▲

Two recent earthquakes in Southern California have reminded residents in the west coast US state that they live in one of the most seismically active regions in the world, an area distinguished by the notorious San Andreas Fault.

The quakes, one a magnitude 6.4, the other a more powerful 7.1, were centered near the small town desert town of Ridgecrest, about 241 km (150 miles) from Los Angeles—two of the biggest earthquakes to rattle the region in decades! And the one question left on everybody's lips? Is the "Big One" on its way?

But what exactly is the San Andreas Fault, and why is it associated with such high-risk earthquake activity?

Click through the gallery for a brief overview of this infamous geological feature.

▲The San Andreas fault zone is a continental transform fault or boundary between the Pacific tectonic plate to the west and the North American plate to the east. (Photo: Wikimedia/CC BY 3.0)

▲The fault zone is not a single line. Instead, it's a system of faults that extend through California, including populous areas such as San Bernardino and Los Angeles counties. (Photo: Wikimedia/CC BY-SA 4.0)

▲Along the boundary, the plates grind past each other horizontally at a rate of about 5 cm (2 in) per year. The slight motion can cause small shocks and tremors.

▲While this motion is almost imperceptible to the naked eye, when rocks get stuck on each other's ledges, pressure builds up until stress overcomes friction. This creates a sudden and overwhelming release of energy that sends seismic waves through the ground, called earthquakes. (Photo: Flickr/CC BY-SA 2.0)

▲Mapping a potential disaster: the San Andreas fault line, showing relative motion.

▲The fault is divided into three segments—Northern, Central, and Southern—each with different characteristics and a different degree of earthquake risk.

▲Much of the length of the San Andreas Fault is lined by a distinct trough. This false-color radar image shows a section of the fault west of San Francisco Bay; the Crystal Springs Reservoir fills the trough that marks the underlying fault.

▲The fault was identified in 1895 by Andrew Lawson (1861–1952), a Scottish-Canadian geologist who became professor of geology at the University of Berkeley, California. He discovered the northern zone, and named it after the surrounding San Andreas Valley.

▲The park, located in the Sierra Pelona Mountains in northern Los Angeles County, is noted for the Vasquez Rocks. These were formed by rapid erosion about 25 million years ago. (Photo: Flickr/CC BY 2.0)

▲The rock formations were later exposed by uplift activity along the San Andreas Fault. Today these distinctive geological features mark the fault line and the tectonic boundary between the Pacific Plate and the North American Plate, and form part of the 4,264-km (2,650-mile) Pacific Crest Trail. (Photo: Flickr/CC BY-NC-ND 2.0)

▲Parkfield, a small town in Monterey County located midway between San Francisco and Los Angeles, lies along the San Andreas Fault. It is the self-proclaimed "Earthquake Capitol of the World." (Photo: Wikimedia/CC BY 2.0)

▲Since at least 1857, Parkfield has experienced a magnitude 6 or greater earthquake about every 22 years. In addition, aseismic creep (the measurable surface displacement along a fault) has bent the Parkfield bridge, which spans the fault, a total of 76 cm (30 in) over the last 85 years or so. (Photo: Flickr/CC BY-NC-ND 2.0)

▲The southern segment of the San Andreas Fault stretches from Parkfield all the way to the Salton Sea. Here, the fault runs through the Carrizo Plain (pictured), a long, treeless plain where much of the fault is clearly visible. The Carrizo Plain is known for its superbloom, when the area is carpeted with colorful wildflowers.

▲The Salton Sea, a shallow, saline lake, is located directly on the San Andreas Fault. The southern segment of the fault line is considered by seismologists as a prime threat for a major earthquake (exceeding magnitude 7)—bigger than any that has occurred in Southern California in modern history. (Photo: Wikimedia/CC BY-SA 4.0)

▲The sea was formed in 1905 when massive flooding caused the Colorado River to burst through an irrigation canal and flow freely into the Salton Basin for 18 months. However, the sea has been shrinking for years due to the silting up of the lake, the evaporation of existing water levels, and the increased salinity of what's left. But what has all this got to do with earthquakes?

