Simultaneous rupture on conjugate faults during the 2018 Anchorage, Alaska, intraslab earthquake (MW 7.1) inverted from strong-motion waveforms

Yujia Guo,Ken Miyakoshi,Masato Tsurugi

doi:10.1186/s40623-020-01315-x

Abstract

An MW 7.1 ~ 50-km-deep 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 recognized sometimes for shallow crustal earthquakes but rarely for deep intraslab earthquakes. Interestingly, one of the conjugate faults had low aftershock productivity. 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 such as the slip amplitude and large slip area nearly equal the global averages derived from source scaling relationships for intraslab earthquakes. Because the source parameters comparable to the global averages were also found for another large intraslab earthquake in the subducting Pacific/Yakutat slab, these source parameters are likely an important source characteristic common to this subduction zone.

Highlights

At 17:29 Coordinated Universal Time (UTC) on November 30, 2018, an MW 7.1 earthquake occurred in southern Alaska ~ 20 km north of Anchorage at a depth of ~ 50 km (United States Geological Survey (USGS 2019); Fig. 1)
Even though source parameters include regional perturbations associated with differences in the subduction environment (e.g., Stirling et al 2013), we suggest that the source parameters of intraslab earthquakes in southern Alaska nearly equal the global average source parameters
Our inversions revealed a simultaneous rupture on two conjugate faults, an eastward shallowdipping fault and a westward steep-dipping fault

Summary

Introduction

At 17:29 Coordinated Universal Time (UTC) on November 30, 2018, an MW 7.1 earthquake occurred in southern Alaska ~ 20 km north of Anchorage at a depth of ~ 50 km (United States Geological Survey (USGS 2019); Fig. 1). To verify if a simultaneous rupture on two conjugate faults occurred during the Anchorage mainshock, we performed waveform inversions for three cases (Fig. 3). Cases B and C assume a conjugate rupture both on the eastward shallow-dipping and westward steep-dipping faults (segment S3).

Results

Conclusion