Explained: Why last night’s big quake rocked some harder than others

• Last night’s strong and shallow 5.3 quake was felt by more than 23,000 people: mostly across the lower North Island.
• A seismologist says it wouldn’t have registered too much to those living around and beyond the Taupo Volcanic Zone, where seismic waves didn’t easily travel through its “spongy” geology.
• The quake also struck in a complex and little-understood environment where some major events have hit in the past.

To people in Manawatū or Wellington, last night’s big quake near Eketahuna felt like a “truck crashing” – but to others farther north, it would have hardly registered at all.

Seismologists have explained why: and it has much to do with the diverse and complex geology that shapes our shaky isles.

More than 23,000 people have reported feeling the magnitude 5.3 quake, which hit just east of the Tararua town, at a depth of about 35km, at about 11.26pm.

A 5.3 magnitude earthquake struck 10km east of Eketahuna. Photo / GeoNet

A quick glance at GeoNet’s “felt map” shows the bulk of those reports came from around the lower North Island, along with Taranaki and Hawke’s Bay – with far fewer stemming from those living in or north of the Taupō Volcanic Zone.

For those in Wairarapa, Tararua and Manawatū, and closer to the epicentre, the quake’s strong shaking owed to its relatively shallow depth, Victoria University seismologist Dr Finn Illsley-Kemp said.

“People in areas of the central North Island, like Taupō, wouldn’t have felt it as much as people to the south towards Wellington, because that part of the country around the volcanoes is much more ‘spongy’ – so the energy can’t transmit as much.”

Local geological conditions also played a major role in amplifying or dampening the felt shaking.

Sedimentary basins, like the Hutt Valley near Wellington, could trap and amplify the seismic waves, leading to more noticeable shaking.

Whether last night’s quake struck on a known fault could prove tough to pin down, he said, given the dense network of faults in the area.

The mainshock hit in a complex “transpressional” fault environment, where the Australian and Pacific plates were converging and sliding past each other – and gradually building the mountain ranges of the central North Island.

GNS Science seismologist Dr Bill Fry said it was also unclear whether the quake struck within the cold and vast “slab” of the Pacific plate diving or “subducting” beneath the North Island - or whether it occurred within the interface between the two plates.

“There are a few little weird, quirky things about it which make me not have confidence that it was either.”

While last night’s jolt would stand out little in the books – more than 60 quakes of that size have been recorded in New Zealand in just the past year – Illsey-Kemp said the region was known to have caused some of our largest known events.

That included the goliath 8.2 Wairarapa Earthquake in 1855, which transformed Wellington’s shoreline and left seven to nine people dead.

“Earthquakes of those sizes are possible all along that mountain belt – so it’s another one of those hazards that are a bit under-appreciated,” Illsley-Kemp said.

Fry said last night’s event also offered a useful way to test a new GNS-led tool called Rapid Characterisation of Earthquakes and Tsunami, or RCET, with which scientists were trying to fully understand a given quake’s complex, three-dimensional nature.

It could eventually allow scientists to quickly gauge an earthquake’s ability to cause widespread shaking, trigger tsunamis and landslides, and damage infrastructure like roads and buildings.

Jamie Morton is a specialist in science and environmental reporting. He joined the Herald in 2011 and writes about everything from conservation and climate change to natural hazards and new technology.

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