What is the "Big One", the devastating earthquake that California is waiting for at any time in the San Andrés Fault

This is how they know the great hypothetical earthquake of catastrophic effects that scientists expect to shake the west coast of the United States, especially California.

"It's not a matter of knowing there will be a big earthquake, but when that happens," say the geologists who studied the area.

The earthquakes of recent days, the most powerful of the past 20 years, have particularly affected the city of Ridgecrest, about 250 kilometers north of Los Angeles.

There were no deaths, although fires broke out and the infrastructure and roads were damaged.

A scenario that has alarmed residents of cities like Los Angeles, who have seen camping gear and first aid kits run out of inventory over the weekend.

Why do they call it "Big One"?

In English, "Big One" can be translated as "the big one".

"In California, we tend to use the term Big One to refer to a very big earthquake in the San Andreas Fault, a 7.8 or 8-magnitude event," said Lucy Jones, a seismologist from reference in the region. .

For his part, journalist Jacob Margolis, presenter of a podcast titled The Big One, tells BBC World that "Big One means that we must not only consider the magnitude of the earthquake, but also the level of damage caused ".

And so large cities such as Los Angeles, San Francisco and San Bernardino were built close to the active and imposing San Andrés Fault, which extends from north to south California over 1,300 kilometers and delimits the Plaque of the Pacific.

That of San Andrés is one of the most studied faults of the planet because, in its practical totality, it is on the surface terrestrial.

This was the cause of the magnitude 7.8 earthquake that destroyed much of San Francisco in 1906, killing more than 3,000 people.

Scientists are particularly interested in the southern part of the fault, as it is estimated that it will take too much time before large amounts of energy are released.

Geological studies show that since the last 1500 to 1400 years, major earthquakes have occurred, with a periodicity of about 150 years in the southern part of the fault.

The largest earthquake that occurred in this region in 1857, this region is therefore considered a likely place for the occurrence of a powerful earthquake in the coming decades.

Seismologists explain, however, that they have no way of anticipating when this will happen.

The most conservative calculations suggest that if there was a magnitude 7.8 earthquake in this section – which would have a direct impact in Los Angeles, the second most populated city in the United States – about 2,000 people would die and there are more than 50,000 wounded.

Property damage would exceed US $ 200 000 million.

Is the "Big One" only badociated with the San Andreas Fault?

Due to its extension and delineation of two tectonic plates, an earthquake in the San Andreas Fault is often cited as the big one that scares many Californians.

But this term could very well be used to talk about other earthquakes that, if they occur, could also cause serious damage.

For example, the Hayward Fault, located east of San Francisco Bay, has been described as "a sort of tectonic time bomb" by the eminent geologist of the USGS, David Schwartz.

The USGS considered in a 2018 report that this failure "is one of the most active and dangerous, as it pbades through a densely urbanized and interconnected region".

About seven million people live in the area.

"Everyone, depending on which area of ​​California you live in, has its own Big One," said Margolis.

The journalist refers to another fault, that of Puente Hills, located under the city of Los Angeles.

USGS scientists point out that a break in this fault is an unusual event, which occurs "every 3000 years".

"Scientists do not care about that because it's not as active as San Andrés, but an earthquake could be very destructive in Los Angeles," warns Margolis.

According to estimates published by the USGS in 2005, a 7.5 km earthquake in Puente Hills could kill 3,000 to 18,000 people and losses estimated at US $ 250,000 million.

However, in recent years, to avoid further damage, Los Angeles authorities have asked building owners to reinforce structures considered weak and likely to collapse during an earthquake.

BBC