By Ross Stein, Temblor
![aleutian-islands-earthquake-kamchatka-commander-islands](https://static.temblor.net/wp-content/uploads/2017/07/aleutian-islands-earthquake-kamchatka-commander-islands.jpg)
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.](https://static.temblor.net/wp-content/uploads/2017/07/17-Jul-2017-Kamchata-quake-map.jpg)
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.](https://static.temblor.net/wp-content/uploads/2017/07/GEAR-Kamchatka.jpg)
Reference
USGS ANSS catalog
- Structures can be both earthquake-resilient and sustainable - July 19, 2024
- Earthquakes rattle Palawan, Philippines: Is the region as stable as we thought? - July 3, 2024
- Seismic retrofit rates highlight inequitable efforts - June 20, 2024