7.5 Earthquake Rocks Alaska, Tremor Swarm With 120 Quakes Since January 2

 The U.S. Geological Survey reports a magnitude 7.5 quake struck at midnight Friday Alaska time and was centered about 60 miles west of Craig, Alaska.   A tsunami warning was in effect for parts of southern Alaska and coastal Canada, but has since been cancelled.  More than 120 earthquakes have been recorded in the area since January 2. 

Nearby Cities

1. 106km (66mi) WSW of Craig, Alaska
2. 304km (189mi) WNW of Prince Rupert, Canada
3. 341km (212mi) S of Juneau, Alaska
4. 404km (251mi) WNW of Terrace, Canada
5. 610km (379mi) S of Whitehorse, Canada

The January 5, 2013 M 7.5 earthquake off the west coast of southeastern Alaska occurred as a result of shallow strike-slip faulting on or near the plate boundary between the Pacific and North America plates.  At the location of this earthquake, the Pacific plate is moving approximately northwestward with respect to the North America plate at a velocity of 51 mm/yr.

This earthquake is likely associated with relative motion across the Queen Charlotte fault system offshore of British Columbia, Canada, which forms the major expression of the Pacific:North America plate boundary in this region. The surrounding area of the plate boundary has hosted 8 earthquakes of magnitude 6 or greater over the past 40 years; In 1949, a M 8.1 earthquake occurred close to the Pacific:North America plate boundary approximately 230 km to the south east of the January 5th earthquake,  as a result of strike-slip faulting. In October of 2012, a M 7.8 earthquake occurred approximately 330 km to the south east of the January 5th 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 January 5th, 2013 earthquake is related to that Haida Gwai earthquake three months previously, and is an expression of deformation along the same plate boundary system.

The Alaska Tsunami Warning Center warned that coastal areas from about 75 miles southeast of Cordova, Alaska, to the north tip of Vancouver Island, Canada could be in danger.   The tsunami warning area extended for about 475 miles.  The tsunami warning has since been canceled. 

The earth’s most active seismic feature, the circum-Pacific seismic belt, brushes Alaska and the Aleutian Islands, where more earthquakes occur than in the other 49 States combined. More than 80 percent of the planet’s tremors occur in the circum-Pacific belt, and about six percent of the large, shallow earthquakes are in the Alaska area, where as many as 4,000 earthquake at various depths are detected in a year.

Early reports of earthquakes in Alaska are fragmentary. The first event in this incomplete record occurred on Sanak and Shumagin Islands, south of the Alaska Peninsula, in July 1788. Apparently no volcanic activity accompanied this event, but the islands of Sanak and Unga and a part of the Alaska Peninsula were inundated by an apparent tsunami (seismic sea wave). The records note, “Some natives lost their lives and hogs drowned.”

Instrumental locations of earthquakes since about 1900 indicate that earthquakes in Alaska center principally in two seismic zones. The most important is the Aleutian Island Arc, one of the planet’s most active seismic areas, which extends about 2,500 miles, from Fairbanks in central Alaska through the Kenai Peninsula to the Near Islands. It maintains a width of nearly 200 miles throughout most of the zone. The second zone begins north of Yakutat Bay in southeastern Alaska and extends southeastward to the west coast of Vancouver Island.

From 1899 to 1969, eight earthquakes of magnitude 8 or more on the Richter scale have occurred in Alaska. Four caused extensive property damage and topographic changes; four centered in areas with no nearby towns, and, except for being recorded by seismographs, went relatively unnoticed.

The Alaskan earthquake that is outstanding in the memory of most occurred in the Anchorage area on March 27, 1964 (March 28, 1964 UTC). The magnitude 8.5 [recalculated to 9.2] shock devastated downtown Anchorage and left homes twisted and broken in the residential section of Turnagain. A tsunami virtually destroyed many of Alaska’s coastal towns and spread death and destruction along the west coast of the United States, Hawaii, and Canada.

Since the temblor occurred on Good Friday, a holiday for schools, and at a time when most people were out of office buildings and on their way home from work, few deaths were caused by the earthquake itself. But 122 persons were drowned by the ensuing tsunami waves: 107 in Alaska, 11 in California, and 4 in Oregon.

The Yakutat Bay area of southeastern Alaska experienced one of the notable earthquakes of the last century on September 10, 1899.Although this shock was preceded one week earlier by a magnitude 8.2 earthquake, most of the effects were associated with the September 10 event which was rated magnitude 8.6 on the Richter scale.

