Why has the shaking from the Taranaki earthquake been described as “light”, even though it is widely felt

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Many people felt a deep earthquake in Taranaki some distance from the epicenter, but to a lesser degree, GeoNet says.

Geonet also described the intensity of the vibration as “light,” but acknowledged that it may not reflect everyone’s experience.

Seismologists have now explained the science behind these deep earthquakes and how we feel about them. The epicenter of the 5.8-magnitude earthquake at 11.49 pm on Wednesday was 25 km east of the city of Taranaki in Stratford, at a depth of 187 km.

Many of the 38,800 people reported feeling they came from the east and south of the North Island — with only a few from the central plateau and further north, GNS Geohazards seismologists Elizabeth Abbott and John Restau said in a GeoNet post.

Read more: * View of the ocean floor to give a clearer picture of earthquakes in the subduction zone * Mayor of the city where the earthquake centered with a magnitude of 5.9 on the Richter scale “I didn’t know it happened” * The earthquake was felt across the North Island

A major part of explaining how the earthquake feels has to do with the way it happened with plate tectonics. The Pacific plate lies in the east under the North Island which lies on the Australian plate.

GeoNet

The M5.8 earthquake struck at 11.49 pm on Wednesday, with its epicenter about 25 km east of Stratford.

Seismologists have said that deep earthquakes usually occur on the Pacific Plate.

The submerged rock slab at the top of the Pacific Plate was solid while the surrounding rock was a mixture of hard and molten rock, much softer.

Around the central plateau with its volcanoes, such as Ruapehu, there were more molten rocks.

“The hard rocks of the Pacific Sloping Plate are better at sending earthquake waves than the softer, semi-magmatic rocks,” said seismologists.

“In fact, the molten rock ‘quenches’ the energy from deeper earthquakes and helps explain why vibrations are felt near the epicenter when earthquakes occur in that region.

“When an earthquake occurs in this plate from or near the subducting plate, most of the earthquake’s energy is transmitted up and along the plate to the surface – near the east and south of the North Island – causing the earthquake-induced vibration to be felt more widely across those regions.”

GNSS Science

On the North Island, the Pacific plate sinks under the Australian plate.

On describing Wednesday night’s quake as “light,” seismologists said the expected shaking intensity of an earthquake on GeoNet was estimated from several factors.

This included vibration recorded by seismic devices, earthquake magnitude, depth, and proximity to population centers. It was estimated for an area close to the epicenter.

“This means that for a deeper earthquake, such as this one in which no shaking is felt near the epicenter, this estimate may not reflect everyone’s experience,” the seismologists said.

“Seismographs only measure vibration at the exact location where it is located.

There are many factors that affect how strongly people perceive vibration. Things like the type and location of the earthquake, local soil conditions, underlying geological structures, the type of building you live in, etc., all change the amount of vibration that is felt or recorded at a particular site.”

The agency has been working on ways to provide maps that would provide information that shook the entire country after a major earthquake.

On the New Zealand Modified Mercalli Density Scale, light is described as: Generally observed indoors, but not outside, as moderate shaking or tremor. Light sleepers may wake up. Walls may crack and shake glassware, crockery, doors, and windows.

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