Earthquakes are among the most terrifying natural events. We take it for granted every day that the ground beneath our feet is solid and unchanging. But as news from Turkey, Taiwan and Mexico has demonstrated in recent weeks, this is not always true.

Around the world, about 20 quakes with a magnitude greater than 7.0 [see chart on Page H6] occur every year. On Aug. 17, one struck western Turkey. Measuring 7.4 on the Richter scale and lasting 45 seconds, it destroyed hundreds of buildings and killed at least 15,000 people.

Forty years to the day earlier, on Aug. 17, 1959, a magnitude 7.5 earthquake rumbled through an area just west of Yellowstone National Park and was felt over much of the western United States. Centered in mountainous terrain and lasting 30 to 40 seconds, it triggered numerous landslides, including a huge one that dammed a river and buried a campground, killing 26 people.

Despite the coincidence of timing, the two quakes were triggered by two completely different types of tectonic (earth-moving) processes, as we shall see. And since this is Earth Science Week, we'll round out the picture by examining a third type of seismic event that caused spectacular destruction last month in Taiwan and 35 years ago in Alaska.

These three types of tectonic movement account for most of the quakes you'll ever read about. All involve the interaction between

large "plates" or blocks, independently moving sections of Earth's crust [see illustration at right below]. One also results from volcanic pressure. All have one thing in common: They release their pent-up energy chiefly around large cracks or fractures called "faults."

Jessica Matthews, a second-grade teacher at Terraset Elementary School in Reston, was visiting a friend in Istanbul, Turkey, last August. Asleep in a fourth-floor apartment, she was awakened by a "loud rattling sound, like a freight train coming through the room."

Matthews recalls that the shaking lasted another 15 seconds and was followed every 10 minutes by a series of aftershocks, so regular that Matthews and her friend knew when to expect the next one.

The aftershocks, mostly in the magnitude 4.0-5.0 range [see chart below], lasted two to three seconds each and felt, she says, like "standing next to a washing machine on spin cycle." The time between aftershocks gradually lengthened to 25 minutes, by which time Matthews and her friend had long since evacuated.

Slip and Strike

The epicenter of the quake -- that is, the point on Earth's surface directly above the main focus of the energy -- was located near the city of Izmit between two strands of the North Anatolian fault zone. This fault is strikingly similar in length and character to California's famous San Andreas fault. Both are "strike-slip" faults.

Strike-slip faults occur where crustal blocks slide past each other in response to plate-tectonic forces [below right].

For the San Andreas, it's the vast Pacific plate being carried northwest past the equally large North American plate at a rate of about one inch a year. In Turkey, smaller crustal blocks are shuffling eastward and westward because of the northward push of the African and Arabian plates against Asia, resulting in a sideways movement along the North Anatolian fault, also of about one inch a year.

Theoretically, movement along such faults could occur steadily and smoothly. In reality, friction on the fault surface prevents the blocks on either side of the fault from moving until enough stress builds. Then the fault surface suddenly ruptures, causing an explosive release of energy and making an earthquake.

The center of this rupture is the earthquake "hypocenter," or focus. Aftershocks following the primary shock complete the crustal adjustment and occur days, weeks or even years later.

Most damaging earthquakes along strike-slip faults have focus depths of six miles or less. Destructive earthquakes can originate at greater depths along other types of faults, but few are deeper than 20 miles.

The deeper an earthquake of a given magnitude, the less likely it is to cause damage at the surface. However, large deep quakes are capable of more widespread damage than shallow ones.

The North Anatolian fault zone is intriguing because, over the last 60 years, earthquakes along the fault generally have moved gradually westward. This has allowed geologists to predict more confidently the location of future quakes there.

In fact, in 1979, researchers pinpointed the area of this year's quake as a likely location because it lies in a "seismic gap" between previous focus points. Such gaps also exist along the San Andreas. But without the same orderly pattern of earthquake migration, geologists cannot predict the location of future events along the San Andreas with the same degree of confidence.

Plumes of Magma

Shortly before midnight Aug. 17, 1959, Anita Harris, a geologist with the U.S. Geological Survey, was sleeping in a trailer not far from West Yellowstone, Mont. Suddenly, the trailer started rocking violently. Dishes fell from cabinets. Hearing a loud rumbling sound and thinking that the propane tank had exploded, she rushed outside and away from the trailer only to find that the earth was shaking like a "glob of jelly."

