The August 2018 Mw=6.4 Kaktovik (North Slope), Alaska earthquake: a further look combining InSAR and relocated seismicity

On August 12, 2018, the North Slope of Alaska experienced its largest ever recorded earthquake, a Mw=6.4 event ~90 km SW of the village of Kaktovik, followed ~6 hr later by a Mw=6.0 aftershock. The Kaktovik sequence demonstrated that large earthquakes do occur in northern Alaska despite its distance from the Pacific-North American plate boundary, and it may provide insights into large earthquakes in other low-strain-rate regions around the world. We use ASF Sentinel-1A interferograms and relocated seismicity to study this sequence, and find the following: 1) As inferred by Gaudreau et al. [2019] and Xu et al. [2020], the main Mw=6.4 earthquake ruptured one or more WNW-ESE-striking faults in the Sadlerochit Mountains, subparallel to but probably not coincident with the previously mapped Sadlerochit Mountains Fault. The Mw=6.0 event occurred to the east, possibly on an extension of the same structure. 2) A previously inferred jog near the midpoint of the Mw=6.4 rupture is likely an artifact of InSAR filtering, and an unfiltered ASF interferogram suggests that the rupture may be geometrically simpler than previously found (pending further dislocation modeling). This is consistent with the linear WNW-ESE trend of relocated aftershocks. 3) Comparison of the InSAR and seismicity suggests that earthquake locations were likely biased ~7 km to the south-southwest, as inferred by Gibbons et al. [2020], perhaps due to elastic heterogeneity that has not yet been incorporated into seismic velocity models. 4) Multiple small earthquakes occurred near the future epicenter of the Mw=6.0 aftershock in the ~6 hr following the Mw=6.4 mainshock. 5) A Mw=5.3 earthquake occurred ~50 km to the south of the Kaktovik sequence on July 3, 2019. We find that the Kaktovik sequence increased the Coulomb stress at the future epicenter of the 2019 event by ~0.05-0.1 bar.