1964 Alaska earthquake

1964 Alaska earthquake

Collapsed street in Anchorage, Alaska
Date	27 March 1964 (1964-03-27) (AKST)
Magnitude	9.2 Mw[1]
Depth	14 miles (23 km)
Epicenter location	Alaska
Countries or regions affected	United States
Max. intensity	XI: Very Disastrous
Casualties	131 killed

The 1964 Alaska earthquake, known also as the Great Alaskan Earthquake, began at 5:36 P.M. AST on Friday, March 27, 1964.^[2] Across south-central Alaska, ground fissures, collapsing buildings, and tsunamis directly caused about 131 deaths. This Alaskan megathrust earthquake is known commonly as The Good Friday Earthquake because of the date of its occurrence.

Lasting nearly five minutes, it was the most powerful recorded earthquake in U.S. and North American history, and the second most powerful ever measured by seismograph.^[3] It had a magnitude of 9.2, at the time making it the second largest earthquake in the recorded history of the world.^[2][4]

The powerful earthquake produced earthquake liquefaction in the region. Ground fissures and failures caused major structural damage in several communities, much damage to property and several landslides. Anchorage sustained great destruction or damage to many inadequately engineered houses, buildings, and infrastructure (paved streets, sidewalks, water and sewer mains, electrical systems, and other man-made equipment). Two hundred miles southwest, some areas near Kodiak were permanently raised by 30 feet (9.1 m). East of Anchorage, areas around the head of Turnagain Arm near Portage dropped 8 feet (2.4 m), requiring reconstruction and fill to raise the Seward Highway above the new high tidemark. In Prince William Sound, a 27-foot (8.2 m) tsunami destroyed the village of Chenega, killing 23 of the 68 people who then lived there; survivors out-ran the wave, climbing to high ground. Post-quake tsunamis severely affected Valdez, Whittier, Seward, Kodiak, and other Alaskan Communities, as well as people and property in British Columbia, Oregon, and California. Tsunamis also caused damage in Hawaii and Japan.

The earthquake

At 5:36 p.m. Alaska Standard Time (3:36 a.m. March 28, 1964 UTC), a fault between the Pacific and North American plates ruptured near College Fjord in Prince William Sound. The epicenter of the earthquake was , 12.4 mi (20 km) north of Prince William Sound, 78 miles (125 km) east of Anchorage and 40 miles (64 km) west of Valdez. The focus occurred at a depth of approximately 15.5 mi (25 km). Ocean floor shifts created large tsunamis (up to 70 feet (20 m) in height), which resulted in many of the deaths and much of the property damage. Large rockslides were also caused, resulting in great property damage. Vertical displacement of up to 38 feet (11.5 m) occurred, affecting an area of 100,000 miles² (250,000 km²) within Alaska.

Type of fault

The Alaska Earthquake was a subduction zone earthquake (megathrust earthquake), caused by an oceanic plate sinking under a continental plate. It was a reverse fault caused by a compressional force. This caused much of the uneven ground.

Death toll, damage and casualties

The Turnagain Arm area of Anchorage was heavily damaged

Various sources indicate that about 131 people died as a result of the earthquake: nine during the earthquake itself, 106 from subsequent tsunamis in Alaska^[4] and 16 from tsunamis in Oregon and California. Property damage was estimated at over $300 million ($2.12 billion in current U.S. dollars).

Anchorage area

Most damage occurred in Anchorage, 75 mi (120 km) northwest of the epicenter. Nine people were killed, the only deaths directly attributed to the earthquake. Anchorage was not hit by tsunamis, but downtown Anchorage was heavily damaged, and parts of the city built on sandy bluffs overlying "Bootlegger Cove clay" near Cook Inlet, most notably the Turnagain neighborhood, suffered landslide damage. Land overlooking the Ship Creek valley near the Alaska Railroad yards also slid, destroying many acres of buildings and city blocks in downtown Anchorage. Most other areas of the city were only moderately damaged. The 60-foot concrete control tower at Anchorage International Airport was not engineered to withstand earthquake activity and collapsed, killing one employee ^[5].

The hamlets of Girdwood and Portage, located 30 and 40 mi (60 km) southeast of central Anchorage on the Turnagain Arm, were destroyed by subsidence and subsequent tidal action. About 20 miles (32 km) of the Seward Highway sank below the high-water mark of Turnagain Arm; the highway and its bridges were raised and rebuilt in 1964-66.

Elsewhere in Alaska

A 2x6 plank driven through a ten-ply tire by the tsunami in Whittier

Most coastal towns in the Prince William Sound, Kenai Peninsula, and Kodiak Island areas, especially the major ports of Seward, Whittier and Kodiak were heavily damaged by a combination of seismic activity, subsidence, post-quake tsunamis and/or earthquake-caused fires. Valdez was not totally destroyed, but after three years, the town relocated to higher ground 7 km (4 mi) west of its original site. Some Alaska Native villages, including Chenega and Afognak, were destroyed or damaged. The earthquake caused the Cold-War era ballistic missile detection radar of Clear Air Force Station to go offline for six minutes, the only unscheduled interruption in its operational history. Near Cordova, the Million Dollar Bridge crossing the Copper River also collapsed. The community of Girdwood was also confined to the southern side of the Seward Highway when water rushed into Turnagain Arm arm and flooded or destroyed any buildings left standing to the north of the highway. Interestingly, only the ground immediately along the highway and that on the north side of the road dropped, prompting geologists to speculate that Girdwood may rest upon an ancient cliff face, now covered by countless thousands of years of sediment and glacial deposits.

Canada

A 4.5 ft (1.4 m) wave reached Prince Rupert, British Columbia, just south of the Alaska Panhandle, about three hours after the quake. The tsunami then reached Tofino, on the exposed west coast of Vancouver Island, and traveled up a fjord to hit Port Alberni twice, washing away 55 homes and damaging 375 others. The towns of Hot Springs Cove, Zeballos, and Amai also saw damage. The damage in British Columbia was estimated at $10 million Canadian ($65 million in 2006 Canadian dollars, or $56 million in 2006 U.S. dollars).

Elsewhere

Twelve people were killed by the tsunami in Crescent City, California, while four children were killed on the Oregon coast at Beverly Beach State Park.^[6] Other towns along the U.S. Pacific Northwest and Hawaii were damaged. Minor damage to boats reached as far south as Los Angeles.

As the entire planet vibrated as a result of the quake, minor effects were felt worldwide. Several fishing boats were sunk in Louisiana, and water sloshed in wells in Africa.^[7]

Aftershocks

Over 10,000 aftershocks were recorded following the main shock. In the first day alone, eleven major aftershocks were recorded with a magnitude greater than 6.0. Nine more occurred over the next three weeks. It was not until eighteen months later that the aftershocks were no longer noticed.

Seward waterfront, mid-1964

See also

• List of earthquakes in the United States

References

General references

External links

• USGS report on the earthquake
• U.S. Geological Survey Photo Library -- earthquake photos
• The Great Alaskan Earthquake & Tsunamis of 1964 -- NOAA report
• The Effects of the March 28, 1964 Alaska Tsunami in British Columbia, Canada
• Eyewitness Accounts
• Info on event from Alaska Earthquake Information Center