Could slow-slip earthquakes cause New Zealand’s volcanic eruptions?

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The mysterious, slow-burning earthquakes implicated in major tsunamis may have a subtle effect on New Zealand’s volcanic activity.

In the two decades since they were first observed, “slow-slip” earthquakes – capable of driving displacements along faults undetected – have come into increasing focus by scientists.

In the aftermath of the 2011 Tohoku earthquake and Japan tsunami, scientists searched data from thousands of earthquakes in the region to find one of these deep-rooted, imperceptible events that may have played a role.

Other studies have linked these slow-sliding episodes to two major earthquakes in Chile and Mexico in 2014 – although the vast majority operate without directly causing disasters.

While marine slow-slide events within our greatest geological threat, the Hikurangi subduction zone, have received much attention, a new study explores whether they may also be able to influence volcanic eruptions.

Scientists have long suggested a link between large earthquakes and volcanic activity, said Dr. Finn Elsley Kemp, a volcanic seismologist at the University of Victoria.

“Slow-slip earthquakes release the same amount of energy as these large earthquakes, but they spread out over a longer period of time, so we don’t feel it,” he said.

“However, in theory, the energy released from them could affect volcanoes in the same way as a large earthquake.

“Often, the initial trigger that causes a volcanic disturbance or eruption is a mystery, and it can serve as one of those triggers.”

Slow-slide events are known to be a relatively common feature of the Hikurangi subduction zone – the offshore margin where the Pacific plate sinks – or descends – westward under the North Island. Specifically, it tends to occur within regions where the subduction zone transitions from being “stuck” below the southern North Island, to a region where the subduction zone is “creeping” to the north, around Gisborne and Hawke’s Bay. Source / GNS Science

The first step in his team’s study, which has just been awarded with a $360,000 Marsden Fund grant, was to understand all of the volcanic activity that has occurred in New Zealand over the past decade — including “upheaval” events that never led to an eruption.

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This will involve building an accurate and highly detailed catalog of tens of thousands of earthquakes – and “a massive amount of rickety data” from the GeoNet seismometer network – to identify those earthquakes that coincided with volcanic disturbances.

Next, they will rely on numerical models that simulate plate tectonics below New Zealand to estimate the stresses caused by slow-slip earthquakes at these volcanoes.

“In a perfect world, we’d see that these match up perfectly, and the pressure from a slow-slip earthquake is immediately followed by volcanic disturbances,” said Elsie Kemp.

“However, nature is seldom that simple and it is very likely that there is a time lag between the stress from a slow earthquake and volcanic disturbances.”

Fortunately, he added, members of the UK-based team were experts at finding patterns in complex data sets, and would use machine learning statistics to analyze the data sets his team created.

For Illsley-Kemp, whose work has revealed how the Taupe-based supervolcano recently experienced its most shaking years in decades, the potential volcanic association with slow-slithering was “puzzling.”

“For example, the disturbances at Taup in 2019 occurred at the same time as a large slow slip event off the east coast and we can see from GPS measurements that the ground around Taup moved in response to this slow slip event,” she said.

“So we really want to investigate this potential link across all of New Zealand’s volcanoes.”

If the link is proven, it could cause a rethink.

“Volcanoes are often treated as isolated systems, but in reality they are located within a much larger and very active tectonic environment,” he said.

“Our work can show that to truly understand them, they need to be studied more comprehensively.”

There can also be major implications for managing natural hazards.

“First, the new earthquake catalog we’ll be producing will really improve our understanding of volcanic activity in New Zealand, and how to differentiate whether an earthquake is related to volcanism or tectonics.”

This would be useful for measures such as the volcanic alert levels that GeoNet uses to classify hazards.

“Perhaps most importantly, if we determine that volcanic activity could be triggered by a slow-slip earthquake, it is very plausible that a large earthquake in the Hikurangi subduction zone would have triggered an eruption, and there is some geological evidence that this may occur in the Hikurangi subduction zone,” he said.

“A double risk scenario would be very difficult to manage but it could be the type of scenario that needs to be planned.”

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