Earthquakes are complicated. They’re hard to predict, sometimes occur nowhere near major fault zones, and on occasion, are caused by nuclear weapons testing. Even the difference between “earthquake” and “aftershock” is fairly arbitrary. When is an aftershock related to a primary quake, and how much time needs to pass for it to become its own, distinct earthquake?

A new study in the journal Science has complicated things even further with a very worrying revelation.

As the authors from the University of California, San Diego (UCSD), point out, a “normal” aftershock is one that occurs on a connected fault line. When a major rupture occurs on one fault, it often directly transfers stress to its network of interconnected faults. If these faults rupture, then this is often referred to as an aftershock.

The team behind this study have found compelling evidence that significantly powerful earthquakes can initiate movement in other fault lines hundreds of kilometers away. These seismic zones can be completely disconnected, geologically speaking, but the seismic wave coming from the first is so energetic that it doesn’t actually matter.

After painstakingly scouring data sets for earthquakes registering at 7 to 8.0M around the world from 2004 to 2015, the team found 48 previously unidentified large aftershocks that occurred within seconds to minutes after the main event – one that took place far, far away from the newly activated faults.

For example, along the Sundra arc subduction zone – the region famous for generating the 2004 tsunami that claimed over 230,000 lives – a magnitude 7.0 tremor triggered two massive aftershocks over 200 kilometers (124 miles) away. This, and plenty of other examples, shows that seismic waves can almost instantaneously transfer stress between faults around the world.

“The results are particularly important because of their seismic hazard implications for complex fault systems, like California,” co-author Wenyuan Fan, a graduate student at UCSD, said in a statement. “By studying this type of triggering, we might be able to forecast hosting faults for large earthquakes.”

^{Devastation in Nepal in April 2015. My Good Images/Shutterstock}

Conventional aftershocks are a common occurrence after almost any earthquake, and they almost invariably come in increasingly diminishing magnitudes. Although these can be damaging and frightening, it’s the transfer of stress from one major fault to another than seismologists tend to worry about.

For example, after the devastating Nepal quake in April 2015, scientists were concerned that a huge fault section in the region remained “aseismic”, in that it didn’t move or rupture at all. This segment appears to be stuck, and every time a major quake occurs with no movement here, it absorbs the additional stress. At some point in the future, this silent zone will rupture and unleash an earthquake at least as powerful as the 2015 one.

In fact, it’s these quiet zones that are worthy of the most attention. Remember, whenever a devastating earthquake happens, it’s highly likely that – despite the destruction on view – it would have been far worse if it happened a decade down the line, as more stress would have been allowed to accumulate.

With this in mind, those living around the San Andreas Fault have to live with the knowledge that, day by day, the “Big One” becomes more likely and, inevitably, more powerful. One 360-kilometer-long (225-mile-long) section hasn’t moved since 1857, and when it does, there’ll be hell to pay.

This study, then, raises two troubling possibilities. If an 8.0M event occurs elsewhere in the country, its seismic waves may reach the San Andreas Fault and actually trigger it to rupture. Alternatively, and more likely, if San Andreas ruptures by itself, it may be so powerful that it could set off distant aftershocks all around the region.

^{Los Angeles awaits the Big One. logoboom/Shutterstock}