Source: Radio New Zealand
All five quakes were centred under Kerikeri Inlet at a depth of around 5km. Supplied / Hauraki Gulf Weather
Experts say a series of tremors around Kerikeri this week are a reminder that nowhere in New Zealand – even the country’s most seismically stable region – is immune from earthquakes.
Five quakes were recorded between 1 and 4 March, all centred below Kerikeri Inlet at a depth of about 5km.
The biggest, with a magnitude of 2.1, struck at 5.05pm on Tuesday.
Residents at Opito Bay, on the northern side of Kerikeri Inlet, described their homes shaking, hearing a “loud thud” or a booming sound like thunder, and their dogs “going crazy”.
Another said it felt like a car had crashed into the side of the house.
The quakes were also felt at Rangitane and Doves Bay, and at Wharau Bay on the opposite side of the inlet.
While small by New Zealand standards, any quakes in Northland get attention due to their rarity.
Seismic duty officer Sam Taylor-Offord said 10 earthquakes, including this week’s Kerikeri cluster, had been recorded in Northland since Earth Sciences New Zealand (formerly GNS Science) expanded its monitoring network in 2022.
He said the Kerikeri quakes were not recorded automatically by GeoNet, the country’s geological hazard monitoring system, and so did not show up immediately online.
That was because the earthquakes were small and occurred at the margins of the monitoring network, which was tailored to large quakes in seismically active areas, such as around the volcanoes of the central North Island.
The shakes were, however, recorded by Northland’s local seismometer network.
The quakes were felt at Opito Bay, Doves Bay, Rangitane (pictured) and Wharau Bay, on either side of Kerikeri Inlet. RNZ/ Peter de Graaf
Using that data, staff at the 24/7 National Geohazard Monitoring Centre were able to pinpoint the locations and manually add the five Kerikeri earthquakes to the record.
“These events correspond to reported times of shaking around Opito Bay and Doves Bay over the last couple of days. We’re now looking to see if there are more earthquakes in the sequence,” he said.
There was no known fault under Kerikeri Inlet so the cause could be best understood as the Earth’s crust breaking under accumulated stress.
Taylor-Offord said he was grateful to Northlanders who had reported the quakes at www.geonet.org.nz.
Such feedback helped Earth Sciences NZ update its records more accurately, especially around the margins of its monitoring network.
Taylor-Offord said earthquakes were rare in Northland because the region was far from the active plate boundary, where the Pacific plate was being forced under the Australian plate.
At its closest point to Northland, the boundary, known as the Hikurangi Subduction Zone, ran parallel to the east coast off Hawke’s Bay and Te Tai Rāwhiti, before continuing northeast along the Kermadec Trench.
That distance meant there was relatively little stress in the Earth’s crust under Northland.
The last big earthquake in Northland, in December 1963, was a magnitude 4.8 shake east of Kaitāia.
Northland’s average of about three earthquakes a year compared to thousands recorded every year along the East Coast (Hikurangi Subduction Zone) and Southern Alps (Alpine Fault) plate boundary regions.
Taylor-Offord said the Kerikeri Inlet shakes were a good reminder to expect earthquakes anywhere in New Zealand.
Anyone who experienced a large earthquake should remember the advice to “drop, cover and hold”.
Anyone who lived near the coast, as was the case for many Northlanders, should also be alert to the possibility of tsunamis.
“If the shaking is long or strong, get gone,” he said.
Northland’s monitoring network had 12 seismometers, with equipment at Whangaruru, Kawakawa and Omahuta closest to the Kerikeri earthquakes.
– Published by EveningReport.nz and AsiaPacificReport.nz, see: MIL OSI in partnership with Radio New Zealand