Exploring Structural Activity Through Gravel Rock Fracture Characteristics: A Case Study of the Milun Fault in Hualien, Taiwan

Hualien City in Taiwan is situated in the northernmost segment of the Longitudinal Valley, transitioning from the subduction of the Philippine Sea Plate to a collision environment. It is one of the most seismically active regions in Taiwan. The Milun Fault, a significant active fault, traverses through the urban area of Hualien, causing an uplift of the Milun Terrace and deformation and fracture of Milun gravel rocks in the hanging wall. The Milun Fault was the seismogenic fault for the 1951 Hualien earthquake (ML 7.3) and experienced approximately 70 cm of horizontal displacement triggered by the 2018 earthquake. The proximity of the Milun Fault to several secondary fault systems indicates complex structural activity.

This study focuses on observing the gravel rock layers beneath the Milun Tableland along the northern coast, utilizing a well-cemented beachrock layer as a key bed to assess variations in uplift across different areas.  Based on the investigations, the gravel rock layers beneath the Milun Tableland can be broadly divided into three zones:(a) Southeast Stable Zone: Characterized by a low beachrock layer height of approximately 3 meters. Minimal evidence of fault activity is observed in this area. (b) Damaged Zone (Middle): Marked by a beachrock height reaching up to 4.5 meters. Multiple fault systems are developed within the gravel rock layers, leading to fragmentation and damage. (c) Northwest Stable Zone: With a beachrock height of less than 1 meter, this area shows observable fractures in the underlying gravel rock but lacks clear structural activity. Measurements of slickenside and identification of fracture axes in the gravel rocks provide the maximum stress direction for each zone, indicating stress orientations ranging approximately between 160-170. The inferred trend of the Principal Displacement Zone (PDZ) aligns with a strike of 010, consistent with the surface rupture observed during the 2018 Hualien earthquake.

The field data from this study can offer additional information about the Milun Fault system. By incorporating the analysis results of surface rupture caused by the earthquake in 2018, it can further confirm the flower structural characteristics of the Milun Fault system. Additionally, it allows for observing other motion features associated with lateral fault movements.