Why have 760 earthquakes shaken a North Island town in the past ...

A “swarm” of hundreds of earthquakes has been shaking the area around the Bay of Plenty town of Kawerau in recent days, with some tremors getting to above magnitude 4.0.

While quake swarms aren’t overly rare in the area, the scale of some of the shaking has caught the attention of scientists. Here’s the latest information from GNS about the swarm and what’s likely to happen next.

How common are such swarms?

The current earthquakes in Kawerau were in line with historic activity in the region. Swarms were a common feature of the Central Volcanic Region, although swarms with as many magnitude 4.0 events as the current episode were “relatively infrequent,” GNS said.

There were swarms in the Kawerau area in 2018 and 2019. Both those swarms lasted for several days, during which several hundred events were recorded.

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How many bigger quakes have there been in recent days?

GeoNet says there were eight earthquakes of magnitude 4.0 or above during the weekend. The last of them was on Sunday morning.

How many earthquakes have there been?

More than 760 had been recorded by 10am Monday, GNS said. Many have been less than 5km deep.

There was no scientific way to know for certain how much longer the swarm would last, but the activity would most likely decrease in the next few days.

GNS Science

Depth and size of earthquakes in the Kawerau area between Friday and Sunday

What is an earthquake swarm?

Swarms are a collection of quakes about the same size, happening in a localised area, usually over a short time period (hours to days to weeks). Swarms usually don’t have a mainshock or larger quake that starts off a sequence, GNS says.

Will this swarm lead to a larger earthquake?

“We can’t predict earthquakes, so we can never say with certainty that a larger earthquake won’t occur – these are the shaky isles,” GNS says.

“However, by far the most likely scenario is that the level of seismic activity will decrease over coming days-to-weeks.”

Supplied

Bottles crashed down off the shelves at Super Liquor Kawerau on Saturday morning.

Is this swarm related to volcanic or geothermal processes?

Although it is a geothermal area, it would be very rare for geothermal activity to lead to such large magnitude earthquakes, GNS says. The location of the swarm is outside a volcanic area, so there is no indication of volcanic unrest, and no relation to the recent unrest at Taupō or Ruapehu.

Given the tectonic setting, history of swarm activity, and the normal-faulting mechanisms identified for some of the larger events, it’s most likely this swarm is due to movement on active faults.

Is this activity associated with a particular fault?

Earthquakes of this size aren’t regularly associated with a particular fault, partly because of uncertainties of location and depth, and partly because even fairly small fractures can host events like these, GNS says.

Many faults and fracture systems had been mapped in the area, which was to be expected given the location.