A silent mystery: the “7.0 earthquake” that Kiwi people didn’t feel this month

Explanation of slow slip earthquakes associated with the Hikurangi subduction zone, New Zealand. / GNS Science

New Zealand’s largest fault zone has just released energy equivalent to a magnitude 7.0 earthquake, through slow-burning ‘silent’ events associated with recent locust swarms over Gisborne.

Scientists have been watching several slow-slip earthquakes–silent but powerful events that roll over days, weeks, or months, rather than seconds–unfolding along the Hikurangi subduction zone.

These slow-moving earthquakes are imperceptible to us and can be observed through the GeoNet network of continuously operating GNSS stations, which are able to track millimeter-level changes in the motion.

They are relatively rare occurrences along a subduction zone – where the Pacific plate sinks westward under the North Island – and usually play out at shallow depths off the east coast, and at deeper levels near the Manawatu and Kapiti regions.

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One event has been happening under Manawatu since early 2022, but it has “gained a little bit of momentum” over the past few weeks, said Dr. Laura Wallace, a geophysicist with GNS Science.

Just in the past month, it has caused a movement of three to four centimeters at the plate boundary, about 30 to 40 kilometers below the surface.

Here’s our model of slow slip events at HikurangiSZ in the past month alone, based on geonet GNSS data. 3 SSEs happening at once in the last few weeks! pic.twitter.com/a7u2dpEvKI

— GNSGeodesy (@GNSGeodesy) January 19, 2023

Events in the area, which used to occur twice a decade and last from one to two years, had previously driven up to 30 cm of movement.

“We expect this to continue well into 2023,” Wallace said.

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“It’s not very big yet, and it’s likely similar to the ones we recorded in 2010 and 2014.”

Since the new year, a separate event off the coast of Tolga Gulf has generated enough motion to turn GNSS stations about 1 cm to 2 cm eastward, likely related to two recent swarms of small earthquakes.

One involved more than a dozen earthquakes between magnitude 1 and 2.5 near Ruatoria, and the other, centered around Tokomaru Bay, numbered 25 – some with a magnitude of 3.5.

“When slow-sliding earthquakes occur, we often see an increase in the number of small earthquakes in the same area,” Wallace said.

“This is because a slow slip event results in stress changes in the Earth’s crust, causing some nearby faults to rupture in small earthquakes.”

Source / NZ Herald

Another event, off the southern coast of Hawke’s Bay, began on January 7.

In all, Wallace said the amount of slow-sliding motion around the boundary over the past month is roughly equivalent to the energy release of a 7.0 earthquake.

“If each of the East Coast slow-slip spots went one by one, you would still be looking at something like 6.8,” she said.

“This shows us that these slow slip events can absorb and release a lot of pent-up energy from plate tectonics in a slow way, rather than earthquakes, which is a good thing.”

The events come several months after a large slow-slide event near Hawke’s Bay last year, on a section of a crevasse between where the most recent events occurred.

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Recent programs that deployed dozens of seafloor sensors along the subduction zone, and attracted some $70 million in funding, have turned New Zealand into something of a global slow-slide event observatory.

“In October of last year, together with American and Japanese scientists, GNS scientists deployed our largest-ever collection of more than 50 seafloor sensors off the coast of Gisborne,” said Wallace.

On land, a large, temporary network of seismometers has been deployed around the southern Hawke’s Bay area to detect earthquakes during slow-slip events, as part of a new project supported by the Marsden Fund.

“So this latest sequence of slow-moving earthquakes is good timing for us to know a lot more than we ever have about slow-moving earthquakes in New Zealand.”

Slow slide events occur in an area where the Hikurangi subduction zone is moving from being ‘stuck’ down the southern North Island, to an area where the subduction zone is ‘creeping’ to the north, around Gisborne and Hawke’s Bay. Image / GeoNet

Scientists have been working on the puzzle from several directions, with studies focused on seafloor measurements, mathematical modeling, and ancient earthquake traces gleaned from tiny ocean creatures that lived thousands of years ago.

They are particularly interested in answering why slow earthquakes tend to occur in regular cycles—something that could be because the fault zone has reached some form of threshold after being continually loaded with pressure through plate motion, or primed through build-up of groundwater.

Advertise with NZME.Source / GeoNet

The Hikurangi subduction zone is believed to be capable of generating monstrous “giant” earthquakes, with the latest research indicating a 26 percent chance of an 8.0 or larger strike down the North Island within the next 50 years.

The potential effects of such a catastrophe were alarming: A report commissioned by the EQC estimated that one event in 500 years could cause tens of thousands of deaths and injuries, along with tens of billions of dollars in property loss.

Because slow-sliding earthquakes have been shown to precede subduction zone disasters — including Japan’s devastating magnitude 9.1 event in 2011 — researchers think they could be key to predicting major disasters.

But their occurrence doesn’t necessarily mean that Hikurangi’s next big shake-up was on its way, with the vast majority of events occurring without cracks.

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