California Is At Risk From A Catastrophic Double-Fault Earthquake

San Diego, one of the cities in the firing line of the San Jacinto fault. welcomia/Shutterstock

The San Andreas fault, the grating, grinding boundary between the gigantic Pacific and North American tectonic plates, is complicated. It has smaller branching faults that cause minor earthquakes, whereas enormous stress build-ups on the main fault line can deliver genuinely devastating tremors to the cities of Los Angeles and San Francisco.

Predicting when the next “Big One” will happen is one of the most difficult tasks facing geoscientists today. Now, a new study published in the journal Science Advances reveals that another fault connected to San Andreas may not solely generate a serious quake of its own, but will cause San Andreas itself to catastrophically rupture.

Last year, NASA predicted that, based on statistical analysis, it is almost certain that a magnitude 5.0 quake will hit Los Angeles before the summer of 2018. This new piece of research looks not into the future, but the past: specifically, at the 1812 quake on the San Jacinto fault, which was estimated to be a magnitude 7.5, close to the strength of the one that rocked Nepal in spring of 2015.

Julian Lozos, an assistant professor of geological sciences at California State University, Northridge and the sole author of this study, noted that no geoscientific records exist for the 1812 event. To get around this, he used a series of computer simulations, backed up by historical accounts, to simulate various ways in which it could have occurred.

Aside from releasing a huge amount of stress itself, the rupture on the San Jacinto fault would have propelled a vast amount of seismic energy towards the nearby, and far larger, San Andreas fault. This caused it to also experience its own rupture, experienced on the surface as one single, powerful tremor.

A map showing where the historical records of the 1812 event took place in southern California. Lozos et al./Science Advances

This historical disaster, which luckily only killed 40 people, was a “double-fault” earthquake. Although these types of earthquakes aren’t necessarily stronger than single-fault earthquakes – see the magnitude 7.8 event that struck off the coast of Indonesia just this month – the danger here is that a smaller tremor can “unlock” a larger fault, causing a larger tremor.

This theory isn’t new: The April quake in Nepal that ended up killing 9,000 people was perhaps close to “unlocking” a far larger fault zone that has been building up stress for an extremely long time without releasing any. Another powerful rupture in the area may suddenly cause this brooding “seismic gap” to cataclysmically burst into life in a terrifying double-fault event.

This new research suggests that a repeat of the 1812 double-fault event is entirely plausible in the same spot beneath California. The population there is far in excess what it was two centuries ago, and another magnitude 7.5 earthquake would unleash 1,000 Hiroshima atomic bombs’ worth of energy in a matter of seconds.

“This study shows that the San Jacinto Fault is an important player that can influence what the San Andreas Fault is doing and lead to major earthquakes,” Lozos said in a statement. “It's important that we learn more about how activity on any single fault can affect other faults.”

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