Triggered: new insights into deep earthquakes

Analysis of two quakes finds the first catalysed the second. Kaya Wilson reports.

Deep earthquakes are rare, and not as well studied as their shallower and more destructive counterparts.

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The two largest ever recorded deep earthquakes in the Tonga-Fiji subduction zone occurred just days apart. Now researchers have found that it is highly likely the first one caused the second.

Earthquakes occurring deeper than 350 kilometres below the Earth’s surface are relatively infrequent and little is known about their rupture mechanisms. On 19 August 2018, a magnitude 8.2 deep earthquake occurred. Just 18 days later and 280 kilometres away, another happened, this time with a magnitude of 7.9. Both lasted about 35 seconds and were among the largest deep quakes ever recorded.

An analysis of the two events and the relationship between them has been published in the journal Geophysical Research Letters. The work is the result of a collaboration between seismologists led by Wenyuan Fan from Florida State University in the US.

The researchers were able to use different techniques to determine the characteristics of both quakes and in turn derive the relevant rupture mechanisms. The magnitude 8.2 event was shown to be the product of two distinct physical processes.

The earthquake was initiated in the Earth’s cold slab core, where plate subduction occurred. It then propagated outwards to the warm thermal “halo” surrounding it. This outward movement demonstrated the transition to a second mechanical process.

“This is interesting because before Tonga was thought to predominantly only have one type of mechanism, which is within the cold slab core,” says Fan.

“But we're actually seeing that multiple physical mechanisms are involved.”

The second, slightly smaller earthquake ruptured slowly in a relatively warmer slab area – until then not known as seismically active. It was followed by relatively few aftershocks. Seismic signatures suggest a complex rupture process, involving multiple sub-events.

The contrasts between the two events are consistent with global differences between deep earthquakes in relatively cold and warm slabs.

A similar quake sequence occurred on a smaller scale in August 2002, suggesting that the Tonga subduction zone may be prone to hosting large deep earthquake in pairs, or “doublets”.

The short time between the two events, their proximity and the previous lack of seismic activity near the second one indicate that they were related, the researchers write. However, they note that the 18-day gap between them was unusually long in terms of quake pairs, and suggest it might be an artefact of the specific mechanics involved.

“It's important that we address the question of how large earthquakes trigger other large earthquakes that are not far away,” says Fan.

"This is a good demonstration that there seem to be physical processes involved that are still unknown. We've gradually learned to identify the pattern, but not to a degree where we know exactly how it works.

“I think this is important to any kind of hazard forecast. It's more than an intellectual interest. It's important for human society.”

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