Aftershock and Bathymetric Evidence for North‐South En Echelon Faulting During the 2018 M _w 7.9 Offshore Kodiak Earthquake

Abstract The 23 January 2018 M W 7.9 Offshore Kodiak Earthquake had a complex rupture, with previous studies proposing strike‐slip motion on intersecting N‐S and E‐W oriented faults. We determine high‐precision aftershock locations using a relative earthquake location method and arrival time data from ocean‐bottom and land seismographs of the Alaska Amphibious Community Seismic Experiment (AACSE) and global catalogs. Additionally, we examine bathymetric and sub‐bottom profiler data gathered by AACSE and other research cruises. We find clear N‐S trends in the seismicity, and no E‐W concentrations, suggesting en echelon strike‐slip movement along many small N‐S faults. This is supported by seafloor bathymetry and sub‐bottom profiles which indicate recent offset along N‐S faults that align with abyssal hill faults. Fracture zones striking WNW to ESE have no seismicity concentrations or evidence of recent fault slip. Static Coulomb stress calculations with slip on the main N‐S fault from previous source studies reveal stress increases west and NE of the epicenter where most of the later mainshock subevents and aftershocks are located. Thus, we propose an alternative scenario for the 2018 Offshore Kodiak Earthquake in which the initial stages of mainshock rupture along the main N‐S fault triggered the later subevents along en echelon N‐S strike‐slip faults east and west of the main fault, and the aftershocks in those regions. Our proposed scenario for en echelon rupture of a large, outer‐rise, strike‐slip earthquake may also apply to similar sequences, like in the Gulf of Alaska, and illustrates the rupture complexity of large oceanic intraplate earthquakes.