Magnitude 7 quake in Tonga - What’s next?

A magnitude 7 earthquake struck Tonga early Monday, prompting tsunami alerts that were lifted later in the morning.

Authorities say the quake occurred around 1.20am local time, with its epicentre located approximately 79 kilometres from Pangai in the Haʻapai group.

Speaking to William Terite on Pacific Mornings, John Ristau, a seismologist at GNS Science, confirms that the quake’s depth was approximately 20 to 25 kilometres.

To put this in perspective for those in Aotearoa, Ristau says that just last week, many people felt a magnitude 6.8 earthquake in the South Island.

“An earthquake of that size you will feel over a large distance, it could be like hundreds of kilometres, a thousand kilometres away, and you feel it,” he says.

“It'll be more of just probably like a slight, slow, swaying, you know, especially if you're in a taller building or something.

“The greater danger for earthquakes like that, of course, is the tsunami, not so much the shaking of the earthquake when you have the offshore earthquakes.”

What’s next?

Since the quake struck offshore, Ristau explains that it was “far enough away from population” centres to limit damage, meaning residents mainly experienced strong shaking.

“If it was close to shore or on shore, that could be a very serious situation, but this was out in the ocean… maybe minor damage like picture frames falling off a wall or something like that, but nothing serious.”

Watch John Ristau’s full interview below.

In terms of future risks, Ristau points to tsunami threats associated with offshore quakes, although an earlier warning has since been lifted.

“There have been a few aftershocks; the largest was just over magnitude six, and there have been a few that have been in the five- to six-range,” he says.

“There's probably a lot more aftershocks. It's just that it's out there in a very remote location in the middle of the ocean, and we can typically only get a good handle on the larger earthquakes.”

He says the greatest risk typically comes from an immediate aftershock following the initial quake, but that the “risk falls off pretty quickly”.

“So if you don't have one in the first 12 hours or something like that, it's not likely that you're going to get another large earthquake.”

Ring of Fire: Large Pacific quakes ‘not unusual’

Ristau says that the Pacific is “quite seismically active” and that earthquakes of magnitude seven and above are not unusual in the region, especially due to the presence of the Ring of Fire (ROF).

The ROF refers to the vast Pacific Plate and its intersections with more minor and major tectonic plates, resulting in frequent tectonic and volcanic activity around the ocean’s perimeter.

Ristau says that New Zealand is situated on the boundary between the Pacific Plate and the Australian Plate, where subduction occurs beneath the North Island. In the south, the plates reverse and slide past each other along the Alpine Fault.

Similar geological conditions extend around the Pacific, from Japan and Alaska to North and South America, creating a ring of intense geological activity where many of the world’s largest earthquakes occur.

Myanmar quake: Not connected to Tonga

Ristau clarifies there is “no connection” between the Tongan quake and the recent magnitude 7.7 event in Myanmar, noting they occurred in different tectonic settings.

“There wouldn't be any connection between [them as] they're too distant from one another,” he says, saying hazardous waves could be possible.

He describes the magnitude 7.7 quake in Myanmar, a larger one than Tonga’s, as the “worst-case scenario,” citing its shallow depth and densely populated location.

“Here in New Zealand, you could think about the Wellington Fault being capable of a similar size earthquake, very shallow type, similar type of earthquake and going right through the middle of a major population centre.”