Seismicity, Segmentation and Structure of the Blanco Transform Fault System in the Northeast Pacific

The Blanco transform fault system (BTFS) in the northwest off the coast of Oregon is highly segmented and one of the newly evolving transform faults. While for most transform systems, no high-resolution seismological data is available, the BTFS was instrumented with a dense network of 54 ocean-bottom-seismometer stations in 2012-2013.  We use one year of ocean-bottom-seismometer data from the Blanco Transform OBS Experiment (network code X9) to compare the seismicity with high-resolution multibeam bathymetry, aeromagnetic, and gravity datasets to study the seismotectonics of the BTFS. 

We determine P and S-phase arrivals using a new machine learning picker trained on OBS data to create a high-resolution local seismicity catalogue. We compare seismicity catalogues based on different picking algorithms and event associators, including an automated phase picker for OBS data (PICKBLUE) using the hydrophone and seismometer channels. 

In total, we locate ~9.000 local events, which reveal lateral deformation along the BTFS in very high detail, including smaller step-overs, transtensional structures, and focused seismicity along the fault trace and several ~15 km long aseismic segments along the BTFS. At segments where the BTFS is linear, most seismicity is very concentrated to the transform fault, suggesting that the deformation is within +/-7 km of the fault trace and, hence, very focused. Furthermore, we will present vp and vp/vs velocity models, which reveal the three-dimensional structure of the BTFS and compare the seismicity with seismic velocities.  

The local seismicity indicates substantial along-strike variations, indicating different slip modes in the eastern and western BTFS. Seismic slip vectors suggest that the eastern BTFS is a mature transform fault system accommodating the plate motion. At the same time, the western BTFS is immature as its re-organization is still ongoing.

The available datasets provide no evidence of either transform faults or fracture zones around the BTFS before 2 Ma, supporting that there were no pre-existing transform faults before the initiation of the BTFS. Therefore, we suggest that the BTFS developed from two broad transfer zones instead of pre-existing transform faults. 

Furthermore, we applied the deep learning phase picker, to analyse an extensive OBS dataset over three years, including a total of 225 OBS deployments (network codes: 7D, X9, and Z5) deployed offshore the south-central Cascadia subduction zone, south of the BTFS. As a result, ~7000 well-constrained local events were derived, providing new insights into offshore fault dynamics and the segmentation of the Gorda ridge  South of the BTFS.