Tsunami scare Tuesday brief but harmless

Wrangell residents were among those in communities ringing the Gulf of Alaska who were alerted to a tsunami warning just after midnight Tuesday, following reports of an 8.2 magnitude earthquake occurring 175 miles offshore from Kodiak city.

Issued at 12:35 a.m. local time, the Tsunami Warning Center in Palmer had forecast tsunami activity after registering the quake, which had occurred at a depth of 12 miles. Part of the National Weather Service, the center projected the rippling effect to first reach Kodiak at around 1:45 a.m., followed by Seward and Elfin Cove 10 minutes later and Sitka at 2 a.m. Tsunami activity was initially projected as far to the northwest as the Alaskan island of Saint Paul, and working its way down the west coast to California, reaching Oceanside by 7:05 a.m.

Residents living within this coastal range were advised to seek higher ground or to move inland, and to tune in to local news reports. Reactions to the alert varied between communities. No action was necessary to take in Wrangell and other Inner Channel communities.

The Anchorage Daily News reported residents of the state's largest city had been shaken awake by the earthquake, about 350 miles away. Sirens went off in both Kodiak and Homer, and limited evacuations of residents and materiel took place in communities from Sitka across the Gulf to Kodiak.

The actual activity was less severe than feared, with waves of less than a foot recorded by the National Tsunami Warning Center in Homer, but most activity ranging at around half that height. No damage or harm was reported, and within several hours warnings were being dialed back or withdrawn. A subsequent 4.7 magnitude tremor was recorded at the area at around 10 a.m., but was not accompanied by any warning of tsunami danger.

A report issued jointly by the Alaska Earthquake Center and other agencies Tuesday morning has subsequently set the earthquake at a magnitude of 7.9. It had occurred as the result of strike slip faulting within the surface of the Pacific tectonic plate, as it converges with the neighboring North America plate. The Pacific plate subducts beneath the latter at the Alaska-Aleutians Trench roughly 55 miles to the northwest of Tuesday's quake, moving at a rate of 2.16 inches per year. The quake occurred on a fault system within the Pacific plate itself, and similar seismic events typically occur over a broad area of 142 by 18 miles.

The Earthquake Center reports large earthquakes are common in the Pacific-North America plate boundary just south of Alaska. In the past 100 years 11 other magnitude 7 or greater quakes have occurred within 370 miles of Tuesday's disturbance. Most of these actually occur on the plates' subduction area, such as the Great Alaska earthquake of March 1964. While those kinds of earthquakes can create tsunami conditions, Tuesday's tremor was of a variety less likely to do so. Quakes of similar magnitude occurring in the vicinity in 1987 and 1988 had also failed to result in either damage or casualties.