Study of the Aftershock Distributions, Moment Tensors and Temporal Evolution of the Stress Field of the Iniskin 2016 & the Anchorage 2018, both Mw7.1 Alaskan Intraslab Earthquakes

We present a detailed study of two Mw 7.1 intraslab earthquakes that occurred in southern Alaska: the Iniskin earthquake of January 24, 2016, and the Anchorage earthquake of November 30, 2018. Both events were followed by hundreds of aftershocks. The aftershock distribution of the Iniskin earthquake suggests that it ruptured up dip within the Pacific Plate and terminated at a stratigraphic horizon, inferred to be either the interface or Moho of the subducting slab. Three earthquakes ruptured the main fault in the preceding two years and had moment tensors similar to that of the mainshock. This evidence suggests that the mainshock occurred on a pre-existing and reactivated outer-rise fault. The Anchorage earthquake aftershock sequence is complex possibly due to its location near the boundary of the subducting Yakutat and Pacific plates. Two main clusters are evident and each favours a separate nodal plane as the rupture plane. One of the clusters is majority explained by dynamic triggering of a fault crossing one of the nodal planes. Stress modelling for both intraslab events indicated a change in the stress ratio after the mainshock, probably linked to change in fluid pressure and/or stress drop.