Simultaneous Rupture on Conjugate Faults During the 2018 Anchorage, Alaska, Intraslab Earthquake (MW 7.1) Inverted from Strong-Motion Waveforms

Abstract

An MW 7.1 intraslab earthquake within the Pacific/Yakutat slab underlying the North American Plate struck Anchorage, southern Alaska, on November 30, 2018. The ground-motion records very close to the source region of the Anchorage earthquake provide an important opportunity to better understand the source characteristics of intraslab earthquakes in this subduction zone. We estimated the kinematic rupture process during this earthquake using a series of strong-motion waveform (0.05–0.4 Hz) inversions. Our inversions clearly indicate that the Anchorage earthquake was a rare intraslab event with simultaneous rupture on two conjugate faults, which are worldwide recognized sometimes for shallow crustal earthquakes but rarely for deep intraslab earthquakes. Interestingly, one of the conjugate faults is located in a zone that had low aftershock activity. This fault extends to great depth and may reflect a deep oceanic Moho or a local low-velocity and high-VP/VS zone within the oceanic mantle. Even though the Anchorage earthquake was a rare event due to the conjugate faults, we found that its kinematic source parameters were not abnormal compared to the average parameters of global intraslab earthquakes. The normal source parameters suggest that the larger low-frequency (0.33-Hz) ground-motion amplitudes than predicted by the ground-motion prediction equation observed in downtown Anchorage were primarily due to site amplification effects associated with a sedimentary basin, not source effects. Because such normality of the source parameters was also found for another large intraslab earthquake in the subducting Pacific/Yakutat slab, this normality is likely an important source characteristic common to this subduction zone.