Earthquake off Alaska spurs tsunami warning for coastal B.C., later cancelled

A 7.5 magnitude earthquake off the west coast of southeastern Alaska on Jan. 5 triggered a tsunami warning in British Columbia that was later rescinded.

Although the West Coast Alaska Tsunami Warning Center issued advisory and warning notices for coastal B.C., these have been were cancelled, notes a Jan. 5 bulletin from British Columbia’s emergency notification system.

“No areas of coastal British Columbia are at risk,” the bulletin stated. “The province will stand down its operations effective immediately.”

It has been reported that Emergency Management B.C. tweeted notifications of the earthquake within about 15 minutes. This follows criticism of the agency’s response time following the earthquake off Haida Gwaii last October.

There were no initial reports of damage from the early-morning earthquake. It occurred as a result of shallow strike-slip faulting on or near the plate boundary between the Pacific and North America plates, notes the tectonic summary from U.S. Geological Survey (USGS). At the location of this quake, the Pacific plate is moving approximately northwestward with respect to the North America plate at about 51 mm annually, the USGS reports.

The latest quake is related to the Haida Gwai earthquake in October and “is an expression of deformation along the same plate boundary system,” the summary states. The October earthquake occurred about 330 km to the southeast of the Jan. 5 event, “slightly inboard of the plate boundary, and was associated with oblique-thrust faulting. The latter earthquake was likely an expression of the oblique component of deformation along this plate boundary system.”

The Jan. 5 earthquake’s closest U.S. location was 94 km west of Craig, Alaska; in Canada, 300 km west north west of Prince Rupert, B.C.

The earthquake broke a fault approximately 50 km in length, and slipped seven to eight metres, notes the USGS. There were a number of aftershocks, with the USGS website listing 15 events on Jan. 5 to 6, all but one citing Craig, Alaska.

The magnitude of the events citing Craig ranged from 3.5 to 5.1, and occurred between 91 km and 144 km away from the community. The final event – 129 km south of Sitka, Alaska – was a 3.8 magnitude quake. 

The USGS reports that the surrounding area of the Pacific/North America plate boundary has hosted eight earthquakes of magnitude 6 or greater over the past 40 years.