ALBERT

Description: The Mackenzie Mountains EarthScope Project—a collaboration between Colorado State University, the University of Alaska, Michigan State University, and Yukon College—deployed a roughly linear, 40‐station broadband seismographic network. This network crossed the actively deforming Northern Canadian Cordillera and the Mackenzie Mountains in Yukon, California; it also extended into the Canadian Shield in Northwest Territories, California. The array was deployed between July 2016 and August 2018 (with four pilot stations installed in July 2015 and three extended stations operating through August 2019) coinciding with and complementing the deployment of the EarthScope Transportable Array to Alaska and western Canada. In this article, we present an overview of project scientific objectives, station configurations, and site conditions; discuss environmental challenges, including those that resulted in station downtime (e.g., spring flooding and encounters with bears); and suggest potential solutions to such subarctic challenges for the benefit of future deployments in comparable regions. We also include an initial characterization of seasonal and geographic variations in ambient seismic noise for the northwestern Canadian Cordillera.

Description: Coulomb stress change is the change in resultant force of shear stress and friction imposed on a receiver fault plane. The resulting stress change is often computed using the Coulomb 3.4 and the postseismic Green’s functions and postseismic components (PSGRN-PSCMP) programs. Notwithstanding both preferences, both have incomplete optimally oriented failure planes (OOPs) and are inconvenient to resolve Coulomb stress changes on various fault planes placed in varying depths. Here, we present an alternative program termed AutoCoulomb. It leverages the shell command-line tool to automatically batch-process Coulomb stress changes on all sorts of receiver fault planes. We first validate the program. We then apply it to the 2020 Mw 7.8 Simeonof Island, Alaska, earthquake, as a case study. Our results show that Coulomb stress changes resolved on fixed receiver faults, using the three programs, are in line with each other. So are those resolved on 3D OOPs using the PSGRN–PSCMP and the AutoCoulomb programs. Nevertheless, Coulomb stress changes on 2D OOPs, generated by the AutoCoulomb program, always outweigh those done by the Coulomb 3.4 program, indicating that 2D OOPs constrained by the latter are not the most optimal. Some nonoptimal 2D OOPs result in the reversal of the signs of Coulomb stress changes, posing a risk of misleading stress shadows with negative Coulomb stress changes. For the case study, the 28 July 2020 Mw 6.1 aftershock received a positive coseismic Coulomb stress change of ∼3.5 bars. In contrast, the compounded coseismic Coulomb stress changes at the hypocenters of the 1946 Mw 8.2, the 1948 Mw 7.2, and the 2020 Mw 7.8 earthquakes are within a range from −1.1 to 0.1 bar, suggesting that coseismic Coulomb stress changes promoted by preceding mainshocks alone are not responsible for these mainshocks. Other factors, such as postseismic viscoelastic relaxation, afterslip, and slow slip, may contribute to promoting their occurrence.