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

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By Ross Stein, Temblor

Check your hazard rank

aleutian-islands-earthquake-kamchatka-commander-islands
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.
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.
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.

 

Reference
USGS ANSS catalog

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