The 2018 MW 7.9 Gulf of Alaska Earthquake: Multiple Fault Rupture in the Pacific Plate

Abstract

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.