The San Andreas fault is a major rupture in the planet’s crust with two tectonic plates sliding next to each other. The fault is associated with large-scale earthquakes, fissures, and landslides, and it’s plausible that a large earthquake could happen relatively soon. Here’s what we know about the fault.

Whose fault is it anyway

In geology, a fault is a fracture or a system of fractures between two large blocks of rock that move relative to each other. Faults can be very large — the San Andreas fault, for instance, is probably the largest in the world, measuring over 800 miles (1,300 km). It’s also common for the two sides of the fault to be offset, or in other words, to show significant displacement from one side to the other.

Faults form because of plate tectonics. Our planet’s crust is rigid, but it lies on top of a mantle that moves around. As a result, the crust is broken into rigid pieces called tectonic plates. These plates move relative to each other, sometimes going underneath each other or sliding alongside each other.

But this sliding isn’t usually smooth: because there’s a lot of friction, the plates can’t always neatly glide or flow past each other, and this process builds up a lot of stress. As a result, fractures and fissures can also appear inside the plate.

These processes also cause earthquakes, and the vast majority of earthquakes happen at the edge of the tectonic plates. This is also the case for San Andreas, as the San Andreas fault marks the boundary between two major tectonic plates: the Northern Pacific plate and the North American plate.

San Andreas is a so-called “transform fault” — a fault in which the relative movement is predominantly horizontal (as opposed to, for instance, subduction areas, where one plate is moving under another). It’s not the only large fault of this type, several other large transform faults exist, and they’re all generally linked to seismic activity.

Earthquakes tend to happen around faults and the edge of the tectonic plates because of the friction between the plates. As they try to move past one another, the edges of the plates are not smooth, so friction opposes the movement. At some points (usually along the fault plane where the plates grind against each other), more and more energy is accumulated and stress builds up. At some point, when stress reaches a critical level that exceeds a threshold, the fault ruptures and releases the accumulated strain energy, sending out seismic waves that we feel as an earthquake.

California earthquakes

The San Andreas fault started forming some 30 million years ago, although the southern section of the fault has ‘only’ existed for around 5 million years. The slip rate along the fault is a fairly average speed, ranging from 20 to 35 mm (0.79 to 1.38 in) per year — about the speed your fingernails grow at.

This speed may not seem like much, and in truth, if you look at it over the scale of a few years, not much will happen. But on larger time scales, the relentless movement of the plates will create the stress mentioned above — and will release it in the form of earthquakes.

The fault was identified over 100 years ago. In 1895, UC Berkeley geology professor Andrew Lawson identified the fault and named it after a nearby lake called Laguna de San Andreas. Little did Lawson know that he also discovered a new type of earthquake called intermediate-depth earthquake. But let’s not get ahead of ourselves.

Everyone knew that California had earthquakes. But plate tectonics only became a mainstream theory in the 1970s, and it wasn’t even clear how related the San Andreas fault was to the earthquakes. There had been a relatively recent earthquake, the Hayward quake of 1868, that caused major damage and fatalities on both sides of the Bay. But not even Lawson was certain what was going on.

Everything changed in 1906 when a massive earthquake struck San Francisco. The earthquake destroyed about 80% of the city, killing over 3,000 people. When the fires were finally extinguished, Lawson assembled a task force of prominent geologists from all around the US to look for more clues about the earthquake and its potential connection to San Andreas. The team documented seismograph records of the earthquake from all around the world, documented the damage on San Francisco buildings, analyzed the geology around San Francisco, and mapped their findings into a comprehensive report (now referred to as the Lawson report). The report was so influential and so comprehensive that to this day, it is considered a benchmark in the investigation of earthquakes in the US. The report also established, beyond the shadow of a doubt, that the San Andreas fault is the “Mother of all Earthquake Faults.”

But the idea that lateral movement was happening alongside the plate still remained very controversial. In fact almost half a century later, in 1953, when geologists Mason Hill and Thomas Dibblee proposed that San Andreas was indeed moving laterally, the idea was considered radical. It took two more decades for plate tectonics to become established and the idea to be vindicated.

