Structural controls on aftershocks of a Mw 7.1 intraslab earthquake in Alaska

On January 24 2016, the Mw 7.1 Old Iliamna earthquake struck at ~130 km depth below Iniskin Bay in southern Alaska. The strike-slip rupture propagated up-dip and was followed by more than 140 aftershocks of Mw > 3, ranging in depth from 95 to 140 km. Double-difference hypocenter relocations indicate that approximately half of the aftershocks define a dominantly linear structure. The orientation of this structure lies within 6 degrees of the intersection of the mainshock rupture plane, as reported by United States Geological Survey (USGS), with the slab model of Jadamec et al. 2010 [1]. This geometrical correspondence suggests that the Old Iliamna earthquake rupture terminated up-dip at a stratigraphic horizon within the subducting plate, such as the oceanic Moho or the subducting plate interface. We recover aftershock focal mechanisms by applying a novel relative moment tensor inversion method, which employs P- and S-principal component waveforms to over 100 3-component stations from several networks in Alaska. The moment tensor solutions form four main clusters: strike-slip, reverse and two mixtures of strike-slip & reverse faulting. Each cluster displays one nodal plane with an average strike within 11 degrees of the strike determined by USGS for the mainshock rupture plane. We plan to use the focal mechanisms within stress inversions and exploit observations of converted body-wave phases to constrain the up-dip limit of mainshock rupture relative to subducting plate stratigraphy.

[1] Jadamec, M.A., and Billen, M.I., 2010, Reconciling surface plate motions with rapid three-dimensional mantle flow around a slab edge: Nature, v. 465, p. 338-342, doi:10.1038/nature09053.