The M_W = 7.8 Earthquake of July 17, 2017 near the Commander Islands: Tectonic Position and Geodynamic Setting

The tectonic position and overall geodynamic setting, as well as the seismological characteristics and peculiarities of the aftershock process, are considered for the source of the strongest Near Aleutian earthquake of July 17, 2017, М_W = 7.8, on the Commander Islands. It is shown that in contrast to the eastern segments of the Aleutian island arc, the subduction of the Pacific lithospheric plate beneath the Commander block is not observed. The analysis has shown that, according to the distribution of the aftershock epicenters in the form of a linear elongated narrow zone with a length of about 400 km, the seismic source occupied almost entire northern slope of the Commander Island rise and spread in the Bering fault zone. It spanned the whole of this seismogenic zone up to the transverse structure west of the Near Islands (Attu islands). The focal mechanism solutions and the pattern of displacements in the sources of the main shock, as well as the strongest foreshocks and aftershocks, suggest that the slip in the source was an almost pure right-lateral shear. The aftershock process of the earthquake on July 17 developed quite feebly for an earthquake of this magnitude. Besides, it has two specific features distinguishing it from the aftershock processes of most of the Kuril-Kamchatka earthquakes: (1) a low release of the cumulative scalar seismic moment (M_{0cum aft}), which, according to different estimates, made up from 0.75 to 1.0% of the main-event seismic moment (M_0me); and (2) a very slow growth of the deficit in the release of seismic moment (M₀). At the same time, the duration of the quasi-stationary phase of the M_0cum release in the aftershocks, which is estimated at approximately half-a-year and which took a considerable span of the total length of the aftershock process from this earthquake, appears to be untypically long. These features of the aftershock process of the Near Island Aleutian earthquake of July 17, 2017 distinguish it from the aftershock processes peculiar to most of the strong Kuril–Kamchatka earthquakes. Overall, its source can be considered as a transform one between the two Benioff zones, the Aleutian and Kuril–Kamchatka ones, rather than as a subduction source characteristic of the last two zones.