Severe shaking after M5.9 earthquake, series of aftershocks hit North Island, New Zealand

A strong and shallow earthquake registered by the GeoNet as M5.9 hit New Zealand’s North Island at 22:16 UTC on April 25, 2023 (10:16 LT, April 26). The agency is reporting a depth of 21 km (13 miles). The USGS reported it as M5.4 at a depth of 25 km (15 miles).

The epicenter was located 45.2 km (28.1 miles) ESE of Dannevirke (population 5 650), and 77.1 km (47.9 miles) SSW of Hastings (population 90 600), Hawke’s Bay, New Zealand.

The mainshock was closely followed by a M5.4 aftershock at a depth of 13 km (8 miles); M5.2 at a depth of 23 km (14 miles), according to the USGS.

By 08:30 UTC on April 26, GeoNet registered 87 aftershocks, including the M5.4 just 3 minutes after the mainshock.

Hawke’s Bay Civil Defence Emergency Management Group controller Ian Macdonald confirmed there had been no major damage sustained in Pōrangahau — the closest populated area to the epicenter, although the local school and some residents had evacuated.

Duty seismologist at GNS Science, Dr. Katie Jacobs​, said the earthquake and many aftershocks had been felt “strongly and widely” across the North Island, with GeoNet receiving more than 22 000 felt reports.

Many people said it was the strongest shaking they have ever felt.

“Pōrangahau is located along the Hikurangi Subduction Zone which drives much of the earthquake activity along the East Coast of the North Island,” GeoNet said. “The region also experiences slow slip events – which are earthquakes that happen very slowly (and are not felt).”

“Scientists are continually doing work to understand more about our seismic activity and the subduction processes behind it. Some of these scientific investigations involve field work, and the deployment of additional sensors and equipment. We’d like to thank all those people that help advance our scientific understanding by hosting our equipment and allowing us access to make observations.”

“Many people reported feeling the shaking as long or strong and may have evacuated to higher ground,” GeoNet added and applauded people that have acted quickly and followed Aotearoa New Zealand’s Long or Strong Get Gone advice. During strong earthquake events, people are advised to head to higher ground if they feel shaking that is long or strong and follow the instructions of their local Civil Defence. In this case, the earthquakes did not invoke a tsunami threat.

The agency issued a warning about possible landslides that may be triggered by heavy rain or earthquakes, with homes near hills or steep slopes most at risk. “If you live near a hill or steep slope, watch out for cracks or movement that could be a warning sign.”

The USGS issued a Green alert for shaking-related fatalities and economic losses.

Overall, the population in this region resides in structures that are highly resistant to earthquake shaking, though some vulnerable structures exist. The predominant vulnerable building types are reinforced masonry and unreinforced brick with timber floor construction.

Recent earthquakes in this area have caused secondary hazards such as landslides that might have contributed to losses.

References:

¹ Severe quakes shake Porangahau – GeoNet – April 26, 2023

² ‘Biggest we’ve ever felt’: Severe earthquake hits Hawke’s Bay, aftershocks likely – Stuff – April 26, 2023