What Caused California’s Napa Valley Earthquake? Faults Explained

Different kinds of faults make different kinds of earthquakes.

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


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.

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.