In an adjacent trailer, geologist Irving Witkind was awakened and thought that his trailer had overrun its wheel chocks and was rolling downhill. Jumping outside, he saw that the trailer wasn't moving. But the trees were whipping back and forth, even though there was no wind.

Loud rumblings in nearby canyons came from rock avalanches producing huge clouds of dust. Realizing that it was an earthquake and that he was not in mortal danger, Witkind timed the aftershocks for 20 minutes before trying to drive his car down the hill.

He went about a quarter of a mile before he was stopped by the newly created cliff face, or "scarp," caused by a fault cutting across the road.

The effects of the earthquake in the surrounding area were awesome. Nearby Hebgen Lake, created by an artificial dam across the Madison River, sloshed back and forth like a huge bathtub for 12 hours, floating lakeside cabins off their foundations and several times sending water over the dam, which fortunately held firm.

Tragedy struck 10 miles west of the epicenter when a huge landslide crashed down the steep mountainside, permanently burying 19 people camped along the Madison and killing seven others. An air blast from the slide blew a man and three of his children to their deaths, while his wife and a fourth child barely survived.

A new lake immediately started forming upstream of the slide and remains today as Earthquake Lake. The Hebgen Lake earthquake was felt in an area from North Dakota to Puget Sound.

Two major faults were active during that quake. Movement along these faults, whose surfaces tilt steeply southwest, was mostly vertical and caused the surface on the southwest side of each fault to drop by as much as 15 to 20 feet.

These "normal" faults, as they are called by geologists, are typical in areas of Earth's crust that are being extended, such as the mid-ocean spreading centers that separate oceanic crustal plates moving away from each other.

The Hebgen Lake earthquake, however, owed its existence to a geologic process not directly tied to plate motion -- a plume of magma (molten rock) arising from deep in Earth's mantle. This mantle plume or "hot spot" also is responsible for the volcanic rocks and heat activity of nearby Yellowstone National Park.

In that part of the country are many faults created by the uplift of the Rockies about 60 million years ago. Heat from the present mantle plume has spread outward under Earth's crust, elevating the land surface, stretching the crust and reactivating some of these faults, causing earthquakes.

Some faults are very active on a geologic time scale. These include the one along the front of the Grand Tetons just south of Yellowstone, which has moved thousands of feet vertically over the last 1 million years. The large number of quakes measured in the greater Yellowstone region, mostly of magnitude 5.0 or less, attest to the ongoing fault movement.

Up and under

Last month's devastating magnitude 7.6 earthquake in Taiwan and the 7.5 event in Mexico a few days later involved yet another geologic mechanism called "subduction." In regions where two tectonic plates move toward each other, one slides under the other. Friction between the two causes the buildup of stresses. When that energy is suddenly released, it produces earthquakes.

Taiwan is located at the junction of two separate subduction zones between the Philippine and Eurasian plates. One extends east to west and plunges to the north. The other runs north to south and plunges east.

The fault-riddled island formed here because it is an area where a volcanic arc, parallel to the east-plunging subduction zone, is beginning to collide with continental crust of the Eurasian plate to the west.

A simpler example of a subduction event -- and one of the best studied earthquakes in history -- occurred on the other side of the Pacific at 5:34 p.m. March 27, Good Friday, in 1964.

The ground in southern Alaska heaved violently for several minutes. Slopes over the region gave way, generating hundreds of landslides and causing large fissures. Part of an Anchorage subdivision slid into Cook Inlet.

The ocean floor tilted, generating huge tsunamis that swamped harbors and carried off dozens of boats. Tsunamis are huge sea waves known incorrectly as "tidal waves" although they have nothing to do with tides.

The southern shore of Cook Inlet's Turnagain Arm, including the town of Hope, dropped five feet. Much of the waterfront in the port town of Seward slid into the bay during the initial quake just before by a giant wave caused fuel tanks to explode, creating a hellish combination of fire and water that demolished the town and killed 12 people.

