Tectonic Summary

The March 4, 2021 M 7.4 earthquake near Kermadec Islands, New Zealand occurred as the result of reverse faulting in the Tonga-Kermadec subduction zone, which extends north-northeast from the North Island of New Zealand for more than 2,500 km through Tonga to within 100 km of Samoa. Focal mechanism solutions for the event indicate rupture occurred as reverse faulting on a moderately dipping plane striking either SSW or NNE.

At the location of the earthquake, the Pacific plate moves westward relative to the Australia plate at a velocity of about 60 mm/yr, subducting to the west beneath the Australia plate at the Tonga and Kermadec Trenches. The eastern edge of the Australia plate may itself be viewed as a collection of microplates whose relative motions help to accommodate the overall Pacific-Australia convergence and associated back-arc spreading. The location and focal mechanism solutions of the earthquake are consistent with it occurring on or near the westward-dipping interface between the subducting Pacific plate and the overriding Australia plate (in detail, the overriding Kermadec microplate).

While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Reverse events of the size of the March 4 earthquake are typically about 70 km long x 35 km wide (fault length x fault width).

The interaction between the Pacific and Australia plates creates one of the most seismically active tectonic environments in the world. Since 1976, a 200-km-long section of the Tonga-Kermadec subduction zone, which includes the epicenter of the March 4th earthquake, has produced more than 50 earthquakes of M 6 or larger. The largest event was a M 8 earthquake in January 1976, more than 100 km to the north, with no associated casualties or damage. The March 4, 2021 M7.4 earthquake occurred ~4 hours after and ~900 km north of a M 7.3 earthquake on March 4 in the same subduction zone. While the two earthquakes occurred in relatively close temporal and spatial proximity to each other, the ~900 km distance between them likely indicates that static stress changes induced by the earlier earthquake did not directly cause the M 7.4 earthquake.

Event Details

Event Map

Tectonic Summary

Tectonic Summary

Historical Magnitude 4.5+ (100km)