A large M6.9 quake has hit near Raoul Island, a remote outpost in the Kermadec Islands chain of New Zealand, and approximately 800 to 1000 kilometers northeast of the country’s north island.
The initial earthquake was followed within minutes by a M6.3; approximately 45 minutes later, the last quake, a M6.2 struck. All of the tremors were located at a depth of approximately 20 kilometers.
No damage on Raoul Island was reported – six Department of Conservation workers were present on the island at the time of the quake, and none were injured.
Authorities at the Pacific Tsunami Warning Centre deemed that there was no widespread tsunami threats present as a result of the quakes, but warned that the local threat of tsunamis within 100 kilometers of the epicenter was a possibility.
What’s behind these large earthquakes in New Zealand?
Tectonic Setting of New Zealand
New Zealand’s tectonic setting is particularly active, and therefore no stranger to seismic activity. Situated between the Australian and Pacific tectonic plates, this region is home to a variety of fault types: convergent faults, where denser crust dives beneath less dense crust, and transform faults, where plate segments are shearing past one another laterally.
Today’s quake was centered along a convergent boundary – specifically, a subduction zone, where the denser Pacific plate is slowly plunging beneath the less dense Australian plate. Zones such as these are hotbeds of seismic activity as these masses of earth override and subduct, or move underneath, one another.
Since 1900, over 40 M7.5+ earthquakes have been recorded on the Kermadec-Tonga trench, the location of today’s earthquake.
Subduction Zone Earthquakes
Because of the sheer size of subduction zones, these seismic zones produce massive earthquakes. Not only are subduction zones a lateral feature on the surface of the Earth, they extend deep down, forming a vertical feature as well.
For contrast, the San Andreas fault is a transform fault in which both plates are moving past each other laterally. This fault may extend down only as deep as the crust, which greatly limits its available area of rupture.
A subduction zone, on the other hand, extends many kilometers downward into the mantle, the semi-molten layer of the Earth found between the crust and the outer core. Earthquakes can occur on this downgoing slab, at the interface between the downgoing and overriding plate and within the plates themselves.
New Zealand’s Seismic Past and Future
While much of New Zealand’s seismic activity occurs in the oceans, and away from its main population centers, the country itself is unfortunately no stranger to earthquakes. The M7.8 Hawke’s Bay earthquake, which occurred on the northern island in 1931, was the country’s largest earthquake ever.
The transform Alpine fault, which bisects the country’s southern island, has not experienced an M7.5+ earthquake in 170 years. Based on studies of the fault’s current strain, however, a similar earthquake is likely again.
With New Zealand’s geologic complexity, it will undoubtedly continue to see a very active seismic future.