Theories abounded until 1912, when German meteorologist Alfred Wegener published two articles on the theory of continental drift. According to the theory, the world's continents originally formed one whole plate, which tore apart 200 million years ago, resulting in various sections floating away. (View USGS interactive tracking continental drift)

The theory of plate tectonics grew out of continental drift theory, and today it is widely accepted that the Earth's outer layer is made up of roughly a dozen plates that are constantly drifting apart from each other, or colliding together.

The Earth is scarred with faults marking the points where tectonic plates are very gradually continuing the processes of tearing apart or crashing together. Along some fault systems, such as in parts of California's famed San Andreas Fault, the area adapts to the constant, slow movement of tectonic plates -- or creep -- by continually adjusting, resulting in numerous small to moderate shocks and tremors.

Conditions are ripe for an earthquake when the Earth does not adjust properly to creep, thereby allowing a great amount of strain to accumulate. As pressure builds in the Earth's outer layer, and the two sections of plate around a fault are forced together, stress and shifting increases in the rocks underground.

If the approaching earthquake is a large one, it may be preceded by foreshocks: small shocks marking the movement of rocks beneath the Earth's surface, occasionally followed by a series of infrequent shocks as the two stirring tectonic plate masses lock together during movement.

The Earth then reacts to this stress buildup by bending, until the pressure becomes greater than the rocks themselves. When this happens, the rocks break apart, a process called faulting. As faulting occurs, the breaking rocks send out vibrations known as seismic waves. The concentric waves spread outward from the source, traveling at different speeds and spreading across a wide range.

The U.S. Geological Survey likens this effect to that of a stone being thrown into water, with ripples moving outward from the point where the stone enters the water. This point of origin for an earthquake is called the "focus," and is often far below ground. Geologists and geographers refer to the aboveground point of the earthquake's origin as the "epicenter."

The zone around an earthquake's epicenter is most vulnerable to damage. In this area, the ground can crack and shift as much as several yards, both horizontally and vertically.

The strength and type of the seismic waves during an earthquake establish the severity of damage a region will sustain during the quake.

There are two classes of seismic waves: body waves, which travel quickly through the dense rock deep in the Earth, and surface waves, which travel more slowly through rock near the Earth's surface. Body waves occur before surface waves.

There are two types of body waves. P, or primary waves, which are the fastest, move between 1 and 5 miles per second, and are capable of moving through rock and liquids. Secondary waves -- or S waves -- move more slowly, but cause more damage, as they make the ground shake perpendicular to the direction in which the waves are traveling.

The other seismic wave category is surface waves, which are even slower than secondary waves, and continue for a longer period of time.

As the waves travel through material, the material's density affects their speed. This material composition determines the extent of vibrations -- and therefore damage -- that the earthquake will ultimately cause.

In areas with a large amount of bedrock, for example, seismic waves are incapable of traveling far, so damage to such areas tends to be more minimal. In areas with loosely compacted soil, waves can travel farther, and therefore incur more damage.

When an earthquake's epicenter is located in or near a densely populated area, extensive damage is possible because the seismic waves can bounce off each other before dispersing. According to the Structural Engineers' Association of Northern California, large amounts of soft soil can increase seismic waves and worsen building displacement.

Aftershocks can occur as the massive slabs of shifted rock settle into their new positions. According to the USGS, aftershocks can cause major difficulties for rescue services, and can also cause further damage to structures damaged in the initial quake.

When an earthquake occurs under the ocean, it can tip off a tsunami: a sea wave that spreads outward from the earthquake's epicenter. A tsunami is generated when the seafloor experiences vertical displacement during earthquakes and volcanic activity. The activity causes the ocean surface to shift downward, and the water eventually springs back violently, approaching a coastline at roughly 500 miles per hour and wreaking havoc on coastal areas.

In Papua New Guinea, a magnitude 7.0 quake struck the north central coast in July 1998, tipping off a series of three tsunamis that, according to USGS, may have been the 20th century's most devastating. The official death toll was 2,182, and 9,199 people were left homeless.

-- By Jessica Moore, Online NewsHour