What is the dreaded San Andreas fault (and why is it so concerned in California)

Every time a big earthquake like the one that just struck Turkey and Syria occurs, many in California wonder: when will the “Big One” happen here?

With the “Big One” they refer to a catastrophic earthquake that, according to seismologists, should have already occurred in southern California.

And it is that this state in the west of the US is prone to earthquakes, since it is located on a series of faults, that is, regions where the tectonic plates meet.

The largest of them -and the potentially most dangerous- is the San Andreas fault, which crosses the state from north to south and extends along 1,300 kilometers.

The fault delimits the North American plate from the Pacific plate and is one of the most studied on the planet, since practically all of it is on the earth’s surface.

On it sit the huge urban centers of Los Angeles, the second most populous city in the US, and San Diego, and 38 million people who live nearby.

Matter of time

The middle part of the fault ruptured about 160 years ago and the northern part in 1906, causing the devastating 7.8 magnitude earthquake that destroyed much of San Francisco in 1906 and left more than 3,000 dead.

But the one that most worries scientists is the southern section of the fault, where there hasn’t been an earthquake in nearly 300 yearsdespite the fact that the geological records indicate that it is the cause of a great earthquake with a periodicity of about 150 years, and in all that time it has been accumulating tension.

The last great earthquake that occurred in that area dates from the year 1700, but there are no details of how it occurred due to the lack of records at that time.

At the National Earthquake Conference held in California in 2016, participating scientists already warned that the southern section of the San Andreas fault is “loaded and ready” to cause a big tremor.

a big risk

Seismologists from the United States Geological Survey simulated the effects of a large earthquake in California for a study program.

one of his computer models assumes the next big event on the San Andreas fault will be a magnitude 7.8, which will start a rupture in southern California near the Salton Sea and then shoot north along the fault to hit Los Angeles .

The most conservative calculations suggest that, if an earthquake of this magnitude occurs in that section, close to 2,000 people would die and there would be more than 50,000 wounded.

About 1% of the buildings in an area of ​​10 million people would collapse and about half of the buildings in the area would have to be abandoned.

Material damage would exceed US$200 billion.

alert system

The viability of communities after such an event depends on preparation.

In California, much of the focus on planning has been reflected in the building regulations.

Following the San Francisco earthquake in Northern California, new regulations were introduced, forcing the reinforcement of concrete-built structures, many of which house schools and hospitals.

In 2014, the Los Angeles City Council proposed a similar regulation.

In early 2019 ShakeAlert LA went live, a early warning system of earthquakes similar to that which exists in countries with high seismic activity such as Japan or Mexico.

This is a mobile app that notifies Los Angeles County residents up to 40 seconds before an earthquake of magnitude 5 or greater occurs. This not only helps to alert the population, but also the authorities.

For experts, the question is not if the San Andreas fault will rupture in southern California, but when it will.