BOX 2.3 Prince William Sound, Alaska, 1964
The earthquake nucleated beneath Prince William Sound at about 5:36 p.m. on Good Friday, March 27, 1964. As the rupture spread outward, its progress to the north and east was stopped at the tectonic transition beneath the Chugach Mountains, behind the port of Valdez, Alaska, but to the southwest it continued unimpeded at 3 kilometers per second down the Alaska coastline, paralleling the axis of the Aleutian Trench for more than 700 kilometers, to beyond Kodiak Island. The district geologist of Valdez, Ralph G. Migliaccio, filed the following report:1
Within seconds of the initial tremors, it was apparent to eyewitnesses that something violent was occurring in the area of the Valdez waterfront … Men, women, and children were seen staggering around the dock, looking for something to hold onto. None had time to escape, since the failure was so sudden and violent. Some 300 feet of dock disappeared. Almost immediately a large wave rose up, smashing everything in its path…. Several people stated the wave was 30 to 40 feet high, or more…. This wave crossed the waterfront and, in some areas reached beyond McKinley Street…. Approximately 10 minutes after the initial wave receded, a second wave or surge crossed the waterfront carrying large amounts of wreckage, etc…. There followed a lull of approximately 5 or 6 hours during which time search parties were able to search the waterfront area for possible survivors. There were none.
The height of the tsunami measured 9.1 meters at Valdez, but 24.2 meters at Blackstone Bay on the outer coast of the Kodiak Island group and 27.4 meters at Chenega on the Kenai Peninsula. The city of Anchorage, 100 kilometers west of the epicenter, was shielded from the big tsunami, but it experienced considerable damage, especially in the low-lying regions of unconsolidated sediment that became liquefied by the shaking. Robert B. Atwood, editor of the Anchorage Daily Times, who lived in the Turnagain Heights residential section, described his experiences during the landslide:
I had just started to practice playing the trumpet when the earthquake occurred. In a few short moments it was obvious that this earthquake was no minor one…. I headed for the door … Tall trees were falling in our yard. I moved to a spot where I thought it would be safe, but, as I moved, I saw cracks appear in the earth. Pieces of the ground in jigsaw-puzzle shapes moved up and down, tilted at all angles. I tried to move away, but more appeared in every direction…. Table-top pieces of earth moved upward, standing like toadstools with great overhangs, some were turned at crazy angles. A chasm opened beneath me. I tumbled down … Then my neighbor’s house collapsed and slid into the chasm. For a time it threatened to come down on top of me, but the earth was still moving, and the chasm opened to receive the house.
Migliaccio and Atwood had witnessed the second largest earthquake of the twentieth century. The plane of the rupture inferred from the dimensions of the aftershock zone was the size of Iowa (800 kilometers by 200 kilometers), and geodetic data showed that the offset along the fault averaged more than 10 meters. The product of these three numbers, which is proportional to a measure of earthquake size called the seismic moment (Equation 2.6), was thus 2000 cubic kilometers, about 100 times greater than the 1906 San Francisco earthquake. Among instrumentally recorded earthquakes, only the Chilean earthquake of 1960, which occurred in a similar tectonic setting, was bigger (by a factor of about 3). Both of these great earthquakes