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TitleAftershock distributions, moment tensors and stress evolution of the 2016 Iniskin and 2018 Anchorage Mw7.1 Alaskan intraslab earthquakes
AuthorDrolet, DORCID logo; Bostock, M G; Plourde, A PORCID logo; Sammis, C G
SourceGeophysical Journal International vol. 231, 2022 p. 199-214,
Alt SeriesNatural Resources Canada, Contribution Series 20220210
PublisherOxford University Press
Mediapaper; digital; on-line
File formatpdf
AreaAlaska; Achorage; Iniskin; United States of America
Lat/Long WENS-160.0000 -145.0000 65.0000 55.0000
Lat/Long WENS-153.5000 -153.0000 60.0000 59.5000
Lat/Long WENS-150.5000 -149.0000 61.5000 61.0000
SubjectsScience and Technology; tectonics; earthquake mechanisms; earthquakes; subduction zones; aftershocks
Illustrationslocation maps; plots; graphs
ProgramPublic Safety Geoscience Assessing Earthquake Geohazards
Released2022 04 28
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 subductingYakutat 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.
Summary(Plain Language Summary, not published)
We present a detailed study of two large (Magnitude 7) earthquakes in southern Alaska: the Iniskin earthquake of 24 January 2016 and the Anchorage earthquake of 30 November 2018. Both earthquakes occurred in the Pacific Plate, which is subducting beneath Alaska, and were both followed by hundreds of aftershocks. We estimate locations and moment tensors (which describe the fault orientation and the slip direction of an earthquake, as well as its magnitude) for the aftershocks. The Iniskin aftershocks end abruptly at a boundary interpreted to be either the top of the Pacific Plate or the contact of its crust and mantle layers. Estimates of tectonic stress from the aftershock moment tensors imply a temporary increase in fluid pressure following the mainshock. The Anchorage aftershock sequence shows a more complicated fault structure and stress history, and its mainshock likely activated two perpendicular faults.

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