March 4, 2021 Earthquake: Gisborne, New Zealand

Reference #1053:

Tectonic Summary

The March 4, 2021 M 7.3 earthquake northeast of Gisborne, New Zealand occurred as the result of oblique reverse faulting at approximately 20 km depth near the plate boundary between the Pacific and Australia plates. Preliminary moment tensor solutions indicate that the earthquake likely occurred on either a north-east dipping reverse fault with oblique right-lateral strike slip motion, or on a south-east dipping reverse fault with oblique left-lateral strike slip motion. The Pacific plate begins its westward subduction beneath the Australia plate at the Kermadec and Hikurangi subduction zone trenches with a velocity of about 47 mm/yr. The location, depth, and faulting mechanisms associated with this earthquake indicate that the earthquake likely occurred within the subducted Pacific lithosphere very close to the oceanic trench between these two tectonic plates.

While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Oblique faulting events of the size of the March 4 earthquake are typically about 85x25km in size (length x width).

The Australia-Pacific plate boundary region east of the North Island of New Zealand has a history of large earthquakes both along the plate boundary proper, within the subducting Pacific Plate, and in the overlying Australia plate. Seismic activity is especially high in the Kermadec Islands region to the north. Within 250 km of the March 4, 2021 event, four earthquakes of M 7 or greater have occurred in the past century, with the most recent being an M7.0 oblique normal faulting earthquake in September 2016. The March 4, 2021 earthquake occurred ~45 km east-southeast of the September 2016 normal faulting earthquake. There have been 29 earthquakes of M 6 or greater within 250 km of the March 4 earthquake over the same time period. Despite the subduction zone environment, previous earthquake mechanisms in the vicinity of the March 4 earthquake exhibit a diverse range of faulting styles including predominantly thrust motion, normal motion, and oblique strike slip motion.