What is the “Big One”—the Mega Earthquake Threatening the U.S. West Coast?

On December 5, 2024, the earth shook off the coast of California, an earthquake which brought back the specter of the earthquake known as the Big One. Even if Man is the dominant species on the planet, he can do nothing against the force of Nature. In the United States, on the West Coast, residents of the San Andreas Fault know that one day they will be hit by this mega earthquake. What would be the consequences of this earthquake and when will it happen next?

Why Big One?

If the scientific community gives it this name, it's quite simple. Indeed, it's all in the name. “Big One” in English simply means “Big”. So when we talk about this earthquake we talk about it as a big earthquake. But given its rare and recurring nature in History, we will therefore speak of the Big One as THE big earthquake.

We mean “big” in the sense of the size it had during its last occurrence, but also in the sense of the level it could reach on the Richter scale. Indeed, when the Big One is felt on the San Andreas Fault, residents living in the San Francisco and Los Angeles region will be faced with an earthquake of at least magnitude 8 on the scale. by Richter.

What would be the consequences of such an earthquake?

The appearance of the Big One in the years to come will therefore be due to plate tectonics and the tension which resides between the Pacific plate and that of the American continent. If the oceanic plate on the West Coast moves a few centimeters per year towards that of the continent, it sometimes happens that this movement is rapid and violent.

Asked by National GeographicJean-Paul Montagner, seismologist at the Institute of Globe Physics in Paris explained that when the Big One makes itself felt, it will be the underground consequence “of a rapid and violent movement of the earth's crust of 5 to 10 meters”.

On the surface, even if it is difficult to estimate what damage such an earthquake could cause, we inevitably expect to be faced with significant damage. And this, even though the population has been prepared from a very young age to react in the event of an earthquake and buildings have been built to withstand such events.

However, when the Big One hits, residents will be able to “reassure” themselves by saying that there is very little, if any, chance of a tsunami occurring. Indeed, during a tidal wave, the earthquake causing it must be underwater. The San Andreas Fault is located on land. In fact, for John Vidale, seismologist at the University of Southern California, in comments reported by Business Insider“there would be some waves generated by an earthquake, but nothing that would be dangerous”.

When will it take place next?

Thanks to the development of technology, it is now possible to predict as accurately as possible when such events might occur. Of course, this doesn't take into account the terrifying variable of the unexpected.

The last occurrences of the Big One date back to 1857 (magnitude 8.3) and 1906 (magnitude 7.8), respectively. If we are to believe existing models and researchers in United States Geological Surveythe Big One would occur approximately every 150 to 200 years. However, it happens, as was the case 118 years ago, that a devastating earthquake occurs sooner than expected.

But for USGS researchers, if the threat from the Big One mainly concerns the inhabitants of Los Angeles because the city is located on an exposed part of the San Andreas fault, the inhabitants of San Francisco have more to fear from earthquakes of a lower magnitude, around 7, a bit like the one that occurred on December 5, 2024 off the coast of California.

Source : National Geographic / Business Insider / USGS