▲

The Salton Sea is the latest in a series of great lakes to form then dry up with the changing course of the Colorado River. Pictured: water bubbles and burps as carbon dioxide escapes from underground.

▲Whereas previously the often changing weight of these lakes is thought to have had a regulating effect by triggering periodic approximately magnitude 7 earthquakes to relieve built up tectonic pressures, the Salton Sea is now bereft of this shifting and vital volume of water, because it is no longer being replenished by fresh water at a sustainable rate: the surface area of the sea is expected to have decreased by 60% by 2021.

▲With no water flowing into the sea, seismologists fear that the historic build-up of pressure underneath this now dying lake will promote the advent of the "Big One"—the huge earthquake California has been expecting for over a century! Pictured: carbon dioxide gas escaping from underground fissures bubbles up from geothermal mud pots, or ''boiling mud pots.''

▲Conversely, Lake Elizabeth—in the Sierra Pelona Mountains, west of Lancaster, Los Angeles County—is one of a series of benign sag ponds created by the motion of the Earth's tectonic plates along the San Andreas Fault.

▲On April 18, 1906, a earthquake struck San Francisco, which lies within the northern segment of the San Andreas fault zone. Contemporary estimations place the earthquake at around 7.9 on the modern magnitude scale.

▲The devastating event left up to 3,000 people dead, and over 80% of the city was destroyed. It remains one of the worst and deadliest earthquakes in the history of the United States.

▲A fence pictured a short distance northwest of Woodville was displaced 2.6 m (8.5 ft) near the San Andreas Fault after the 1906 San Francisco earthquake. The image clearly illustrates the immense forces of nature at work during the tremor.

▲The Earthquake Trail at the Point Reyes National Seashore, an hour north of San Francisco. The San Andreas Fault runs adjacent to the trail. During the 1906 San Francisco quake, the land along the fault in the Point Reyes area moved approximately 6.5 m (21 ft).

▲The epicenter of the 1989 Loma Prieta earthquake was approximately 16 km (10 miles) northeast of Santa Cruz on a section of the San Andreas Fault. Two moderate foreshocks occurred in June 1988 and again in August 1989—warnings of what was to come! Pictured is San Francisco's Marina district disaster zone after the quake.

▲Loma Prieta was regarded as a wake-up call: this segment of the San Andreas fault zone had been relatively inactive since the events of 1906. Pictured are the collapsed sections of the Cypress Viaduct on Interstate 880 in Oakland.

▲Flames shoot from a ruptured gas main near Balboa Boulevard as homes nearby were destroyed by fire following the magnitude 6.7 Northridge earthquake that hit the Los Angeles area on January 17, 1994. The Northridge event took place well away from the San Andreas Fault, but again demonstrated California's vulnerability to seismic shock.

▲Layers of earthquake-twisted ground are seen at dusk where the State Route 14 highway crosses the San Andreas Fault near Palmdale, California. In 2006 scientists were warning that after more than 300 years with very little slippage, the southern end of the San Andreas Fault north and east of Los Angeles had built up immense pressure that could trigger a massive earthquake at any time.

▲On July 4, 2019, a 6.4 magnitude earthquake struck the area near Ridgecrest in Southern California. While no loss of life was reported, damage to infrastructure was widespread, and 3,000 people in Ridgecrest and the surrounding area were left without power.

▲Two days later another quake rattled parts of the same region, this time registering a powerful magnitude 7.1. It left residents wondering whether the "Big One" was on its way. Californians remain on permanent alert for a catastrophic earthquake that seismologists say is well overdue.

▲Set up in 2002 with the aim of collecting geological data about the San Andreas Fault for the purpose of predicting and analyzing future earthquakes, SAFOD, located at Parkfield, has installed geophone sensors and GPS clocks in a borehole that cuts directly through the fault.

▲

A map displaying each of the seven major fault lines in the San Francisco Bay Area, and the probability of a magnitude 6.7 earthquake or higher occurring on each fault line between 2003 and 2032.