5.8 quake: Why a sleep-shattering shake in the night was 'light'

The light earthquake was felt all the way through the lower half of the North Island. Image / GNS Science

Many Hawke's Bay residents were surprised to read that Geonet reported an overnight 5.8 magnitude earthquake as "light".

The epicentre of the earthquake was 25km east of Stratford, with an estimated depth of 187km.

The shake was reported to have been felt from the central and lower North Island, to as far south as Christchurch.

After the quake, many Hawke's Bay residents took to community Facebook pages talking about the "massive shake", with some questioning how a 5.8 earthquake is considered light.

The quake was rated as "severe" on Geonet's "felt reports" feature by several people in Hawke's Bay, Manawatu, Whanganui, Wairarapa and Wellington, as well as near its Taranaki epicentre.

GNS Science Seismic Duty Officer Elizabeth Abbott said GNS records peak ground acceleration on strong motion stations around the country.

Wednesday night's earthquake was reported as light as it did not record the "movement intensity" needed for a higher classification.

"How an earthquake is felt by people depends on the magnitude, the depth, and the path the energy from the earthquake takes to get to someone's location," Abbott said.

"Whether the energy is traveling along the subducting plate or not makes a big difference in how much of the earthquake's energy someone might feel."

People in Hawke's Bay may have felt stronger shaking as the energy from the earthquake travelled along the plate closer to the East Coast.

On social media, many Hawke's Bay residents said they were alerted seconds before the shake by the new Android Earthquake Alerts System.

While some found the alert helpful, others said "it was a little scary just waiting for it".

Not everyone got an alert via the Android Earthquake Alerts System which is still in ''development stage''.

Across the country, Kiwis reported that they had received an Android Earthquake Alerts System notification on their phone a few seconds before shaking started.

Google has chosen New Zealand and Greece to trial its new system, which uses mobile phones to detect seismic waves, then centralises and assesses the information and warns users according to their location.

GNS Science was not involved in the development of the Android Earthquake Alerts System.

However, they welcome any innovation that helps build New Zealand's resilience to earthquakes, and that complements the crucial work done by GNS Science and GeoNet, Abbott said.

The National Emergency Management Agency (NEMA) is the official agency for providing advisories and warnings for New Zealand.

Most importantly, everyone in New Zealand needs to remember what to do when they get the natural warning signs of an earthquake, she said

"Drop, Cover and Hold, and 'Long or Strong, Get Gone' if you are near the coast."