THE GREAT ALASKA EARTHQUAKE OF 1964

THE GREAT ALASKA EARTHQUAKE OF 1964

EPICENTRAL PARAMETERS

Origin Time: 3/28/64 03:36:14.0 GMT (3/27/64 05:36:14.0 p.m., local time)

Location: Lat. 61.04 N. - Long. 147.73 W. Depth approximately 25 km. The epicenter was about 10 km east of the mouth of College Fiord, approximately 90 km west of Valdez and 120 km east of Anchorage.

Magnitude: 9.2 (Moment Magnitude) Second largest earthquake ever recorded. The largest was a 9.5 earthquake in Chile in 1960.

Duration of rupture: approximately 4 minutes (240 sec.)

AFTERSHOCKS

The aftershock zone of this earthquake was about 250 km wide and extended about 800 km from Prince William Sound to the SW end of Kodiak Island. The mainshock and its aftershocks occurred on a fault which is part of the boundary between the Pacific and North American plates. Thousands of aftershocks were recorded in the months following the mainshock. In the first day there were 11 aftershocks with magnitude greater than 6.0 on the Richter scale; in the next three weeks there were 9 more. Smaller aftershocks continued for more than a year.

CAUSE

The northwestward motion of the Pacific plate at about 5 to 7 cm per year causes the crust of southern Alaska to be compressed and warped, with some areas along the coast being depressed and other areas inland being uplifted. After periods of tens to hundreds of years, this compression is relieved by the sudden southeastward motion of portions of coastal Alaska as they move back over the subducting Pacific plate.

As a result of the 1964 quake, the Latouche Island area moved about 18 meters to the southeast. Also, the patterns of uplift and subsidence which had been slowly developing prior to the earthquake were suddenly reversed, with areas around Montague Island being uplifted 4-9 meters and areas around Portage down-dropped as much as 3 meters. The hinge line (line of no vertical change separating the uplift and subsidence zones) extended from near the epicenter in Prince William Sound to the SE coast of Kodiak Island. This vertical deformation affected and area of approximately 250,000 km2 (100,000 miles2). The end results was the movement of the Pacific plate under the North American plate by about 9 meters on average.

OUTCOME

Damage

The area where there was significant damage covered about 130,000 km2. The area in which it was felt was about 1,300,000 km2 (all of Alaska, parts of Canada, and south to Washington). The four minute duration of shaking triggered many landslides and avalanches. Major structural damage occurred in many of the major cities in Alaska.

Losses

Deaths: 115 in Alaska, 16 in Oregon and California

The death toll was extremely small for a quake of this size, due to low population density, time of day (holiday) and type of material used to construct many buildings (wood)

Monetary: 300-400 million dollars (1964 dollars)

WATER WAVES

Much of the damage and most of the lives lost were due to the effects of water waves. These were mainly of two kinds: the tsunami of open-ocean sea wave, generated by large-scale motion of the sea floor; and the local wave, generated by underwater landslides in bays of fiords.

The 1964 Alaska tsunami was the second largest ever recorded, again following only the one caused by the 1960 Chile earthquake (4 meters at Sitka). Of the 119 deaths attributable to the effects of the ocean, about one-third were due to the open-ocean tsunami: 4 at Newport Beach, Oregon; 12 at Crescent City, California; and about 21 in Alaska.

Local waves claimed at least 82 lives. Maximum height reported for these waves were 70 meters in Valdez Arm.

Seiches, a sort of sloshing of water back and forth in a small body of water like a boat harbor or swimming pool, were observed as far away as Louisiana where a number of fishing boats were sunk. Oscillations in the height of water in wells were reported from as far away as South Africa.

SEISMIC WAVES

In addition to damage in the epicentral region immediately following the quake, long period seismic waves traveled around the earth for several weeks. Basically the whole earth vibrated (rang) like a church bell during this time. States as far away as Texas and Florida were affected with vertical motions of up to 5 to 10 cm.

(adapted from D. Christensen, UAF)

USCG Radio Traffic Transcript during the 1964 Earthquake

PHOTOGRAPHS OF DAMAGE ASSOCIATED WITH THE 1964 ALASKA EARTHQUAKE

Collapse of Fourth Avenue near C Street, Anchorage, due to earthquake caused landslide. Before the earthquake, the sidewalk at left, which is in the graben, was at street level on the right. The graben subsides 11 feet in response to 14 feet of horizontal movement. Anchorage district, Cook Inlet region, Alaska. 1964.

House in Turnagain Heights, Anchorage

Government Hill elementary school, which was destroyed by the Government Hill landslide. Anchorage, Alaska.

View southwest along the Hanning Bay fault scarp on southwest Montague Island in Prince William Sound. The Hanning Bay fault was reactivated during the earthquake. Its trace is marked by 10 to 15 feet high bedrock scarp which trends obliquely across the field of view from the right foreground to the left background. The fault trace lies between the uplifted wave cut surface that is coated white by desiccated calcareous marine organisms and borders the open ocean and the area of brown sand and silt in the cove. The ground northwest of the fault (right side of photo) was displaced upward as much as 16 feet with respect to the ground southeast of the fault during the earthquake, but both sides of the fault were uplifted with respect to sea level due to general tectonic uplift of the region. The fault plane dips steeply NW, or is vertical.

Close-up view of tsunami damage along the waterfront at Kodiak.

A subsidence trough (or graben) formed at the head of the "L" Street landslide in Anchorage during the earthquake. The slide block, which is virtually unbroken ground to the left of the graben, moved to the left. The subsidence trough sank 7 to 10 feet in response to 11 feet of horizontal movement of the slide block. The volume of the trough is theoretically equal to the volume of the void created at the head of the slide by movement of the slide block. A number of houses seen in this photograph were undercut or tilted by subsidence of the graben. Note also the collapsed Four Seasons apartment building and the undamaged three story reinforced concrete frame building beside it, which are on the stable block beyond the graben.

The rails in this approach to a railroad bridge near the head of Turnagain Arm were torn from their ties and buckled laterally by channelward movement of the river banks during the earthquake. The bridge was also compressed and developed a hump from vertical buckling.

Photos and descriptions from the USGS