Ring-Shaped Seismicity Structures in the Region of Southwestern Alaska: A Justified Forecast of the Location and Magnitude of the Chignik Earthquake of July 29, 2021 (Mw = 8.2).

We consider some characteristics of seismicity in the region of Southwest Alaska. The aftershock zones of the Simeonof (July 22, 2020, Mw = 7.8) and Chignik (July 29, 2021, Mw = 8.2) earthquakes are identified. The characteristics of ring-shaped seismicity structures, which formed in a few decades prior to the Chignik earthquake, are described. The location and magnitude of a large event, which could be prepared to the east of the Simeonof earthquake rupture zone, was predicted in (Kopnichev and Sokolova, 2021a) based on the parameters of these structures. An estimate of the expected magnitude (Mw = 8.2 ± 0.2) was obtained using correlation dependences between the parameters of ring-shaped structures and the magnitudes of large and great earthquakes that occurred in the East Pacific region. This forecast, which was made on the basis of the data obtained prior to October 21, 2020, was presented in the mentioned paper, which was accepted for publication before July 29, 2021. We consider possible changes of the parameters of ring-shaped structures immediately prior to the Chignik earthquake and discuss geodynamic processes that can lead to the formation of ring-shaped structures. [ABSTRACT FROM AUTHOR]