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We combine aftershock relocations, source mechanisms, teleseismic P wave backprojection, and Global Positioning System data inversion to constrain complex faulting geometry of the 2018 M-W 7.9 offshore Kodiak earthquake. Relocated aftershocks delineate several N-S trends including a prominent 110-km-long segment, as well as broad NE-SW trends. Global Positioning System modeling and backprojection indicate that the NE-SW trending left-lateral strike-slip segments released most energy dominating far-field crustal deformation and radiated wavefield. Backprojection infers fast E-to-W rupture propagations superimposed on a slower S-to-N migration. We propose a five-segment model of the rupture that was partially driven by dynamic triggering.

Plain Language Summary In the early morning hours of 23 January 2018, a magnitude 7.9 earthquake struck similar to 300 km offshore Kodiak Island, Alaska, in the outer rise region of the Alaska-Aleutian subduction zone. While the moment tensor for the earthquake suggests predominantly strike-slip faulting, the true complexity of the source has only become evident through analysis of multiple data sets (aftershock locations and source mechanisms, Global Positioning System deformation, and teleseismic P wave backprojection). Our analysis indicates simultaneous rupture on a system of N-S and NE-SW oriented conjugate faults. The N-S faults correlate with plate bending faults imaged by seismic imaging and high-definition bathymetry. The NE-SW faulting trends do not coincide with locations of oceanic plate fractures but correlate with orientations of fault planes of previously recoded earthquakes in the area. Rupture propagation during the earthquake did not follow a continuous path and was partially driven by dynamic and static stress triggering. The modeling results indicate that left-lateral strike-slip segments released most energy thus dominating the radiated wavefield and the resulting crustal deformation. The Mw 7.9 offshore Kodiak earthquake is another example of a complex rupture of the oceanic lithosphere.