M7.2 - FOX ISLANDS, ALEUTIAN ISLANDS, ALASKA

Tectonic Summary

The June 24, 2011 Fox Islands (Aleutian Islands, Alaska) earthquake occurred as a result of faulting within the down-going Pacific slab as it thrusts beneath the North America plate at the Aleutian Trench, in the subduction zone extending to the southwest from Alaska. Though the June 24 earthquake is near the subduction thrust interface in the region, its depth (currently estimated as 63 km) and mechanism (either very shallow or steep normal faulting) both suggest that the earthquake occurred within the subducting plate, beyond the down-dip edge of the coupled zone between North America and the Pacific. At the location of this event, the Pacific plate converges with North America at a rate of approximately 71 mm/yr in a northwesterly direction.

Historically, the section of the Aleutians arc in the vicinity of the June 24 earthquake is less active than other parts of this subduction zone, though it has hosted 11 events of magnitude 6 or greater since the early 1970's, all shallower than the June 24 quake. Few instrumentally recorded earthquakes at the depth of the June 24 event have been as large as M6, though two nearby shocks of M > 7 near the beginning of the twentieth century were also thought to have occurred within the subducting Pacific slab. The shallow subduction zone up-dip of the June 24 earthquake ruptured in the M 8.6 Central Aleutians earthquake of 1957, which spawned a large and damaging tsunami locally, that also impacted the shores of Hawaii and California.

Increased attention is being paid to this event due to the fact that the recent M 9 Tohoku, Japan earthquake was preceded two days earlier by a nearby M 7.3 foreshock. We note that while both of those Japanese earthquakes occurred on the subduction thrust plate boundary between the Pacific and North American plates, the June 24 Fox Islands earthquake is likely to have occurred within the lithosphere of the down-going Pacific slab, and thus any link to future events on the plate boundary itself is less clear.

Statistically, any large earthquake raises the probability that other events may occur nearby (typically within two fault lengths, or approximately 100 km in this case). While we are unable to precisely evaluate the probability of a larger earthquake occurring in the near future close to the June 24 event, such probabilities are typically low. The USGS will continue to carefully monitor seismicity in the area, paying particular attention to the number and size of subsequent earthquakes that may increase our understanding of the earthquake sequence and of the likelihood that larger, more damaging events may follow.

Earthquake Information for Alaska