What Caused the Chilean Earthquake? Faults Explained

Even planet Earth has its faults. Movement along those fault lines causes earthquakes, big and small, to rattle the globe every day, most recently making news this week with a powerful quake, and tsunami, striking Chile.

The latest magnitude 8.2 earthquake struck 59 miles (95 kilometers) northwest and offshore of Iquique, Chile, on April 1, according to the U.S. Geological Survey. (Related: "Massive Chile Earthquake May Not Be the 'Big One.'")

The powerful quake's center was about 12.5 miles (20.1 kilometers) underneath the Pacific Ocean, where ocean crust and continental crust clash along an earthquake-prone fault that has seen powerful quakes in the past.

The quake spawned a tsunami that led to evacuations and saw seven-foot-high (two-meter-high) waves strike the northern Chilean coast. At least five people are reported killed by the quake.

Whole Lotta Shaking

All earthquakes spring from faults deep underground, but what kind? It can take scientists some time to answer that question for specific quakes.

The Earth's crust is made of a jigsaw puzzle of continental and oceanic plates that are constantly ramming each other, sliding past each other, or pulling apart. Along the Ring of Fire girding the Pacific Ocean, for example, the seafloor plunges beneath Asia and the Americas, building mountains, feeding volcanoes, and triggering earthquakes.

Most earthquakes arise along such fault zones. The ground first bends and then snaps—an earthquake—to release energy along faults. Here are a list of the various ways Earth can shake.

Strike-Slip

When portions of the Earth's crust moves sideways, the result is a horizontal motion along a "strike-slip" fault.

The most famous example is California's San Andreas Fault, which stretches some 600 miles (1,000 kilometers) from southern California to north of San Francisco. The sideways motion of the fault's branches is caused by the Pacific Ocean's crustal plate moving to the northwest under North America's continental crust.

Dip-Slip

Up-and-down motions in earthquakes occur over so-called "dip-slip" faults, where the ground above the fault zone either drops (a normal fault) or is pushed up (a reverse fault). A normal fault occurs where the deeper part of the crust is pulling away from an overlying part. A reverse is, well, just the reverse.

An example of a normal fault is the 240-mile-long (150-kilometer-long) Wasatch Fault underlying parts of Utah and Idaho, again caused by the Pacific plate driving under western North America. One magnitude 7.0 quake along the fault perhaps 550 years ago dropped the ground on one side of the fault by three feet (a meter). The U.S. Geological Survey sees the fault as posing a risk of more magnitude 7.0 earthquakes.

The Chilean quake was spawned by a straightforward dip of the Nazca plate (ocean crust undergirding the Pacific Ocean off South America's coast) as it slips eastward under the continent's crust, according to the USGS.

Oblique

Faults that combine sideways with up-and-down motions are called oblique by seismologists. The Santa Clara Valley south of San Francisco holds a fault prone to oblique motions, for example, seen in a 1999 quake.

Human-Induced

It really takes the movement of crustal plates to uncork a massive earthquake, such as the magnitude 9.0 quake off the coast of Japan in 2011, which was caused by the Pacific plate moving under Asia. But humanity has figured out ways to trigger small quakes as well.

Temblors can be triggered by pumping wastewater onto faults in deep disposal wells, as seen in quakes that occurred in Oklahoma, Texas, and Ohio in recent years.

Seattle Seahawks football fans gained their own notoriety during an NFL Superbowl-winning playoff run this year, triggering so-called beast quakes detected by seismologists across the Pacific Northwest. The height of the seismic activity caused by fans came during a touchdown run.

The only control that humanity has over most quakes, however, is in preparing for them.

Chile has experienced a series of quakes in the past decade, including a very powerful, magnitude 8.8 one that killed about 700 people in 2010. Evacuations in the latest event were described as orderly from coastal towns in northern Chile, now declared a disaster zone.

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