One of the strongest deep earthquakes on August 19, 2018, MW = 8.2, h = 600 km, near the Tonga Islands is considered. Its feature is the presence of a powerful aftershock sequence, which is generally uncharacteristic of deep earthquakes and allows us to consider it as a unique seismic event. Based on the first day aftershocks, its three-dimensional geometric model of the source was obtained, the seismic history and features of the aftershock process were studied. It is shown that the source of the earthquake on August 18, 2018 belongs to the NDC type. The duration of the aftershock process and the value of the scalar seismic moment released in aftershocks were estimated. Some characteristics of the August 19, 2018 earthquake were compared with those of the strong deep Sea of Okhotsk earthquake of May 24, 2013, MW = 8.3 off the western coast of Kamchatka. The main results of the work can be summarized as follows. 1. It is shown that the source of the strong deep earthquake of August 19, 2018, MW = 8.2 near the Tonga Islands has a shape close to isometric with upper estimates of its dimensions in the directions of 155 km (N–S), 126 km (E–W), 130 km (z). Similar estimates of the shape and size of the source were also obtained for the deep Sea of Okhotsk earthquake on May 24, 2013, МW = 8.3. At the same time, judging by the aftershock fields of the first day, the linear dimensions of the sources of these earthquakes are approximately two times smaller than the linear dimensions of crustal earthquakes with the same magnitudes. 2. The focal mechanisms of the earthquakes of August 19, 2018, МW = 8.2, h = 600 km and the earthquake in the Sea of Okhotsk of May 24, 2013, МW = 8.3, h = 611 km are guaranteed belong to the NDC-type sources; the source of the earthquake of 2018 off Tonga islands can be classified as a normal fault which characterizes the conditions of uniaxial extension. At the same time, the source of the Sea of Okhotsk earthquake of 2013 belongs to the thrust type and characterizes the conditions of uniaxial compression. 3. The consideration of the available seismic history in the presumed preparation region of the deep earthquake of August 19, 2018, МW = 8.2 near the Tonga Islands showed that its preparation period probably noticeably exceeds 100 years. 4. The duration of the aftershock process can be estimated at about 8 months, and the value of the scalar seismic moment released in aftershocks was estimated as M0 ≈ 9.85∙1019 N∙m that is of about 3.9% of the Main Shock’s scalar seismic moment. The aftershock process of the earthquake in the Sea of Okhotsk of May 24, 2013, МW = 8.3, is estimated to have lasted for about 10.5 months, which is comparable to the duration of the aftershock process of the Tonga Islands earthquake.
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