Of course, the 1868 and 1906 earthquakes weren’t the only big ones that happened. This triggered a deluge of studies on the San Andreas, which substantially enhanced our understanding of the fault — and of geology in general. In the past 24 years alone, over 3,000 studies were published on the fault.

For instance, researchers found that the fault produces a magnitude 6.0 earthquake approximately once every 22 years. Such events happened in 1857, 1881, 1901, 1922, 1934, and 1966, with remarkable periodicity. But it’s impossible to predict exactly when earthquakes will happen, and this was exemplified here too when the subsequent earthquake from 1966 happened all the way in 2004.

As mentioned, earthquakes happen when enough stress is accumulated and then released in a burst. But while estimating the general behavior of such a system is possible, knowing exactly when an earthquake will happen is not possible. Still, researchers have some clues.

The next big earthquake

The great majority of California’s population lives around the San Andreas fault; some cities and development projects are actually built on it. Despite some protective measures, millions and millions of people are exposed to a potential earthquake, and understandably, a lot of people are worried.

While earthquakes with a magnitude of 6 can cause a lot of damage in densely populated areas, they’re not normally devastating. But an earthquake of 7, or even worse, an earthquake of magnitude 8, could be totally devastating.

Earthquake magnitude is a logarithmic scale in the base ten — what this means is that for every order of magnitude, the earthquake is 10 times stronger. A magnitude 8 earthquake is 10 times stronger than a magnitude 7 and 100 times stronger than a magnitude 6.

So when can we expect a truly big earthquake in San Andreas?

Let’s emphasize, once again, that predicting the exact time of an earthquake is impossible — all we can get is general expectations.

This being said, a 2006 study published in Nature concluded that the San Andreas fault has reached a sufficient stress level for a 7+ magnitude earthquake. The risk, the study noted, is most concentrated on the southern section of the fault — ie the area around Los Angeles. In fact, several studies suggest Los Angeles could be way overdue for an earthquake.

Because strong earthquakes have occurred relatively recently on the central (1857) and northern (1906) segments of the fault, some of the stress in those areas has been removed, but there hasn’t been a major earthquake in the Los Angeles area for over 300 years. An earthquake in this area would cause major damage to the Palm Springs–Indio metropolitan area and other cities in San Bernardino, Riverside, and Imperial counties in California. It’s hard to even put into words how damaging such an earthquake would be — and older buildings and those built in coastal areas or on unconsolidated gravel or where water tables are high.

However, it goes to show just how challenging it is to predict such earthquakes that in the 16 years since the study was published, there was no substantial quake in the Los Angeles area. Subsequent USGS reports have also made varying predictions regarding the risk of future events, and while the risk does indeed seem to be high, it’s hard to put a timeline on the problem.

To make matters even more complex, San Andreas produces intermediate-depth earthquakes, which means the hypocenter (the point within the earth where an earthquake rupture starts) is at 70–300 km deep — most large earthquakes in the world tend to be more shallow. These intermediate-depth earthquakes are harder to study and less well understood than their shallow counterparts.

So the bottom line is it’s impossible to predict an earthquake on the San Andreas fault, but it’s plausible that one can happen in the near future.

What can be done

Well, we can’t really stop earthquakes from happening, but we can learn to live with them — Japan and Chile are two examples of countries where strict building codes saved lives. Buildings in earthquake-prone areas should be designed carefully and constructed to resist earthquake shaking. Some geological areas are simply unsuitable for earthquake resilience — and they should be avoided.

Still, reinforcing existing buildings is much harder and more expensive, and progress is slow. Simply put, California is unprepared for a big earthquake, and a big earthquake may not be that far off.

Ultimately, it’s up to policymakers and communities to implement protection measures — research can only inform us. Our understanding of the San Andreas fault and the seismogenic mechanisms associated with such faults isn’t perfect, but it’s improved massively over the decades. Hopefully, this knowledge will be translated to healthy policies that will save lives and prevent damage when an earthquake does come.