Characteristics and disaster implications of offshore active faults around Nan'ao Island, the northern South China Sea

Abstract. Frequent earthquakes in China's Nan'ao Sea, the fault structure and its activity pattern in this region have been the focus of attention. Using high-resolution seismic reflection data, this study uncovers a large graben offshore Nan'ao island. The northwest boundary of the graben is a seaward-dipping normal fault, which spreads upward in the form of fault-propagating folds and forms a gentle submarine slope. The southeast boundary of the graben is a landward-dipping normal fault, manifesting as a growth fault that cuts to the seabed. Analysis of activity shows the fault/fold activity rates on both sides of the graben are similar, suggesting they are part of a unified active tectonic system. Simulations of fault propagation folds on the northwest side of the graben show that after three phases of normal dip sliding (approximately 20 m per phase), a prolonged period of quiescence ensues between the sliding phases, this result supports the periodic and intermittent character of the fault activity and indicates the presence of seismic-scale strain. Based on these insights, the research explores the activity mode of fault association in the Nan'ao sea area, it proposes a seismogenic model that emphasizes the controlling and driving role of gravity in shallow deformation characteristics.