South America's western coast is a seismic hot spot

The same forces that formed the Andes Mountains produced the magnitude 8 earthquake that struck Peru's southern desert region Wednesday night: the collision of two massive tectonic plates along South America's western coast.

The Nazca plate under the eastern Pacific Ocean is ramming into the larger South American plate at a rate of about 3 inches per year, one of the fastest rates anywhere in the world, according to geophysicist Paul Earle of the U.S. Geological Survey's National Earthquake Information Center.

As the Nazca plate dives under the coastal plate, it forces the ground upward, forming the mountains and releasing tremendous amounts of energy. That makes the region one of the most seismically active in the world and the source of frequent, massive temblors.

The two plates are merging, a process called subduction, offshore. That process has formed the 5-mile-deep Peru-Chile trench about 100 miles off the coast.

The earthquakes that result from the subduction are called mega-thrust quakes because Earth's surface is being thrust upward, and they are the most powerful earthquakes on the planet. Typically, land levels will rise a few yards in the region of the epicenter.

A magnitude-8.4 quake struck in 2001. The great Peru earthquake of 1868 had a magnitude of 9.0. It killed 25,000 people and caused tsunami damage as far away as Hawaii.

"People think of the San Andreas fault as very active, but there are places in the world like this that are much more active," Earle said.

Although the 50-mile-thick Nazca plate is sliding under the South American plate at a more or less constant rate, relatively small sections of the convergence zone get hung up and the subduction is halted until the pressure builds and the blocked section snaps free, catching up with the rest of the plate and producing an earthquake.

In Wednesday's quake, a section about 120 miles long broke free, Earle said. In general, the longer the section that breaks, the greater the magnitude of the quake. The epicenter was about 25 miles below the surface, Earle said.