The swarm of earthquakes plaguing Bay of Plenty residents could continue for weeks, a GNS seismologist says.

More than 760 earthquakes have been recorded so far over the last few days, with the largest a 4.8 magnitude.

"The earthquakes are shallow, with many recorded as less than 5km deep," says GNS Science Seismic Duty Officer John Ristau.

"A swarm is the term used to describe earthquakes of similar size that are clustered together in space and time, and they are not unusual.

"The current swarm is in keeping with historic activity in the region and, though there is no scientific way to know for certain, we think the most likely scenario is that we will see this activity decrease over the next few days," says John.

"We believe this swarm to be tectonic in nature, due to the movement of active faults, and not related to volcanic unrest or geothermal activity."

Some Kawerau businesses had to shut down with stock repeatedly thrown from shelves, and some people decided to leave for a few days until the shaking stops.

A crack may have formed in the side of nearby Putauaki, a dormant volcano on the edge of the Taupō Volcanic Zone.

The possible crack in the side of nearby Putauaki would be investigated further today. Photo: Supplied.

The Institute of Geological and Nuclear Sciences -GNS - says earthquake swarms were quite common in New Zealand.

While the current frequent quakes were unsettling for those across the Bay of Plenty region, they were reminiscent of similar events in 2018 and 2019, it says.

Duty seismologist John Ristau told Morning Report the usual earthquake pattern was one big quake followed by a group of aftershocks until activity tapered off over time.

But the Bay of Plenty region is experiencing a swarm which is a series of earthquakes of similar size and often shallow which was normal for the Taupō Volcanic Zone. Typically they would be within 10km of the surface.

Location of the earthquakes during Saturday's swarm near Kawerau. Image: GNS Science.

So far there have been several over 4 magnitude and many others of a lesser magnitude, Ristau says.

Seismologists were still puzzling over the differences between the two kinds of earthquakes but in both cases the stress built up, however, a swarm of quakes occurred because the stress could not be released in one big shake.

"There's no fault large enough for that one large earthquake to happen. So instead it triggers earthquakes a little bit smaller on other faults in order to release that stress.

"These can go on for several days, they can go on for weeks or months unfortunately and it then tapers off over time."

It was not necessarily a harbinger of a big earthquake to come although he was not promising this, he says.

There would be some people who would recall the series of small earthquakes that preceded the magnitude 6.5 Edgecumbe earthquake on 2 March 1987.

He expects the possible crack in the side of nearby Putauaki would be investigated further today.

Risk of landslides

GNS said there has been some landslide activity in the area related to these earthquakes, including rockfalls. This extent of landsliding is to be expected for M4-5 earthquakes. The ground is likely more vulnerable to landslides now due to the recent cyclone and rain, and there are many landslides still visible from Cyclone Gabrielle.

Landslides can be triggered by heavy rain or earthquakes and can occur with little or no warning. Some can occur without any obvious trigger. Homes near hills or steep slopes and cliffs are most at risk - so residents should be on the lookout for cracks or movement that could be a warning sign, and get quickly to safety.

GNS said given the tectonic setting, history of swarm activity, and the normal-faulting mechanisms identified for some of the larger events, it was most likely that this swarm was tectonic in nature.

Although it is a geothermal area, it would be very rare for geothermal activity to lead to such large magnitude earthquakes. 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.

GNS teams are continuing to monitor the activity and expected the most likely scenario was for the swarm activity to continue decreasing over the next few days.

Since the EQ swarm in Kawerau began yesterday, we have located over 600 earthquakes. We know the shaking has been disconcerting for folks in the area and our teams continue to monitor. Check out our new story covering some of your FAQs about this event: https://t.co/ixWvby8mFx pic.twitter.com/ndGwwOaQqu

— GeoNet (@geonet) March 19, 2023

We know ongoing earthquake activity is unsettling and encourage people that are wanting information and advice to keep an eye on our GeoNet app and web news.

Remember to drop, cover and hold in an earthquake and follow advice from Bay of Plenty Civil Defence.>

More information about the earthquake swarm is available here

RNZ