OBS Data Mining and Earthquake Swarms Analysis Reveal the Complex Structure and Dynamics of the Blanco Fracture Zone

Oceanic transform faults (OTFs) accommodate deformation through a combination of quasi-periodic Mw > 6 earthquakes, aseismic creep, and microseismicity (Mw 0–5), including swarms, foreshocks, aftershocks, and background events. These seismic patterns result from the interplay of tectonic processes and fluid circulation, as well as magmatism within intra-transform spreading centers. Hydrothermal and magmatic fluid circulation at OTFs significantly alters the lithological and rheological properties of fault systems, shaping their seismic and aseismic behavior.

In this study, we analyze seismic data from a 55-station ocean-bottom seismometer (OBS) network (X9: 2012–2013) deployed around the Blanco Fracture Zone (BFZ), located offshore Oregon. Using advanced machine-learning pickers trained on OBS data, the PyOcto 1D associator, and high-resolution location algorithms, we significantly enhance the detection of low-amplitude / low-SNR microseismic events (Ml 0–2), augmenting the seismic catalog by approximately 20,000 events.

Our high-resolution seismic catalog reveals significant along-strike variations in earthquake depths, density and moment release, with main part of the earthquake density concentrated near the transform fault offsets (East Blanco Depression, Surveyor Depression, Cascadia Depression, and Gorda Depression), some of which may be spreading centers. Comparing our seismic catalog with recent 3D thermal and shear-wave velocity models highlights the along-strike variations in fault properties, providing new insights into the geological, lithological, and rheological complexities of the BFZ system. While most of the seismicity lies above the ~600°C isotherm located in the upper mantle, some deeper events might suggest deep seawater infiltration further in the mantle. Additionally, spatial correlations between slow shear-wave velocity anomalies and microseismic activity may indicate partial melting beneath intra-transform spreading centers and/or active hydrothermal fluid circulation through fault networks.

Spatio-temporal analyses of the seismic catalog identify over 1,000 earthquake clusters, including migrating seismic swarms with rates of up to 1 km/hr. These complex swarms, observed at the East Blanco Depression, West Blanco Depression, Cascadia Depression, and Blanco Ridge Transform, point to the involvement of hydrothermal and possibly magmatic fluid circulation, as well as slow-slip transients.

The western segment of the BFZ system is characterized by prominent swarms, including those associated with Mw 5 earthquakes in 2008, 2015, and 2021. Building on insights from the 1-year OBS dataset, we plan to refine and validate methods that use only permanent land-based stations along the coast. By developing this workflow, we aim to produce a multi-year seismic catalog that enhances the spatio-temporal resolution of BFZ seismicity. This future work will allow us to revisit the recurrent West Blanco swarms and explore their triggering mechanisms, which might involve slow-slip transients, fluid dynamics, and the interplay between tectonic and magmatic processes.