Should we expect more shakes following 6.8 earthquake off lower South Island?

• A 6.8 magnitude earthquake struck 160km northwest of Snares Islands at 2.43pm on Tuesday.
• Over 4500 people felt the quake, with reports of items falling and buildings swaying.
• It hit near a tectonic plate boundary where the Australian plate dives below the Pacific plate.

Scientists say the South Island quake region is still “very poorly understood” despite continued warnings of the high possibility of a devastating earthquake in the near future.

A 6.8 magnitude quake hit 160km northwest of Snares Islands this week, according to the GeoNet website, at a depth of 12km at 2.43pm.

GNS Science seismic duty officer Sam Taylor-Offord said the quake was widely felt throughout the lower South Island, with nearly 5000 people returning reports to GeoNet.

Officials quickly assessed the threat and determined evacuations weren’t necessary.

The event occurred near a tectonic plate boundary where the Australian plate dives below the Pacific plate – the opposite way to how the two plates “subduct” in the North Island.

The coastal areas at risk from strong currents and unpredictable surges. Image - Nema

A recent study highlighted the tsunami threat facing Southland, where a nearby earthquake zone is thought capable of unleashing 12m-high waves.

“This area is actually very poorly understood,” said seismologist Dr Finn Illsley-Kemp, whose Victoria University colleagues have been carrying out work in the region.

“There’s been far less research done here compared to other regions, leaving many unknowns about the exact tectonics – but it’s hosted some of our biggest earthquakes.”

Illsley-Kemp said GNS Science experts would be focusing on this week’s earthquake’s focal mechanism, telling them how the fault moved during the earthquake.

That information helped assess potential tsunami risks, which are an ever-present danger facing Southland.

GeoNet said the plate boundary south of the South Island was active and indicated the potential for large, tsunamigenic earthquakes.

“The quake occurred in an area of active subduction and at this point and was hard to say whether this earthquake happened on the plate interface or an overlying fault,” GeoNet said.

Subduction occurs when the edge of a plate is forced below the edge of another.

“Both the uncertainty in the earthquake depth and a limited understanding of the shape of the plate interface at depth contribute to this.”

Earthquakes like this have occurred in the past and will occur again, GeoNet said.

“The plates are converging at about 35 mm per year at this boundary and earthquakes, like we felt yesterday, are a consequence of this converging movement.

“While we know a lot less about the tectonics of this region owing to its remote and underwater location, we should treat it just like we do our more familiar plate boundaries: as a region where large earthquakes and tsunamis are expected and should be prepared for.”

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