Stress-Drop Variability in Aftershocks of the 2018 Mw 7.9 Gulf of Alaska Outer-Rise Earthquake

Abstract

Stress drop is a fundamental parameter for understanding earthquake source processes, yet its variability and dependence on magnitude, depth, and tectonic setting remain debated. The Alaska–Aleutian subduction zone is one of the most seismically active subduction systems and provides an exceptional natural laboratory for studying stress accumulation and release in a complex tectonic setting. In this study, we systematically analyze 1175 small-to-moderate aftershocks of the 23 January 2018 Mw 7.9 Gulf of Alaska outer-rise strike-slip earthquake recorded by the Alaska Amphibious Community Seismic Experiment (AACSE) between May 2018 and September 2019. The combined offshore and onshore station coverage of AACSE provides high-quality observations that enable estimation of earthquake source spectra by applying a spectral decomposition approach to P waves. Assuming a Brune source model, stress drop shows no systematic dependence on earthquake magnitude, consistent with self-similar scaling. We observe a clear depth dependence of stress drop, with median values at deeper depths (>25 km) being approximately three times higher than those at shallower depths (<25 km). Temporal analysis reveals no significant evolution of stress drop, but the depth-dependent pattern persists throughout the observation period. Lateral variations in stress drop are found within the 12–25 km depth range but are absent at shallower or deeper depths. These results suggest that bending-related stresses primarily control the depth dependence of stress drops, whereas depth-dependent and laterally heterogeneous mechanical properties play an important role in governing stress-drop heterogeneity in an outer-rise setting.