Yesterday’s M=7.7 Kamchatka strike-slip earthquake was preceded by foreshocks

By Ross Stein, Temblor

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Yesterday’s M=7.7 earthquake off the Commander Islands in eastern Russia was preceded by a foreshock sequence. (Photo © Jenny E. Ross)


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.

USGS epicenter map of the sequence as of 11:15 am PDT on 17 July 2017. Aftershocks are seen

to extend 400 km along strike (the NW-SE axis). The 1906 San Francisco earthquake ruptured

for about 475 km and is variously assessed to be M=7.7-7.9. The mainshock is turquoise.


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.

One can see from the Temblor earthquake forecast (the Global Earthquake Activity Rate model of Bird et al., Bull. Seismol. Soc. Amer., 2015), that earthquakes of M~7.7 would be expected at a rate of about 1% per year, making this event unexceptional. One also sees that still larger quakes would be expected to the east or west of today’s sequence. A 1929 M=7.8 earthquake struck 200 km (120 mi) to the east of today’s shock.



USGS ANSS catalog