Intraplate deformation and mantle shear zones in the Gorda plate driven by the northward migration of the Mendocino triple junction

Since 28 Ma, the North American, Pacific, and Gorda plates have been coupled, with the Mendocino triple junction (MTJ) migrating northward and exerting significant influence on coastal California and the northern San Andreas Fault system. While deformation within the continental domain has been well-documented, the dynamics of the oceanic lithosphere remain poorly understood. The subducting Gorda plate, constituting the southern block of the Juan de Fuca plate, is known as a nonrigidly deforming zone bounded by the Gorda ridge to the west, the Cascadia deformation front to the east, and the Mendocino transform fault to the south. However, its northern boundary—delineating the limit of deformation—remains contentious due to insufficient offshore seismic evidence. Furthermore, earthquake locations within the Gorda plate, derived primarily from land-based seismic networks, are inherently biased, further constraining insights into deformation patterns and styles in the offshore region. Enhanced offshore seismic observations are essential to resolving these uncertainties and better understanding the plate's geodynamic behavior driven by the northward migration of the MTJ.

We utilized ocean-bottom seismometer (OBS) networks (network codes 7D, X9, Z5, OO) deployed as part of the Cascadia Initiative to investigate offshore micro-seismicity in Cascadia between July 2012 and October 2015. We firstly evaluated the performance of multiple deep-learning pickers, including EQTransformer, PhaseNet, and PickBlue. Among these, the OBS picker, PickBlue, demonstrated superior event detection performance when applied to OBS data compared to pickers trained on onshore datasets. Using PickBlue, we further derived 2,253,059 P-phases and 1,405,180 S-phases with confidence values exceeding 0.6, enabling us to locate 14,057 local earthquakes offshore Cascadia. The significant enhancement in the detection of offshore seismic events allowed us to reveal more detailed patterns of seismic activity, particularly in the Gorda plate and its surrounding boundaries, which had been largely absent in earlier studies. We observed a high density of intraplate micro-seismicity, underscoring the complex tectonic interactions within the Gorda plate. The spatial distribution of micro-events delineates the northern limit of deformation, which aligns precisely with pseudofault traces between 42°N and 42.5°N. These micro-earthquakes are well-focused and closely correspond with spreading fabrics and magnetic anomalies, providing seismic evidence for the reactivation of spreading-related faults responsible for much of the internal nonrigid deformation of the Gorda plate. Additionally, a spatial trend of increasingly deeper seismic events is evident with greater distance from the Gorda ridge and closer proximity to the Cascadia deformation front. This pattern reflects the combined influence of north-south compression from the Pacific plate and plate bending prior to subduction. Notably, clusters of deep micro-earthquakes reaching depths of up to 40 km are observed along the southern boundary, the Mendocino transform fault. These findings offer the first seismological evidence for the development of shear zones in the uppermost mantle of the Gorda plate, likely resulting from active asthenospheric flow influenced by the northward migration of the MTJ and advection due to the larger Pacific plate.