Crustal stability in Nanpu sag: Insights from the temporal and spatial characteristics of historical earthquakes

The evaluation of regional crustal stability plays a crucial role in the planning and construction of major projects. Given that the Nanpu sag is the site for China's first offshore gas storage facility, its crustal stability naturally becomes a focal point of concern. Based on historical seismic data for the region, this study summarizes the spatiotemporal distribution of seismic activities in the Nanpu sag and its surrounding areas. It analyzes the rupture modes of the Tangshan–Hejian–Cixian fault zone and the Zhangjiakou–Penglai fault zone, and explores the crustal stability of the Nanpu sag. The results indicate that the Nanpu sag is a relatively stable "safe island" near active tectonic zones. Regionally, stress accumulated around the periphery of the Nanpu sag is mainly released through the activity of faults such as the Luanxian–Laoting and Baodi–Ninghe faults, with limited impact from seismic activities on the periphery affecting the interior of the Nanpu sag. Internally, a large-scale extensional deformation system has developed within the Nanpu sag. Given the current tectonic stress background, only a small magnitude of stress accumulation is required to trigger unstable sliding of pre-existing normal faults within the Nanpu sag. This suggests that stress cannot accumulate over long time scales, thus the interior of the Nanpu sag lacks conditions for major earthquakes to occur.