The MW 7.8 Earthquake of July 17, 2017 off the Commander Islands and Other Large Earthquakes at the Western Segment of the Aleutian Island Arc

Abstract

This paper discusses the tectonic setting, seismological characteristics, and the aftershock process for the great (MW = 7.8) Near-Islands Aleutian earthquake of July 17, 2017 off the Commander Islands. Our analysis showed that the rupture zone as inferred from the distribution of aftershock epicenters in the shape of an elongate narrow zone approximately 400 km long completely covered the north slope of the Commander Islands uplift and was confined to the Bering Fault. The rupture zone involved this entire earthquake-generating area as far as a transverse structure west of the Near Islands group (Attu Island). The fault-plane solutions and the inferred displacements in the rupture zones of the main shock and of the largest foreshocks and aftershocks revealed the rupture to be a practically pure right lateral movement. The aftershock process of the July 17 earthquake was occurring at rather a sluggish pace for such a large earthquake. In addition, the process involved two other features to distinguish it from the aftershock processes of most Kuril–Kamchatka earthquakes: (1) a low release of the cumulative scalar seismic moment (M0cum aft), which amounted to between 0.75% and 1.0% of the mainshock seismic moment (M0me), according to several estimates; and (2) a very slow increase in the deficit of seismic moment (M0) release. At the same time, the duration of the quasi-stationary phase in the release of M0cum by aftershocks, which is estimated to have lasted approximately half a year and which occupied much of the entire duration of the aftershock process for this earthquake seems to be unusually long. These features in the aftershock process of the Near-Islands Aleutian earthquake of July 17, 2017 distinguish the process from those common to most large Kuril–Kamchatka earthquakes. Overall, the rupture zone can be regarded as a transform feature between two Benioff zones (The Aleutian and the Kuril–Kamchatka ones) rather than as a subduction event, as is typical of the two latter arcs.