Segmentation and Seismicity of the Eastern Gofar Transform Fault System Revealed by 30-Month Ocean Bottom Seismometer Deployment

Oceanic transform faults offset spreading axes by tens to hundreds of kilometers and are among the most prominent tectonic features in deep ocean basins. The Gofar transform fault system (GTFS) is a major left-lateral ridge-crest discontinuity connecting segments of the fast-spreading East Pacific Rise. This highly segmented transform fault system is characterized by high-relief flanks, J-shaped structures at ridge-transform intersections, and deep troughs connecting three fault segments (G1, G2, and G3, from east to west). Over the past two decades, the western G3 segment has been extensively studied through multidisciplinary approaches including near-field observations and numerical modeling, revealing along-strike variations in seismicity patterns, slip behavior, and potential governing factors. However, the segmentation of the entire GTFS and its relationship with intra-transform spreading centers and/or pull-apart basins remain poorly understood, as seismic behavior of the eastern G1 and G2 segments has not been sufficiently well constrained by near-field observations.  

Between November 2019 and February 2022, 30 ocean bottom seismometers (OBS) were deployed to monitor seismic activity along the eastern GTFS (G1 and G2 segments). We first evaluated the performance of multiple deep-learning phase pickers on this OBS dataset, including EQTransformer, PhaseNet, and PickBlue. PickBlue, specifically trained for OBS data, demonstrated superior event detection performance compared to pickers trained on onshore datasets. We then applied the non-linear oct-tree grid-search algorithm (NonLinLoc) with source-specific station terms (SSST) to obtain precise absolute event locations. Our results reveal high seismicity density along the G1 and G2 transform segments, as well as distributed deformation within the deep trough connecting these segments, showing features resembling continental pull-apart basins. Notably, the OBS network captured a magnitude 6 earthquake in the study area, providing unique insights into fault slip behavior before and after the mainshock at oceanic transform faults.