Morphological analysis of volcanic cones and its implication to Quaternary tectonics of the Jeju Island (South Korea)

The morphological characteristics and distribution of volcanic cones of Jeju Island in Korea could be controlled by several factors, including stress field in the substrate and subsurface fractures. Controlling fractures, however, might not be visible on the outcrop in volcanically active areas like Jeju Island, because of the layer of volcanic rocks covering them. Furthermore, inferring the paleostress is much more difficult. Hence, it is essential to identify the concealed feeding fractures through an indirect approach based on a morphometric analysis. Thus, Jeju Island is one of the best places to study this issue. Measurements of ellipticity, maximum crater diameter, breaching azimuth, coeval cone alignment, and the line connecting depressed points on 152 Quaternary volcanic craters, were taken to identify magma-feeding fractures. Jeju Island in SW Korea comprises Quaternary volcanic rocks overlying the Tertiary similar to Quaternary sediments of the Seogwipo Formation, Tertiary unconsolidated sediments (U Formation), and Cretaceous granite. This study identified two dominant types of Pre-Pleistocene magma-feeding fractures including E-W and NE/ENE-trending fractures consistent with previously identified Pre-Neogene E-W and Miocene NE-trending fractures around Jeju Island. These fractures might be associated with the rotation of maximum horizontal stress direction (sigma(Hmax)). Identifying subsurface structures is critical for seismic hazard assessment because their reactivation could result in destructive earthquakes.