Bay of Plenty earthquakes could continue for weeks - GNS

By Contributor

A swarm of earthquakes is proving unsettling for those in Bay of Plenty. Photo / Geonet

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

Hundreds of tremors have been occurring over the last few days, with the largest a 4.8 magnitude.

Some Kawerau businesses had to shut down because stock repeatedly fell 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 Mt Putauaki, a dormant volcano on the edge of the Taupō Volcanic Zone.

The Institute of Geological and Nuclear Sciences (GNS) said 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 said.

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.

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

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 said.

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

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

Risk of landslides

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

Landslides could be triggered by heavy rain or earthquakes and could occur with little or no warning. Some couldoccur without any obvious trigger. Homes near hills or steep slopes and cliffs were 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 were continuing to monitor the activity and expected the most likely scenario was for the swarm activity to continue decreasing over the next few days.