By Ross Stein, Temblor

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The mainshock stuck on July 17 at 23:34 UTC. It followed a M=6.2 by 12 hours earlier only 12 km (7 mi) away. Although this is adjacent to a subduction zone, where the Pacific Plate is being shoved under the Aleutian and Kamchatka islands, the plate motion here is almost purely right-lateral. And so, this quake is similar to, or perhaps a bit smaller than, the 1906 San Francisco earthquake.

Twelve-hour foreshock sequence

Some 12 hr before the mainshock, there was a M=6.2 earthquake about 12 km to the northwest, which was followed by M=5.1 and M=4.4 aftershocks or subsequent foreshocks within the next few hours. Undoubtedly there were smaller shocks, but the detection limit in this remote area is probably about M=4.5. Foreshock sequences are rare among any earthquakes, and strike-slip shocks and did not precede the 1906 M=7.8 or the 1989 M=7.0 Loma Prieta shocks.

Reference

USGS ANSS catalog