ANCHORAGE -

To understand what happened that Good Friday 50 years ago, it's important to know what was going on beneath the surface.

In the five decades since, scientists have used the 1964 earthquake to better understand Alaska's unstable ground.

Peter Haeussler is a research geologist for the United States Geological Service. He studies the history of active faults.

“On the surface of the Earth there are these plates that move with respect to each other,” Haeussler said. “The reason they're moving is there's hot in the center of the Earth and cold on the outside. That heat from the middle of the Earth wants to get out somehow.”

State seismologist Michael West studies earthquakes in Alaska. When the Earth moves, sensors send data to the Earthquake Research Center at the Geophysical institute at University of Alaska Fairbanks. West said when a quake occurs, the center receives information in a matter of seconds.

Scientists now know the ‘64 earthquake was caused by a buildup of pressure between two tectonic plates: the Pacific Plate and the North American Plate that converge along the Aleutian Chain. The two plates meet in what is called a subduction zone. That means there is pressure pushing the plates together that pushes the Pacific Plate under the North American Plate.

Hundreds of years of pressure built until there was a rupture between the plates. “When that happened, a piece of that interface between the two plates began to rupture beneath Prince William Sound,” West said.

The epicenter, in the middle of Prince William Sound rippled along the subduction zone, an area about 500 miles long and 150 miles wide. The earth shook, the ground undulated for four-and-a-half minutes.

Like the ripples from a pebble thrown into a pond, that rupture caused the Earth's crust to move in waves. In other smaller earthquakes such waves would be barely detectable.

“People who went through the earthquake, a number of them felt two pulses of shaking, in that there was an initial one where things were shaking like crazy and then it kind of quieted down for a little bit and then it shook a lot more again,” Haeussler said.

The earthquake measured 9.2 on the moment magnitude scale. The force of the shake, according to the USGS, was greater than the atomic bomb dropped on Hiroshima. In fact, it was like dropping 63,000 of those bombs, Haeussler said. The Earth rang like a bell, sinking boats in Louisiana and moving water in wells all the way in South Africa. There hasn’t been a bigger earthquake in the world since that day.

When the shaking stopped, the land stopped moving. In some places the land sank, like in the Portage area where forests filled with saltwater, killing the trees and leaving behind “ghost forests.” In other places the land rose up drying out swamps. Those dramatic changes in elevation leave behind clues, West explained. Scientists can take core samples to look for evidence of past shifts in elevation that may hold clues to other times the ground moved.

Scientists use those clues of a changing landscape from the past to try to predict what might happen next.