Seismotectonic characteristics and stress regimes of interplate and intraplate earthquakes in southwestern Saudi Arabia

Southwestern Saudi Arabia lies at the confluence of multiple rift systems, including the Red Sea, Gulf of Aden, and East African rift, within a structurally complex Arabian shield. This study investigates the seismotectonic and stress regimes of 14 interplate and intraplate earthquakes with moderate-magnitude ( M_L ≥ 3.5) along the southern Red Sea rift axis and inland regions. Broadband waveform data from the saudi national seismic network (SNSN) were processed for full-waveform moment tensor inversion to determine faulting mechanisms, seismic moments, focal depths, and moment magnitudes. Interplate events exhibit predominantly northwest-southeast (NW–SE) and northeast-southwest (NE–SW) trending normal faults with minor strike-slip components, shallow focal depths (3–19 km), and moment magnitudes M_W range from 4.2 to 5.2, reflecting extensional tectonics associated with Red Sea rifting. In contrast, intraplate earthquakes with M_W range from 3.5 to 4.4, display mixed normal–strike-slip mechanisms, consistent with the reactivation of inherited Precambrian NW–SE and ENE–WSW faults, with some events occurring in previously unrecognized seismic zones. Stress tensor inversion confirms a stable normal-faulting regime for interplate earthquakes (shape ratio R = 0.60, NE–SW extension) and a transtensional regime for intraplate earthquakes (R = 0.74, NNE–SSW extension), indicating oblique deformation inland. These results demonstrate that contemporary deformation in southwestern Saudi Arabia is governed by the interplay of active rifting at plate boundaries and pre-existing basement structures, producing a complex, mixed normal–strike-slip stress field. Findings provide critical constraints for regional seismotectonic modelling and seismic hazard assessment along the Red Sea margin.