Ancestors of the Good Friday Earthquake

Like a forest of ghosts, the lifeless gray trees along Turnagain Arm are a silent reminder of Alaska's Good Friday earthquake. While studying the same area, an Alaska researcher unearthed less obvious clues that chronicle Alaska's violent past and point to the state's unstable future.

The earthquake that rocked Southcentral Alaska on March 27, 1964, was the second-largest ever recorded. The magnitude 9.2 earthquake trails only a 9.5 recorded in Chile in 1960. Alaska's largest earthquake shook the ground for four to seven minutes. During those hundreds of seconds and shortly thereafter, 115 Alaskans died. The tsunamis generated by the earthquake spread the deaths southward; four people died at Newport Beach, Oregon, and 11 more at Crescent City, California.

The killer earthquake was caused when the Pacific plate slipped beneath the North American plate, relieving pressure the two masses built up by pushing against each other for centuries. The release of energy changed the elevation of much of Southcentral Alaska. Portage, at the southern tip of Turnagain Arm, sunk as much as eight feet. Other areas to the east rose as much 38 feet, a fact confirmed when researchers measured the height of barnacles that clung to rocks well above sea level.

About two weeks after the earthquake, a high tide flooded the forest floor near Portage and began the forest’s transformation from healthy spruce and cottonwoods to whitish sticks drowned by salt water.

In the late 1980s, Rod Combellick was studying the Portage and Girdwood areas for signs of liquefaction, a process in which earthquake motion upsets the structure of saturated sand so the sand acts as a liquid rather than a solid. Combellick, of the Alaska Division of Geological and Geophysical Surveys, found plenty of liquefaction evidence, but he also found something else. While taking soil cores, he noticed several dark areas under gray layers of silt. The pattern intrigued him because it seemed to mirror what he saw in Turnagain Arm soils as a result of the 1964 earthquake: beneath ocean-carried silt was a dark layer of peat--vegetation that was alive in 1964 but died quickly when the earthquake deluged the plants in salt water.

Combellick performed another study in which he pulled up soil cores from as deep as 60 feet below the surface. He again saw the repeated pattern of peat covered with silt, which told him large earthquakes in the past had also dropped the elevation of Turnagain Arm and killed the plants that formerly lived near sea level.

By radiocarbon dating the peat samples, Combellick discovered that earthquakes with a magnitude similar to the Good Friday earthquake shook Southcentral Alaska at least six times in the past 5,000 years. The most recent giant earthquake before 1964 probably occurred between the years A.D. 1000 and A.D. 1200.

Using the information in the peat layers, Combellick figures a 1964-type earthquake occurs in the Southcentral region every 600 to 800 years, but that doesn't mean Anchorage is safe until 2564. Smaller earthquakes can be just as destructive as the Good Friday earthquake if they occur near Anchorage or Fairbanks. A relatively small 1994 earthquake in southern California destroyed more than 20 billion dollars worth of property just because it happened near where a lot of people live.

"They were worried about the big one on the San Andreas fault in Southern California, and a 6.7 happened on a hidden fault right under their feet," he said. "The next damaging earthquake in Anchorage may be much different than the one in 1964."