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

Area of Science:

• Geophysics
• Tectonics
• Seismology

Background:

• The Mendocino triple junction is a complex tectonic zone where the Pacific, North American, and Gorda plates meet.
• This junction facilitates the reconciliation of Cascadia subduction with San Andreas transform motion through various faults.

Purpose of the Study:

• To analyze the seismic and geodetic signatures of the 2022 Ferndale earthquake and its aftershocks.
• To understand the earthquake's role within the complex tectonic setting of the Mendocino triple junction.
• To investigate the interaction between intra-slab earthquakes and the subduction megathrust.

Main Methods:

• Analysis of seismic data from the mainshock and aftershock sequence.
• Analysis of geodetic data to measure ground deformation.
• Modeling of postseismic deformation to infer aseismic slip on the subduction interface.

Main Results:

• Aftershocks occurred on faults within the upper Gorda slab, with no significant seismicity on the subduction interface.
• Postseismic deformation was observed and modeled as aseismic slip on the subduction interface.
• The slip was likely triggered by stresses from the mainshock at the updip limit of the rupture.

Conclusions:

• The Ferndale earthquake sequence highlights the potential for interactions between intra-slab earthquakes and subduction megathrusts.
• Subduction zones should be viewed as coupled systems of interacting faults.
• Understanding these interactions is crucial for seismic hazard assessment in complex tectonic regions.