Aftershocks of the April 3, 2023, Mw 6.6 Vilyuchinsk Earthquake, Kamchatka

Abstract—This paper analyzes the aftershock seismicity observed in the southern Avacha Bay, Kamchatka after the Vilyuchinsk earthquake of April 3, 2023, MW = 6.6 (φ = 52.58° N, λ = 158.78° E, h = 105 km). The total duration of this activation is estimated at 450 days. The linear dimensions of the study area are limited horizontally by an ellipse with axes a = 34 km (azimuth α = 117°) and b = 22 km in the depth interval h = 95–105 km. The orientation of the ellipse corresponds to one of the two nodal planes of the main event focal mechanism. Two stages are distinguished in the aftershock seismicity decay rate: (1) Omori decay lasting 8.4 days and (2) exponential decay (τ = 140 days) until reaching background seismicity level. The two stages are comparable in terms of the number of earthquakes. The change phase between these two decay modes maps on the time interval between the strongest aftershocks, ML5.5 April 11, 2023 and ML4.9 April 21, 2023. These two earthquakes are shown to be a doublet as suggested by the high correlation of their waveforms and closeness of their epicenters. A sharp postseismic decrease in the Gutenberg–Richter b-value by a factor of 1.5 after the largest ML5.5 aftershock of April 11, 2023 is followed by a subsequent recovery throughout the exponential decay phase of aftershock seismicity. No b-value variations are revealed during the Omori decay phase.