Machine Learning-Based New Earthquake Catalog Illuminates On-Fault and Off-Fault Seismicity Patterns at the Discovery Transform Fault, East Pacific Rise

Oceanic transform faults connect spreading centers and are imprinted with previous tectonic events. However, their tectonic interactions are not well understood due to limited observations. The Discovery transform fault system at 4 degrees S, East Pacific Rise (EPR), represents a young transform system, offering a unique opportunity to study the interplay between faulting and other tectonic events at an early phases of an oceanic transform system. Discovery regularly hosts M5-6 characteristic earthquakes, and the seafloor north of Discovery includes a 35 km-long rift zone that records a complex history of rifting, faulting and volcanism, suggesting that the transform faults likely interact with regional tectonic activity. We apply a machine-learning enabled workflow to locate 21,391 earthquakes recorded during a 1-year ocean bottom seismometer experiment in 2008. Our results indicate that seismicity on the western Discovery fault is separated into seven patches with distinct aseismic and seismic slip modes. Additionally, we observe a patch of off-fault seismicity near where seafloor abyssal hills intersect the rift zone. This seismicity may have been caused by varying opening rates as spreading rate decreases from north to south in the rift zone. Our findings suggest that the Discovery system is still evolving, and that system equilibrium has not been reached between rifting and faulting. These results reflect the complex yet rarely observed interactions between fault slip, plate rotation, and rifting which are likely ubiquitous at oceanic transform systems. Plain Language Summary Oceanic transform faults are major plate boundaries connecting mid-ocean ridges. Despite their important role in plate tectonics, their interactions with adjacent mid-ocean ridges and surrounding oceanic plates are not well understood. The Discovery transform fault system at 4 degrees S, East Pacific Rise, is a young oceanic transform system formed approximately 1 My ago, offering a unique opportunity to study the interplay between faulting and other tectonic events at an early phase of an OTF. Discovery faults have quasi-periodical magnitude (M) 5-6 earthquakes. Using ocean bottom seismometer data recorded over 1 year, we find that seismicity of the western Discovery fault can be grouped into seven patches, indicating division of alternating slip modes that either releases tectonic strain by M > 5 earthquakes or creep steadily. North of the western Discovery fault, a similar to 10 km wide rift zone, abundant seamounts, and abyssal hills form an interactive tectonic complex. We observe a patch of off-fault seismicity coinciding with seafloor abyssal hills near their intersection with the rift zone. This off-fault seismicity indicates ongoing deformation within the oceanic plate and possible spatial variations in rifting rates. Our results suggest that the Discovery system is still evolving with rifting and faulting accommodating plate spreading simultaneously.