The 1964 Earthquake-Tsunami Event

At 5:36pm (Alaska standard time) on 27 March 1964 (or 03:36:11.9 to 12.4 GMT on the 28 March 1964) an earthquake with a body magnitude of Mw9.5 (or Richter magnitude of 8.3 to 8.75) occurred with its epicentre located beneath Prince William Sound 80 miles east southeast of Anchorage. The exact co-ordinates for the epicentre of this event are recorded as 61degrees 4'N, 147degrees 73'W. The earthquake originated at a depth between 20 and 50 km and is the largest to have struck the North American continent in history. The earthquake occurred as a result of a northward thrust of the Pacific Ocean sea floor beneath the continental landmass of Alaska. Duration of strong ground motion (earthquake shaking) was on average 3 to 4 minutes and may have been as great as 7 minutes. Vertical changes in the earth's surface elevation that accompanied the Alaska earthquake are the largest in recorded history. 48,000 square miles of land and sea floor dropped up to seven feet and more than 60,000 square miles of the land surface were uplifted. In some places, uplift of the land surface and sea floor exceeded 50 feet (seriously affecting maritime activity). The whole of the contiguous continental mass of North America rose and fell several inches as the earthquake waves traveled out from the epicentre. The energy release was equivalent to 12,000 Hiroshima-type atomic bombs or 240 million tons of TNT.

The enormous deformation and uplift of the seafloor caused the generation of a large tsunami within the Gulf of Alaska which propagated out across the Pacific Ocean as a teletsunami. This tsunami also impacted the southern coast of Alaska. This tsunami caused the deaths of many people and inflicted massive damage on many locations far from its point of origin (e.g. California and Washington States and in Hawaii). Whilst significant interruption of the social fabric of these places as well as major economic losses were incurred, they lie outside the study area of this report and as such are not dealt with. It is noteworthy however, that in Crescent City, California, damage was estimated at more than US$84 million and in places, the tsunami inundated more than 800 metres from the coastline with wave run-ups of more than 10 feet.

In addition to the major teletsunami, more than 20 local tsunamis were generated. These local tsunamis were initiated by a combination of submarine and subaerial landslides at the heads of river deltas and in fjords all along the coast and were triggered by the large earthquake. All of the locally generated tsunamis arrived at the coastline within 2 to 5 minutes of the onset of the earthquake and were responsible for the majority of the damage and deaths reported in the various communities affected in Alaska.