Subduction intraslab-interface fault interactions in the 2022 Mw 6.4 Ferndale, California, earthquake sequence.

The Mendocino triple junction-the intersection of the Pacific, North American, and Gorda plates-activates a collection of disparate faults that reconcile Cascadia subduction with San Andreas transform motion. The 20 December 2022 Mw 6.4 Ferndale, California, earthquake occurred within this complex zone as strike-slip faulting within the subducting Gorda slab. Here, we analyze the seismic and geodetic signatures of the mainshock and aftershock sequence to illuminate its role within complex tectonic surroundings. We find aftershocks on varied fault structures within the uppermost Gorda slab, yet seismicity on the subduction interface itself was notably absent. Nevertheless, we identify small but coherent postseismic deformation that is well modeled by aseismic slip on this interface, likely triggered by stresses generated at the updip limit of coseismic rupture. This sequence demonstrates the potential for interactions between intra-slab earthquakes and slip on the subduction megathrust, highlighting the need to consider this and other subduction zones as coupled systems of interacting faults.

2019 M7.1 Ridgecrest earthquake slip distribution controlled by fault geometry inherited from Independence dike swarm.

Faults often form through reactivation of pre-existing structures, developing geometries and mechanical properties specific to the system's geologic inheritance. Competition between fault geometry and other factors (e.g., lithology) to control slip at Earth's surface is an open question that is central to our knowledge of fault processes and seismic hazards. Here we use remote sensing data and field observations to investigate the origin of the 2019 M7.1 Ridgecrest, California, earthquake rupture geometry and test its impact on the slip distribution observed at Earth's surface. Common geometries suggest the fault system evolved through reactivation of structures within the surrounding Independence dike swarm (IDS). Mechanical models testing a range of fault geometries and stress fields indicate that the inherited rupture geometry strongly controlled the M7.1 earthquake slip distribution. These results motivate revisiting the development of other large-magnitude earthquake ruptures (1992 M7.3 Landers, 1999 M7.1 Hector Mine) and tectonic provinces within the IDS.

Localized fluid discharge by tensile cracking during the post-seismic period in subduction zones.

It is thought that extensional structures (extensional cracks and normal faults) generated during the post-seismic period create fluid pathways that enhance the drainage of the subducting plate interface, thus reducing the pore pressure and increasing fault strength. However, it remains to be elucidated how much pore fluid pressure decreases by the extension crack formation. Here we examined (i) the pore fluid pressure decrease, and (ii) the degree fault strength recovery by the extension crack formation during the post-seismic period by analyzing extension quartz veins exposed around the Nobeoka Thrust, southwestern Japan. The Nobeoka Trust is an on-land analog of the modern splay fault at shallow depths (~ 8 km) in the Nankai Trough. The poro-elastic model of extensional quartz vein formation indicates that the formation of extensional cracks only releases up to ~ 7-8% of the total pore fluid pressure at ~ 8 km depth. The pore pressure around the Nobeoka Thrust was close to lithostatic pressure during the entire seismic cycle. The estimated effective frictional coefficient along the Nobeoka Thrust after this small fluid-loss by the extensional crack formation does not exceed 0.15. Hence, the pore fluid pressure reduction due to the post-seismic extensional cracks contributes little to increase the fault strength of the megasplay fault.

Stress orientations in subduction zones and the strength of subduction megathrust faults.

Subduction zone megathrust faults produce most of the world's largest earthquakes. Although the physical properties of these faults are difficult to observe directly, their frictional strength can be estimated indirectly by constraining the orientations of the stresses that act on them. A global investigation of stress orientations in subduction zones finds that the maximum compressive stress axis plunges systematically trenchward, consistently making an angle of 45° to 60° with respect to the subduction megathrust fault. These angles indicate that the megathrust fault is not substantially weaker than its surroundings. Together with several other lines of evidence, this implies that subduction zone megathrusts are weak faults in a low-stress environment. The deforming outer accretionary wedge may decouple the stress state along the megathrust from the constraints of the free surface.