. Trench exposures document a steeply east‐dipping principal fault (∼80°) accompanied by several branch faults, defining a zone of distributed deformation. Stratigraphic restoration, growth strata, colluvial wedges, and liquefaction features indicate five late Holocene surface‐rupturing earthquakes, including the 1951 and 2018 events and three prehistoric earthquakes dated to 783–352, 2361–1302, and 3185–2303 cal yr BP. Borehole correlations indicate ∼18.6 m of vertical separation since ∼5.7 ka, corresponding to a long‐term vertical slip rate of ∼3.2–3.6 mm/yr. Stratigraphic relationships further indicate that the Milun Tableland emerged above sea level at ≈3.2 ka, implying cumulative uplift through repeated seismic events. Inter‐event times range from decades to more than a millennium, and overlapping age constraints preclude definition of a characteristic recurrence interval. These results show that deformation along the Milun Fault is accommodated by a steep principal fault together with multiple branch faults, and that earthquake recurrence in this transpressional plate‐boundary setting is temporally irregular.