Nor was the destruction confined to Alaska because water can carry energy long distances. More than eight hours after the initial shock, a tsunami crashed into Crescent City, Calif., devastating the waterfront and drowning 10 people. Severe aftershocks continued for three days, followed by lesser ones for 18 months.

Amazingly, this magnitude 8.4 earthquake, one of the most powerful ever measured, resulted in only 115 deaths, primarily because the epicenter region was sparsely inhabited.

The big story of the Alaskan earthquake, geologically speaking, lies in the lasting changes in elevation. It produced the greatest total amount of earthquake-induced ground movement ever measured-more than 100,000 square miles of Earth's surface had permanently shifted up or down, as was readily visible in drowned forests and suddenly elevated tide lines.

Unlike the Turkey and Hebgen Lake earthquakes, this massive movement occurred in the absence of significant surface faulting. Only two fault scarps were discovered, both on an island in Prince William Sound.

Geologists noted the parallelism between the zones of uplift and subsidence, the distribution of aftershock epicenters, the trace of a canyon on the ocean floor called the Aleutian trench and the chain of Aleutian volcanoes. Analysis of the main quake and aftershocks suggested that the fault slip occurred along a very large thrust fault that arcs gently underneath Alaska.

At that time, the theory of plate tectonics was in its infancy. But soon a plate-tectonic explanation for the Alaska quake would become clear: subduction. The thin oceanic crust of the Pacific Ocean floor was diving under Alaska's thick continental crust along the Aleutian Trench. At deeper levels, melting triggered by the diving plate generates the magma that has risen to form the Aleutian volcanoes.

Local Motion

What about the possibility of earthquakes in our area? The metropolitan Washington region is not situated at a plate boundary. Yet "intraplate" (that is, within the same plate) earthquakes also occur in the eastern United States, just as they do around Yellowstone.

The exact cause of intraplate quakes in the East is not known, although it is generally believed that they occur along ancient buried faults being reactivated by regional compressive stresses.

Intraplate quakes generally are felt over a larger area than quakes along plate margins, as was the one at Hebgen Lake. The four great quakes near New Madrid, Mo., in 1811-1812, with magnitudes of about 8.0, were reported to have rung church bells 1,000 miles east in New England.

The largest earthquake in the Southeast occurred in Charleston, S.C., in 1886, with an estimated magnitude of about 7.0. It killed 60 people, caused widespread damage and was felt as far away as Chicago. A magnitude 5.8 quake in Giles County, Va., in 1897 toppled chimneys throughout the southwestern part of the state.

So it is not unreasonable to think that we be shaken in the Washington area in our lifetimes. Ellicott City, for example, had a mild earthquake a few years ago that rattled windows.

Since we don't really know what's producing these earthquakes, we cannot rule out the possibility of a damaging quake sometime in the unforeseeable future. But it's not something to lose sleep over.

William C. Burton is a geologist with the U.S. Geological Survey. For updated Web-based information on recent earthquakes and general earthquake information, go to http://quake.wr.usgs.gov/

CAPTION: Making Waves (This graphic was not available)

Relationship Between Earthquake Magnitude and Energy

Earthquake magnitude is measured using the Richter scale, named for American geologist Charles Richter (1900-85).

Each whole number increase represents 10 times as much measured ground movement.

The spheres are proportional to the amount of energy released in quakes of Richter magnitude 1, 2 and 3.

SOURCE: Australasian Institute of Mining

Ocean-continent subduction zones are a type of tectonic plate boundary where, as two plates converge, the ocean plate slides underneath the continental plate. The tremendous stress generated greatly deforms the continental plate; the ocean plate descends into the interior and melts. Subduction zones form mountains and volcanoes and cause many of the world's significant earthquakes.

Earthquakes occur when the build-up of stress causes Earth's crust to break. This break or fracture is called a fault. Several kinds of faults occur. Three of the most common are shown here: a reverse fault caused by the compression of a subduction zone (top); a transform, or strike-slip, fault caused by lateral, crustal movement (middle); and a normal fault caused by crustal extension (below).

The exact underground location along the fault where the crust ruptures is called the focus (lower red spots). The spot on the surface directly over the focus is called the epicenter (upper red spots).

SOURCES: U.S. Geological Survey and Time-Life Books