Magnitude 6.3 quake south of Nikolski brings no tsunami threat

A magnitude 6.3 earthquake struck the seafloor south of Nikolski in the central Aleutians on Thursday. About a dozen aftershocks followed, at least three of them above magnitude 4. Federal tsunami centers found no threat to Aleutian communities. The U.S. Geological Survey rated expected losses at its lowest level.

USGS recorded the mainshock 84 kilometers south-southwest of Nikolski at 11:17 UTC Thursday, at a depth of 35 kilometers. The reading came from 136 seismic stations. The U.S. Tsunami Warning Centers event page states there was "No Tsunami Warning, Advisory, Watch, or Threat in effect." USGS assigned the quake a green PAGER alert, meaning no expected fatalities and less than $1 million in economic loss, according to agency documentation.

The quakes struck offshore in sparsely populated areas. Weak shaking is estimated to have reached up to several thousand people, with little expectation of significant economic loss.

Other quakes clustered along the same stretch of the Aleutian arc. USGS weekly feed data list a magnitude 5.5 event 100 kilometers south of Nikolski, a 5.3 at 104 kilometers south-southwest, and three separate magnitude 5.2 events. A magnitude 4.9 quake struck near Nikolski, 81 kilometers southwest, on Thursday. A separate magnitude 3.0 quake hit 65 miles southeast of Herbert Island the same day, at a depth of 11 kilometers. A seismologist had not yet reviewed that reading.

The Alaska Earthquake Center describes shallow quakes in this stretch of the Aleutians as tied to volcanic processes and crustal faults in the overriding North American Plate. "They occur regularly and may produce vigorous aftershock or swarm-like sequences," the center said.

The sequence follows weeks of activity in the same area. That activity included a swarm of 34 earthquakes southwest of Nikolski over roughly a day, and four offshore quakes there topping out at magnitude 5.3. KUCB reported that a magnitude 6.1 quake southwest of Nikolski on Feb. 22 triggered more than 60 aftershocks. Residents in Unalaska, 173 miles away, reported feeling the shaking.

Alaska Volcano Observatory status pages showed no change in alert levels for Aleutian volcanoes as of Thursday.