Disasters
Great Alaskan Quake of 1964
Good Friday Earthquake

The Good Friday Earthquake in Alaska was the largest recorded temblor on the North American continent, and the second largest ever recorded (largest: Chile, 1960).

The earthquake occurred at 5:36 p.m. on March 27, 1964, Alaska Standard Time. The epicenter was in the Northern Prince William Sound, about 75 miles east of Anchorage, or about 55 miles west of Valdez. Nine immediate deaths occurred in the Anchorage area: five in downtown Anchorage, three in Turnagain Heights, and one at the International Airport.

The reported Richter magnitudes ranged from 8.4 to 8.6. The depth, or point where the rupture began was about 14 miles within the earth's crust.

The strong ground motion reported in the Anchorage area lasted about four to five minutes, which triggered many avalanches and landslides, some being tsunamigenic.¹ Ground deformations were extensive, with some areas east of Kodiak elevated by 30 feet and areas around Portage lowered by eight feet.

The maximum intensity reported was XI on the modified Mercalli Intensity scale,² indicating major structural damage, and ground fissures and failures. This 12-point scale is usually shown in Roman numerals, ranging from I, (not felt/no damage) to XII (total destruction/ many lives lost). From this event, significant damage covered an area of about 50,000 square miles. Intensities of IV-V (felt by most people/minor damage) were reported as far away as Cold Bay, Bethel, McGrath, Kotzebue, Deadhorse, Fort Yukon, Eagle, and Skagway.

Earthquake-generated tsunamis

The earthquake caused a total of 115 deaths in Alaska, with 106 of them due to tsunamis that were generated by a tectonic uplift of the sea floor, and by localized subareal³ and submarine landslides.

The temblor caused at least five local slide-generated tsunamis within minutes after the shaking began. In general, slide/slump-induced tsunamis are generated within a few minutes after an earthquake begins. These five occurred at Valdez (two), Seward, Whittier, and Kachemak Bay.

To summarize: In Seward, a 3,511-foot section of the waterfront slid into Resurrection Bay. That created a local tsunami, causing much damage and flaming oil floating on the water. About 20 minutes after that occurred, the first wave of the main tsunami arrived. The 11 to 13 fatalities in Seward were due to the local and the main tsunamis.

Tsunamis generated by the 1964 earthquake, and their subsequent damage and loss of life, were recorded throughout the Pacific. It was the most disastrous tsunami to hit the U.S. West Coast and British Columbia. The largest wave height for that tsunami was reported at Shoup Bay, Valdez Inlet, at 220 feet.

The number of lives lost and cost of damage for Alaska, British Columbia, Washington, Oregon and California are:

¹Tsunami. A tsunami (pronounced soo-nah-mee) is a series of waves generated when water in a lake or the sea is rapidly displaced on a massive scale. Earthquakes, landslides, volcanic eruptions and large meteorite impacts all have the potential to generate a tsunami. The effects of a tsunami can range from unnoticeable to devastating.

² Mercalli Intensity scale. In seismology, a scale of seismic intensity is a way of measuring the effects of an earthquake at different sites. The Modified Mercalli Intensity Scale is commonly used in the United States by seismologists seeking information on the severity of earthquake effects. Intensity ratings are expressed as Roman numerals between I at the low end and XII at the high end. The Intensity Scale differs from the Richter Magnitude Scale in that the effects of any one earthquake vary greatly from place to place, so there may be many Intensity values (e.g.: IV, VII) measured from one earthquake. Each earthquake, on the other hand, should have just one magnitude, although the several methods of estimating it will yield slightly different values (e.g.: 6.1, 6.3).

³ Subareal. Above water.

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