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A photo of a highway patrol officer redirecting traffic away from a part of the highway that was damaged in a 6.0 earthquake.

A California Highway Patrol officer redirects traffic from a buckled section of California's Highway 12 after Sunday's early morning earthquake.

Photograph by Peter DaSilva, EPA

Dan Vergano

National Geographic

Published August 24, 2014

The magnitude 6.0 earthquake that struck California’s Napa Valley north of San Francisco on Sunday morning—collapsing older buildings, sparking fires and causing scores of injuries—fell along a series of cracks in the Earth tied to the famed and feared San Andreas Fault.

The early morning event centered about 6.7 miles (10.8 kilometers) underneath Northern California's wine country. There, like most locales along the Pacific rim, ocean crust and continental crust clash to create numerous faults and quakes.  (Related: "Massive Chile Earthquake May Not Be the 'Big One.'")

Felt from San Francisco to Sacramento, the quake was one of the largest to strike the region since 1989's magnitude 6.9 Loma Prieta quake. Early news reports on Twitter suggest that a local hospital has received at least 70 patients with injuries.

All earthquakes spring from faults deep underground, but it can take scientists some time to identify the particular type of fault-line activity behind a specific earthquake. That will likely be the case with Sunday’s Napa Valley quake, where some early reports suggested the quake was perhaps provoked by the Franklin Fault, a crack in the Earth that was thought to be dormant for 1.6 million years.

Check out National Geographic's Earthquake 101 video.

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 "Great Quake" of 1906 that destroyed San Francisco, making Northern California famous for quakes, struck along this fault. (Watch: "The Great Quake.")

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.

Similarly, the 22 mile long (35 kilometers) series of faults in the Napa Valley, including the Franklin Fault, have tended the move sideways, in a north to northwest fashion indicative of strike slip faults.

A small magnitude 3.6 aftershock has been reported from the Sunday quake, a typical occurrence after a temblor.

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 (400-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.

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. Thrusts from this fault may power later sideways slip ones similar to Sunday's 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. (RELATED: “Oklahoma Grapples With Earthquake Spike—And Evidence of Industry's Role.”)

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

According to USGS reports, no quakes larger than magnitude 6.0 seem to have struck the Napa Valley in historical records prior to Sunday. Mapping of faults there has informed building codes throughout the region, likely helping to limit injuries on Sunday.

Follow Dan Vergano on Twitter.

7 comments
Vistar H.
Vistar H.

This is a poorly amateurish written article that ends more with a question mark rather than with an answer - or at least  an intelligent bluff ! 

Deborah Rhein
Deborah Rhein

This article has too many mistakes in it and is not up to the standards I expect from an article from the National Geographic brand.

Roger Schellekens
Roger Schellekens

"An example of a normal fault is the 240-mile-long (150-kilometer-long) Wasatch Fault" 

1 mile = 1.6 km, 240 miles = 400 km. 

Sean Ellis
Sean Ellis

I've loved the NatGeo magazine since I was a little kid, amazed at the world captured in its pages.  This article, however, is about as click-bait as it can get. "What Caused California’s Napa Valley Earthquake? Faults Explained" You ask a question in the title that isn't answered, as you yourself report that it won't be understood until more data can be reviewed.  You proceed to explain just three basic fault lines in what is probably the most rudimentary way you could possibly explain them and include a thinly veiled stab at fracking that has nothing to do with the story (and isn't a fault type, by the way). I followed the link hoping to learn a little bit about what happened with the earthquake and learned less than I did from my local nightly news.  I understand it's the internet, but please stick to what has made National Geographic one of the longest running magazines in the world and leave the click-bait to the less prestigious publishers.

Corina Forson
Corina Forson

The arrows in the last diagram of the San Andreas Fault are going the wrong way. The San Andreas is  a right lateral strike-slip fault and those arrows indicate a left lateral fault.

Karen Eicher
Karen Eicher

I think the highly interesting article is overlooking a series of small, man-made quakes in the Denver-Boulder area that occurred  in the late 1960's. This series of small quakes was significant because adding & withdrawing liquid produced predictable results. Some people thought that the process indicated that there was hope for manipulating faults.

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