Seismicity in the Zabargad Fracture Zone, Northern Red Sea and its tectonic implications: insights from an Ocean Bottom Seismometers Network

The Red Sea is one of the youngest ocean basins on Earth and is classified as an ultra-slow spreading ridge, with spreading rates decreasing from 15 mm/year in the Southern Red Sea to 7 mm/year in the Northern Red Sea. The Zabargad Fracture Zone (ZFZ), the largest rift-axis offset (~100 km) in the Red Sea (23.5^oN to 26^oN), separates the Central and the Northern Red Sea. The proximity of the seismically active ZFZ to coastal cities and infrastructure in the region has implications for the regional seismic hazard. However, thick salt and sedimentary covers in the ZFZ obscure the exact geometry of the oceanic spreading axes, and any potential transform faults or non-transform offsets, resulting in ambiguous interpretations. Seismological studies to date have relied on onshore recordings, yielding limited earthquake location accuracy that has impeded detailed analysis.

We deployed the first-ever broadband ocean-bottom seismometer network in the Red Sea, which was augmented with land-based stations, for a period of 12 months to improve the seismic data coverage in the ZFZ. The deployment resulted in a recovery rate of over 90% for the continuous seismic recordings. Using this new dataset, we applied a deep-learning-based algorithm for automatic earthquake detection and phase picking. The results were manually verified and refined, enabling the development of a high-resolution earthquake catalog. These processing steps yielded over 3,900 local earthquakes, with magnitude ranging from M_L -0.4 to M_L 2.5.  We further optimized a 1-D seismic velocity model for the ZFZ and improved earthquake locations using a double-difference relocation algorithm. Focal mechanisms for selected events were determined using polarity and amplitude ratios.

Our findings reveal two major seismicity clusters in the northern part, near the Mabahiss Deep, a deep with exposed oceanic crust, and in the southern part, around the ZFZ. The hypocenter distribution is consistent with NNW-SSE trending normal faults parallel to the ridge axis, indicating ridge segmentations and at least one ~25 km long NE-SW transform fault with strike-slip mechanisms. Variations in seismicity depth highlight changes in the brittle-ductile transition zone: shallower near Mabahiss Mons, an axial Mid-Oceanic Ridge Basalt volcano, reflecting elevated temperatures, and deeper further south, suggesting lower temperatures due to fluid circulation. These results provide new insights into the ZFZ's tectonic structure and seismic activity, improving our understanding of oceanic spreading dynamics in the northern Red Sea and the associated earthquake hazard.