Aftershock distributions, moment tensors and stress evolution of the 2016 Iniskin and 2018 Anchorage Mw 7.1 Alaskan intraslab earthquakes

Abstract

SUMMARY We present a detailed study of two Mw 7.1 intraslab earthquakes that occurred in southern Alaska: the Iniskin earthquake of 24 January 2016, and the Anchorage earthquake of 30 November 2018. We have relocated and recovered moment tensors for hundreds of aftershocks following both events, and inverted for stress histories. The aftershock distribution of the Iniskin earthquake suggests that the rupture propagated updip along a fault dipping steeply into the Pacific Plate and terminated at a stratigraphic horizon, inferred to be either the interface or Moho of the subducting slab. In addition, four earthquakes ruptured the main fault in the preceding two years and had similar moment tensors to the mainshock. This evidence suggests that the mainshock likely reactivated a pre-existing, outer-rise fault. The Anchorage earthquake sequence is complex due to its location near the boundary of the subducting Yakutat and Pacific plates, as evidenced by the aftershock distribution. Aftershock hypocentres form two main clusters that appear to correspond to orthogonal, conjugate faults, consistent with the two nodal planes of the dominant focal mechanisms. Both geographic groups display many focal mechanisms similar to the mainshock, which could indicate simultaneous rupture on conjugate planes. The time dependence in stress ratio for the Iniskin sequence can be interpreted in terms of pore-pressure evolution within the mainshock fault zone. In particular, our observations are consistent with a dehydration-assisted transfer mechanism where fluids are produced during rupture through antigorite dehydration and raised to high pore pressures through matrix collapse and/or thermal pressurization. The Anchorage sequence exhibits a more complex stress ratio evolution that may be associated with stress adjustments within a distributed fault network, or reflect a strongly heterogeneous stress field.