Seismicity, Repeating Earthquakes,and Tomographic Imaging of the Blanco Transform Fault System, Northeast Pacific

The Blanco transform fault system (BTFS) represents an evolving transform plate boundary in the Northeast Pacific Ocean. Its seismic behavior was captured with a dense network of 54 ocean-bottom seismometers (OBS) operated for one year. We created a high-resolution earthquake catalog based on different machine-learning onset pickers. The high-resolution seismicity catalog has 12,708 events outlining the current deformation and stress release. Seismicity indicates seismic and aseismic fault patches or segments, as well as complex along-strike and off-axis deformation, step-overs, and internal faulting within pull-apart basins. Along simple linear fault strands, earthquakes are localized within 2 km of the seafloor expression of the fault. By applying cross-correlation techniques, we identified bursts and repeaters along the BTFS. Most bursts have interevent times of less than five minutes. Repeaters are predominantly found in the west of the BTFS and along the Gorda Depression, and to a smaller extent beneath the eastern Blanco Ridge. Slip rates estimated from repeaters exceed the geological slip rate by approximately 4 times, suggesting small seismic patches that release their slip every ~4 years. Along the BTFS, fault coupling varies between fully locked and creeping. Local earthquake tomography shows elevated vp/vs values exceeding 2, suggesting significant serpentinization from seawater entering the transform faults, the oceanic crust, and the mantle. The study shows how modern machine learning pickers applied to OBS data yield essential insights into the physics of faulting along major plate boundary faults in time and space, including the partitioning of deformation between seismic and aseismic slip.