Both of the shocks were felt at villages over 400 miles from Yakutat Bay. The only settlement in the area was Yakutat village, over 30 miles form the Bay. The shaking there on September 3 was described by eye-witnesses at “violent, and impossible to stand without holding on to something.” Prospectors in Disenchantment Bay, an arm of Yakutat Bay, described the September 3 shock as “slight,” compared to the earthquake a week later. Eight men camped near Disenchantment Bay during this violent shock barely escaped with their lives. Behind one camp the water in a small lake left its banks and swept down toward the beach, carrying masses of rock with it. The prospectors described a wave immediately afterward to be 20 feet high; it washed inland over the beach and swept everything away but a few provisions and one boat. All managed to escape to Yakutat.

There is little doubt that changes in land level, chiefly uplifts, occurred at the time of these earthquakes. During June 1899, three months before the shocks, the Harriman Scientific Expedition visited the region to study glaciers and did not report unusual land-level changes. Also, photographs taken in 1895 showed coasts and islands as they had been previously mapped. A field investigation in this area was undertaken in 1905 by a U.S. Geological Survey party. They reported the largest uplifts in land ranged from 30 feet to about 47 1/2 feet on the west coast of Disenchantment Bay. Changes of 17 feet or more affected a large area, and, in a few cases, 1 to 7 foot depressions occurred.

In October 1900, a magnitude 7.9 earthquake was felt from Yakutat Bay to Kodiak, and probably farther westward. On Kodiak Island chimneys were downed, and a man was thrown from his bed. The shock probably centered near Cape Yakataga in southeastern Alaska. Property damage was very moderate for such a great shock, due to the sparsity of population.

The Andreanof Island sustained a magnitude 8.8 earthquake in March 1957 that caused very severe damage on Adak and Unimak Islands. A damaging tsunami was generated, and a wall of water 40 feet high smashed the coastline of Scotch Cap on Unimak Island. Sand Bay, near Adak, reported 26 foot waves inundated its shores.

On Adak, this earthquake destroyed two bridges, damaged some housed, and left gaping cracks in the road. Some cracks were reportedly 15 feet wide, but this is probably an error. At Umnak, part of a dock was destroyed, a cement mixer turned upside down, and other heavy equipment was scattered about. In addition, Mount Vsevidof erupted after being dormant for 200 years. At Sand Bay, the tsunami waves washed away several buildings and damaged oil lines. Millions of dollars in property damage occurred in Hawaii and Japan as a result of the tsunami; minor damage was sustained in southern California and in Central America. This earthquake initiated a series of aftershocks that extended more than 700 miles along the southern edge of the Aleutians.

During the period 1899 to 1969, eight great earthquakes occurred in Alaska, numerous major earthquakes (magnitude 7 to 7.9) centered in the State. Thirteen occurred in or near populated regions and caused minor to severe damage – eight in the intensity (Modified Mercalli Intensity scale) VI category; one, intensity VII; three, intensity VIII; and one, intensity XI. Probably 150 or more occurred in uninhabited areas. Some of the more significant shocks are described below.

On July 22, 1937, a magnitude 7.3 earthquake occurred in central Alaska, about 25 miles southeast of Fairbanks, that was felt over most of Alaska’s interior, about 300,000 square miles. About ten years later, on October 15, 1947, a magnitude 7.3 shock occurred in the same region. It was preceded by a swarm of shocks, some very minute, others violent.

On April 7, 1958, a magnitude 7.3 shock centered in central Alaska near Huslia. Within a 40 to 50 miles radius of Huslia, cracks in lake and river ice, and many ground cracks and mud flows, were observed. Evidence of pressure ridges, lakes thawing, numerous lakes filled with black slimy mud, and craters 20 feet across and 6 feet deep were reported. Some minor damage to log structures was sustained in Huslia.

The strongest shock since those of September 1899 at Yakutat hit southeastern Alaska on July 9, 1958. It was rated magnitude 7.9 on the Richter scale. Three persons were killed on Khantaak Island, and two were missing and presumed dead after being caught in a huge wave generated by the shock in Lituya Bay.

This magnitude 7.9 shock was felt by residents over 400,000 square miles of Alaska, as far south as Seattle, Washington, and east to Whitehorse, Yukon Territory, Canada.

The largest magnitude earthquake in the central interior of Alaska since October 1947 occurred on October 29, 1968. Rated magnitude 6.5, the shock centered southeast of the village of Rampart, on the Yukon River. This area was badly shaken, but no damage was sustained, since most buildings at Rampart were of log construction. Most residents were frightened from buildings, goods toppled from shelves, and equipment not bolted down shifted. Greatest evidence of the shaking was in the Hunter Creek area near Rampart. Many landslides occurred, most on south-facing slopes. Lake ice cracks were extensive in some areas and were observed some 50 miles from the epicenter in the Minto Lakes area. Ground cracks were noted at Nenana, about 50 miles southeast of Rampart, and plaster cracked and fell. During the first 24 hours after the earthquake, College Observatory recorded over 2,000 aftershocks.

Alaska has been hit with a swarm of earthquakes in the past week.