The latest magnitude 6.9 earthquake struck 50 miles (80 kilometers) offshore of Eureka, California, on March 9, according to the U.S. Geological Survey.
The focus of the quake was about 10 miles (16 kilometers) underneath the Pacific Ocean, where a triangle of ocean and continental crust meets to form earthquake-prone faults.
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.
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.
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 150-mile-long (240-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.
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.
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.
And Seattle Seahawks football fans have gained their own notoriety during a NFL Superbowl-winning playoff run this year, triggering "Beast Quakes" detected by seismologists across the Pacific Northwest. The height of their seismic activity in one game came during a touchdown run.
Correction: The original story misplaced the focal center of the earthquake, which was 10 miles underneath the surface of the Pacific Ocean.
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