Seismicity and active tectonics of northern Borneo

Sabah, the easternmost state of Malaysia, is the most tectonically active region of Borneo despite being distant from  active plate boundaries. Global earthquake catalogues record recurrent earthquakes of Mw ≈ 5.0 at roughly five-year intervals, primarily concentrated along the northwestern and southeastern flanks of Sabah. Seismicity along the northwestern flank is particularly focused around Mount Kinabalu and the offshore Baram Delta, where normal faulting and half-graben basin development coexist with thrust faulting. In contrast, seismicity along the southeastern flank is dominated by thrust-faulting earthquakes at  depths up to 30-40 km. Numerous studies suggest that this intraplate seismicity reflects the reactivation of post-subduction structures inherited from the Proto–South China Sea subduction and subsequent Celebes Sea subduction beneath Sabah in the Neogene. Despite this activity, detailed seismicity studies remain sparse due to historically limited seismic station coverage. Recent expansion of the Malaysian National Seismic Network and the temporary Northern Borneo Seismic Network (nBOSS) between 2018-202  provide new opportunities to develop an enhanced earthquake catalogue with improved source characterisation. This study aims to produce a refined earthquake catalogue for Sabah, with particular focus on the Mount Kinabalu and Darvel Bay regions. We integrate machine-learning-based tools, including PhaseNet, together with in-house software (QuakeMigrate and MTfit), to automatically analyse spatial and temporal patterns of seismicity and focal mechanisms. The goal is to improve our understanding of the active tectonics of northern Borneo and assess the implications for regional seismic hazard in this post-subduction setting.