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

The slight earthquake was felt in the lower half of the North Island. Image / GNS science

Many Hawke’s Bay residents were surprised to learn that Geonet reported a 5.8 magnitude earthquake overnight as “light”.

The earthquake’s epicenter was 25 km east of Stratford with an estimated depth of 187 km.

The shock is said to have been felt from the central and lower North Island to Christchurch in the south.

After the quake, many Hawke’s Bay residents went to the community’s Facebook pages and talked about the “massive tremor,” with some asking how a 5.8 magnitude earthquake is considered minor.

The quake has been rated “severe” in Geonet’s “Felt Reports” feature by several people in Hawke’s Bay, Manawatu, Whanganui, Wairarapa, and Wellington, as well as near the Taranaki epicenter.

GNS Science Seismic Duty Officer Elizabeth Abbott said GNS records the maximum ground acceleration on stations with heavy movement across the country.

Wednesday night’s earthquake was reported as minor as it did not record the “movement intensity” required for a higher classification.

“How an earthquake is felt by people depends on the strength, depth and the path that the earthquake’s energy takes to get to a person’s location,” said Abbott.

“Whether or not the energy moves along the descending plate makes a huge difference in how much of the earthquake’s energy someone could feel.”

The people of Hawke’s Bay may have shivered more as the earthquake energy migrated along the plate closer to the east coast.

On social media, many Hawke’s Bay residents said they were alerted by the new Android earthquake warning system seconds before the shock.

While some found the warning helpful, others said, “It was a little scary just waiting for it”.

Not everyone has received an alert about the Android Earthquake Alerts system, which is still under development.

Across the country, kiwis reported receiving a notification on their phone through the Android Earthquake Alerts system a few seconds before the shaking began.

Google selected New Zealand and Greece to test its new system, which uses cell phones to detect seismic waves, then centralize and analyze the information and alert users to their location.

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

However, they welcome any innovation that helps strengthen New Zealand’s earthquake resilience and complements the crucial work of GNS Science and GeoNet, Abbott said.

The National Emergency Management Agency (NEMA) is the official agency for providing advice and warning to New Zealand.

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

“Drop, Cover and Hold and ‘Long or Strong, Get Gone’ when you are near the coast.”

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