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.

Houston Uplift. Much stronger were the short-lived waves that ran along the surface. All over the globe they knocked seismograph recorders off their scales, but Seismologist Jean-Claude de Bremaecker of Rice University had special instruments that could measure the actual height of the waves as they passed through Texas at thousands of miles per hour. He estimates that they lifted Houston by about four inches. Since they were several hundred miles long, they set the city down again so slowly and gently that nothing was broken, and no human sense could detect what was happening.

Some seismologists doubt de Bremaecker's figure, but the waters of Texas felt some motion. While the earth waves were passing, the level of the Houston Ship Canal rose and fell. At Sabine Pass on the Gulf of Mexico, the Coast Guard reported a tide three feet higher than normal. Unusual surges were noted at Corpus Christi and in the Colorado River, and a tugboat captain in the Intercoastal Canal near Port Arthur called by radio and reported unnatural waves five feet high.

If the Alaska quake had happened in thickly settled country, it might have killed thousands of people instead of a few score, and its nearby effects would have been observed more accurately. As it is, the seismologists can only say at this time that it probably came from slippage along a fault associated with the young, still-growing mountains of Alaska. The fault may be wholly buried, or it may reach the surface in some remote place. During the Yakutat earthquake of 1899, a fault in the Alaska panhandle moved vertically in one swift, high-rising jump, forming a new cliff 47 ft. high.

Tsunami. There must have been some surface changes because a tsunami, a seismic sea wave commonly miscalled a tidal wave, spread swiftly southward from Alaska. Hawaii and Japan were warned to prepare for trouble, but there was little damage to their shores. Except for points near the epicenter, the only place seriously hurt was Crescent City in northern California, where the shape of the harbor and the bottom near shore efficiently focuses the energy of Alaskan tsunamis.

In the open ocean, tsunamis seldom rise more than a few inches, and they are usually unnoticeable. But they may be more than a hundred miles long, and their speed, which depends on the depth of the ocean, may reach up to 500 m.p.h. When they hit a shore line, they generally cause a gentle rise of water level; only when the shape of the shore line is just right do they build into enormous waves that rise up and toss raging water high on the land.

Drifting Continents. Though seismologists agree that big, shallow earthquakes are caused by faults, they are not sure where the energy comes from to make the faults move. The most popular modern theory holds that a layer of hot, plastic material lies just below the earth's cool and brittle crust. Heat generated by radioactivity makes the plastic expand and rise toward the surface like water heated in a saucepan. The plastic rock rises in some places, moves horizontally in others, and sinks back to the depths. Circulation is extremely slow, an inch or so per year, but it is so powerful that it moves continents as if they were icebergs floating in an ocean current.

This theory of continental drift, though not universally accepted, goes far to explain the ring of active volcanoes and earthquake-prone mountain ranges that surrounds the Pacific Ocean. The original villain is a great mass of plastic rock that is slowly rising under the Atlantic. One hundred and fifty million years ago, all the continents were bunched together, but the rising rock current split them apart, moving North and South America away from Europe-Africa. The split has now grown into the Atlantic Ocean, and down through its center, keeping equidistant between the two continents, runs the mid-Atlantic ridge, where the ocean floor is still cracking and separating. In highly volcanic Iceland, where the ridge comes to the surface, is a belt of brand-new land made of basaltic lava that the rising rock current has brought up from deep in the earth.

Arcs & Ranges. If the continents are moving away from each other across the Atlantic, they must be moving toward each other across the Pacific, because the earth is a sphere and they have nowhere else to go. As they move, their leading edges push against the crust of the ocean bottom, sometimes thrusting it down in deep trenches, sometimes bending it upward to form curving arcs of islands, like Japan. High mountain ranges like the Andes rear up behind the edges of the advancing continents, and where the rocks bend and break, lines of volcanoes spout their fire.

Alaska is a churning focus of just such action. A rock current under the crust is pushing North America into the Pacific, where another current is moving toward the northwest. The two currents are at right angles to each other, and their force makes the crust yield sideways, forming the great Fairweather Fault running up the Alaskan coast. The fault is a prolific spawning ground for earthquakes, and at its northern end is another source of seismic trouble: the great Aleutian Arc, which was formed by Siberia pressing southeastward into the Pacific and is dotted with active volcanoes. The Fairweather Fault and the Aleutian Arc intersect near Anchorage --which, as Good Friday proved, makes the site a shaky place for building a city.

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