Abstract

The Alaska earthquake of March 27, 1964, caused four phenomena at the head of Puget Bay in south-central Alaska. A large rock-snow avalanche fell from Puget Peak and slid into the bay. Sea waves struck the coast, transporting debris inland to elevations of 7 m. Tectonic warping uplifted Puget Bay 1.7 m, and earth cracks formed in the surficial valley alluvium.

The Puget Peak avalanche transported 1.8 × 106m3 of rock, snow, soil, and plant debris downslope. The avalanche began as a large rockfall of jointed and fractured bedrock from Puget Peak. The rockfall reached the cirque traveling at a speed of more than 100 kph; there, it set in motion a large volume of snow. Most of the debris was deposited on the beach and in the bay.

Undisturbed 1963-1964 snow in the cirque, areas of undisturbed vegetation and soil, many fresh grooves and scars on bedrock surfaces, and large areas stripped of surficial vegetation and soil along the avalanche track indicate that the avalanche mass slid on snow, soil, and rock from the cirque to the bay. Evidence along the avalanche track indicates that the avalanche mass traveled at high speed along its entire extent.

Similar avalanches have occurred at Puget Bay in the past and will occur again, because shattered bedrock and steep slopes remain on Puget Peak.

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