Microseismicity delineates active plate boundaries and intraplate faults at Discovery transform fault system, East Pacific Rise

Oceanic transform faults play a fundamental role in plate motions at mid-ocean ridges, yet their slip partitioning mechanisms and formation processes are poorly understood. The Discovery transform fault system at 4°S on the East Pacific Rise consists of two fault segments, D1 and D2 from east to west, connected by a short intra-transform spreading center. Here we use microseismicity recorded by eight ocean bottom seismometers that were deployed along segment D2 in 2008 to examine the fault structure and regional deformation pattern of the Discovery system. Several fault patches are known to rupture in M5-6 earthquakes quasi-periodically with a recurrence interval of 5-6 years, while the remaining segments rarely host M>5 earthquakes. The Discovery system is surrounded by complex tectonic structures, with seafloor topographic relief suggesting episodes of rifting, faulted seamounts, and small-scale rotation, indicating multiple stages of tectonic activity during its formation and development. The lack of fracture zones extending beyond the two transform fault traces indicates that the Discovery system was formed later than its neighbor transform fault systems, Quebrada and Gofar. We detect and locate ~20,000 microearthquakes in one year that primarily occur along the D1 and D2 fault traces. The microearthquakes show strong along-strike variations in their spatiotemporal evolution, which implies along-strike segmentation in both fault structure and associated mechanical properties. We also find seismicity bursts occurring frequently near the ends of D1 and D2, indicating possible interactions between transform faults and nearby spreading centers. Intriguingly, there is a northeast-southwest trending band of seismicity ~10 km north of D2 that is oriented sub-parallel to a complex area of faulted and rotated abyssal hills. This off-fault seismicity likely represents on-going deformation between different tectonic subdomains that accommodates the plate spreading between the Quebrada and Discovery transform systems.