Earthquakes Can Trigger Aftershocks In Faults Hundreds Of Kilometers Away

The seismic destruction of Amatrice, a historical town in Italy, was plastered over the news this August. Antonio Nardelli/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

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