The 4,000-mile tsunami that killed people while they slept in California
In short: On Good Friday in 1964, the ocean floor beneath Alaska tore open for 600 miles. While the shaking lasted only minutes, the resulting underwater landslides turned harbors into wave machines. These waves traveled across the Pacific, remaining nearly invisible until they struck distant shores with lethal force.
The Great Alaska Earthquake of March 27, 1964, remains the most powerful seismic event ever recorded in North America. With a magnitude of 9.2, it caused the ocean floor to rupture over an area of 600 miles. In downtown Anchorage, the intensity was so great that entire neighborhoods underwent liquefaction, essentially turning solid ground into a fluid that flowed toward the sea. However, the most devastating impact was not the shaking itself, but the secondary geological reactions triggered by the initial rupture.
Underwater landslides transformed nearby harbors into immediate wave generators. These events launched a tsunami across the Pacific Ocean. Because tsunamis are composed of long-period oscillatory waves, they can travel at speeds up to 500 miles per hour in deep water. In these conditions, the waves have very low amplitudes—only about 1 to 2 feet high—making them virtually undetectable to ships and nearly invisible to observers on the open sea. They move like a pebble dropped into a pool, propagating outward in ever-widening circles.
The danger lies in the transition from deep to shallow water. As the waves approach a coastline, friction with the rising sea bottom reduces their velocity, causing the wavelengths to shorten and the amplitudes to surge. Coastal waters can rise as high as 30 meters above sea level in just 10 to 15 minutes. In 1964, this process killed 131 people, many of whom were nowhere near Alaska. The waves reached Oregon and Crescent City, California, still carrying lethal energy 4,000 miles from the source.
The destruction is often caused by three to five major oscillations. These 'run-ups' can uproot trees and pull buildings off their foundations, but the subsequent outflow of water—the receding tide—is frequently just as destructive. Because these waves are reflected and refracted by seafloor topography, they can behave unpredictably, continuing to oscillate for several days until the ocean finally reaches equilibrium.
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Sources
- Summary by Intellect · source: ALASKA 1964 — The Earthquake That Kept Killing for 4,000 Miles
- Also: Alaska earthquake of 1964 | Magnitude 9.2, Tsunami, Destruction, & Facts | Britannica
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