Almost 9,000 people have told GeoNet they felt a magnitude 4.6 earthquake that struck 30 kilometres north-west of Porirua at 11.09pm on Friday, with reports arriving from across the lower North Island and the top of the South Island.
GeoNet put the quake at a depth of 38 kilometres and graded the location quality as best, which is its highest confidence rating. The agency’s own summary of the event described the shaking as light close to the quake.
The epicentre sat offshore, in the water west of the Kāpiti coast, which is a large part of why nothing more came of it. Nobody was standing directly above the point where the rock moved.
By Saturday morning GeoNet had logged 8,971 felt reports for the quake, and the detail inside that number tells you more than the headline figure does. A total of 4,731 reports were graded weak and another 3,760 were graded light, which together account for about 91 percent of everything submitted. A further 454 were graded moderate, 13 strong, four severe and nine extreme. The small cluster at the top of the scale is the sort of scatter that turns up in any large batch of public reports, and it sits well outside the light rating GeoNet assigned to the event as a whole.
Newswire compared the location of every one of those reports against the epicentre. About 85 percent came from within 50 kilometres of it, and roughly 96 percent from within 100 kilometres. Almost all of the remainder arrived from the next 50 kilometre band out. Beyond 150 kilometres the reports fell away to a trickle, with the most distant sitting about 650 kilometres from where the quake began.
That is a tight, ordinary footprint for a quake of this size and depth. 1News reported that the shake was felt across Wellington, Manawatū-Whanganui, Taranaki and the upper South Island, and gathered reactions posted online in the minutes afterwards. One person described it as a “nice decent shake here in Wellington”. The New Zealand Herald reported isolated reports from as far north as Auckland and as far south as Christchurch.
The depth explains more about Friday night than the magnitude does. The faults that people in the capital think about most, including the Wellington Fault, sit within roughly the top 20 kilometres of the crust. At 38 kilometres, this quake was below that layer. Energy released that far down passes through more rock before it reaches the surface, so the shaking tends to arrive softer directly above the source but carries further out. The same magnitude at a depth of five kilometres under a suburb would have been a much sharper experience for the people living on top of it.
A second and far smaller quake followed about an hour later, at 12.13am, centred 10 kilometres south-east of Collingwood in Golden Bay at a depth of eight kilometres. GeoNet measured that one at magnitude 2.7. It drew 48 felt reports, a fraction of the Porirua total, and it was several hundred kilometres away from the first.
Friday was also a reminder of how much of New Zealand’s seismic activity happens where almost nobody lives. GeoNet logged nine earthquakes of magnitude 4 or greater in the 36 hours to Saturday evening. Only one of them, the Porirua quake, was anywhere near the populated mainland. The other eight were in the Kermadec region, between 450 and 1,225 kilometres north-east of Whangārei and Whakatāne, and they passed without anyone on land noticing.
The country has had a run of notable quakes in recent weeks. A magnitude 5.9 quake near Taumarunui woke much of the North Island before dawn on 25 July. That quake was both shallower and considerably larger, and it produced a very different morning for the people closest to it. Friday night’s quake released a tiny fraction of the energy of the Taumarunui event, because each full step up the magnitude scale represents about 32 times more energy released.
Nothing about Friday night points to damage. The light classification, the depth and the offshore location all say the same thing, and no reports of damage had emerged by Saturday morning.
Quakes in this range are the ordinary background noise of living on a plate boundary, and the honest value in them is as a prompt rather than a warning. The advice has not changed. Drop, cover and hold while the ground is moving, then check on the people around you. Anyone near the coast should move inland or to higher ground straight away if shaking is long or strong enough that it is hard to stand, without waiting for an official message, because a locally generated tsunami can arrive faster than any warning system can reach a phone.
It is also worth knowing that the felt reports themselves are what make an analysis like this possible. GeoNet’s shaking maps are built almost entirely from what ordinary people submit in the minutes after a quake, which is why 8,971 responses from a Friday night is genuinely useful data rather than just a curiosity.
You can see where Friday’s quake sits alongside every other recent shake, and file your own account of what you felt and where you were, on our live New Zealand earthquake map.
Did the Porirua quake wake you on Friday night, and what did it feel like where you were? Tell us in the comments below.
This article was written by AI, briefed to report the facts, hopefully without some of the bias people bring to the job 🙂
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