Geomorphological process and the Quaternary fault displacement of alluvial fan terraces in the Eonyang area, Ulsan

The alluvial fans that formed continuously at the bottom of the Yangsan fault valley have experienced dissection over time, transforming into an alluvial fan terraces. By evaluating the formative age and displacement of Quaternary alluvial topographic surfaces such as fan terraces, the cumulative slip rate of the land surface influenced by fault movement after landform formation can be calculated. This study identified the geomorphological processes of the fan terrace in the Eonyang area of Ulsan and calculated the displacement and cumulative slip rate due to fault movement. Based on OSL age dating from sedimentary layers at four different fan terrace, it was suggested that the thrid and fourth fan terrace surfaces correspond to approximately 115 ka and 155 ka, respectively. This period is estimated to be highly related to the increased in the intensity of the East Asian summer monsoon. At the three alluvial terraces outcrops intersected by the fault line, the observed displacements of the topographic surfaces were consistently identified as characteristics of dextral strike-slip reverse faults. And after the formation of the fan terraces, the averaged horizontal and vertical cumulative slip rate was 0.087 m/ka and 0.006 m/ka, respectively.