Geophysics: Why Anchorage Rocked

Whether its builders knew it or not, the construction of Anchorage was always a risk. Set as it is in southern Alaska, it is deep in an earthquake zone. To make matters worse, most of the city was built on a glacial-outwash plain, which rides on thick beds of slippery clay. When earthquake waves raced through Anchorage on Good Friday, they shattered many a brittle, modern concrete building, but their worst effect was to crack the underlying clay and start the whole place sliding toward the sea.

Seismologists are still analyzing the wiggly lines with which their instruments recorded the quake, and their work will go on for months or years. But already they know that the epicenter (the place on the earth's surface that is directly above the underground source of trouble) was located somewhere between Anchorage and Valdez in a wild, uninhabited region of glaciers and high, rugged mountains. Caltech's famed Seismologist Charles F. Richter thinks that the focus—the point where the shock originated—was at the comparatively shallow depth of 20 miles below the epicenter.

Racing Rupture. Such shallow earthquakes, which are apt to be the most violent and do the most damage, are usually caused by sections of the earth's crust slipping past each other along great cracks called faults. Most of the time, a fault is motionless, its two rock faces pressed tightly together, cemented, perhaps, by chemical action. During these quiet periods, tension builds up along the fault. If the fault finally yields at one point, the rupture races along it at several miles per second. Hundreds of miles of rock relax like a broken spring, releasing the gigantic energy that was stored inside them. Most of the energy turns into waves in the rock, and some of the waves plunge downward to pass through the earth to the opposite side. The most powerful waves run along the surface, making the solid crust shake and jump.

This is what happened in Alaska, where active faults are numerous. The amount of rock movement that took place has not yet been estimated, but Dr. Richter believes that the quake registered at 8.4 on the Richter energy scale, which he invented. By his reading, it ranks among the most powerful of recent earthquakes, exceeded in strength only by the Tibetan quake of 1960(8.5).

Big Bell. Since the earth acts like a solid object, it can be made to vibrate all over if it is hit hard enough. This behavior was predicted more than 80 years ago, but it was first detected with certainty after the Chile quake, when new instruments were ready and watching for it. The whole earth rang like a great, silent bell for two weeks. Its fundamental note had a period of about 54 minutes, which is more than 20 octaves below middle C, vastly too low for human ears to hear.

The Alaska quake was a bell ringer too. Seismologist Jack Oliver of Columbia University's Lamont Geological Observatory says that the whole earth vibrated every 54 minutes. The maximum surface movement was about one-third of an inch and was very gradually diminishing toward the fadeout point.