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A major (MW 7.9) intraplate earthquake ruptured the Pacific plate seaward of the Alaska subduction zone near Kodiak Island on 23 January 2018. The aftershock seismicity is diffuse, with both NNW- and ENE-trending distributions, while long-period point source moment tensors have near-horizontal compressional and tensional principal strain axes and significant non-double-couple components. Backprojections from three large-aperture networks indicate sources of short-period radiation not aligned with the best double-couple fault planes. A suite of finite-fault rupture models with one to four faults was considered, and a four-fault model, dominated by right-lateral slip on an SSE trending, westward-dipping fault, is compatible with most seismic, GPS, and tsunami data. However, the precise geometry, timing, and slip distribution of the complex set of faults is not well resolved. The sequence appears to be the result of intraplate stresses influenced by slab pull, the 1964 Alaska earthquake, and collision of the Yakutat terrane in northeastern Alaska.

Plain Language Summary On 23 January 2018 a very large earthquake, with magnitude 7.8, ruptured in the Pacific plate southwest of the Alaskan subduction zone. There are multiple indications of complex faulting for this event: The point source moment tensor is not consistent with a single fault rupture; the aftershock distribution is diffuse, with nearly orthogonal trends in seismicity; the aftershock mechanisms are diverse; backprojections of short-period seismic waves show complex patterns of high-frequency energy release not on a single plane; and teleseismic waveforms are complex. Inversions of the teleseismic signals for a variety of models with from one to four different faults being allowed provide slip models that are used to predict regional GPS observations from Alaska along with deepwater tsunami recordings from seafloor pressure sensors at Deep-ocean Assessment and Reporting of Tsunamis (DART) stations. The primary rupture occurred on a fault trending SSE, dipping to the west, and several nearly perpendicular faults appear to have ruptured as well, but the limited spatial extent of the rupture makes it difficult to resolve the details of the faulting.