Transform Faulting in the Northern Red Sea Revealed by Ocean Bottom Seismometers Deployed in the Zabargad Fracture Zone

Abstract

The Zabargad Fracture Zone (ZFZ), the largest rift-axis offset in the Red Sea, extends from 23.5°N to 26°N. It is covered by thick sedimentary layers that have hindered the ability of geophysical data sets to image the geometry of the normal faults and any potential transform faults. Passive seismology observations in the region have been limited to onshore recordings, leading to high uncertainty in earthquake locations and hampering detailed seismicity analyses. To address this limitation, we deployed a network of ocean bottom seismometers (OBSs) and land-based stations for a duration of 12 months. From these data, we built the first high-resolution earthquake catalog for this region of the Red Sea by applying a deep-learning-based algorithm to automatically detect earthquakes and pick P- and S-phases. To obtain reliable earthquake locations, we derived a 1-D seismic velocity model of the ZFZ. We established a calibration between $M_L$ measured on land and on OBSs to overcome magnitude overestimation using the OBS data alone. Additionally, we computed focal mechanisms of selected events using waveform polarities and amplitude ratios. The hypocenter distribution reveals two major seismicity clusters, the North and South clusters. The North cluster highlights NW–SE trending normal faults reflecting the extensional tectonics of the Red Sea. In the South cluster, we discover a NE–SW transform fault, possibly in its early development stage and NW–SE normal faults. The depth of seismicity reveals variations in the brittle-ductile transition zone depth, with shallow depth in the north indicating higher shallow lithospheric temperature.