M 7.7 - 198km ESE of Nikol'skoye, Russia
USGS
Tectonic Summary
The July 17, 2017 M 7.7 earthquake ESE of Nikol'skoye, Russia occurred as the result of transform faulting on or near the plate boundary between the Pacific and North America plates in the Northwest Pacific Ocean. The focal mechanism solution of the event indicates the earthquake occurred either on a right lateral fault oriented NW-SE, or on a left lateral fault striking NE. At the location of the earthquake, the Pacific plate moves towards the NW at a rate of ~73 mm/yr with respect to North America, effectively moving horizontally past the North America plate at this the western extent of the Aleutian Trench. Along the arcuate Aleutian Trench, the motion of the Pacific Plate with respect to the North America Plate changes from almost purely convergent near south-central Alaska, to almost purely translational in the western most Aleutians where the Aleutian Trench meets the Kamchatka Trench. The location, depth, and focal mechanism solution of the earthquake are consistent with rupture on the main plate boundary, as right lateral faulting.
While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Strike-slip-faulting events of the size of the July 17, 2017 earthquake are typically about 170x20 km (length x width).
The Aleutian Trench is seismically active, and the region within 250 km of the July 17, 2017 event has experienced 14 M 6.5 and larger earthquakes since 1900. The largest event was a M 7.8 earthquake that occurred December 17, 1929, approximately 200 km to the ESE of todays earthquake. The July 17, 2017 earthquake was preceded by a M 6.2 foreshock roughly 12 hours earlier, and 15 km to the NE. In the 2.5 hours since the M 7.7 mainshock, 10 M 4.4+ aftershocks have been recorded.
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https://earthquake.usgs.gov/earthquakes/eventpage/us20009x42#executive