Tectonic Summaries
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.
The March 4, 2021 M 8.1 earthquake near Kermadec Islands, New Zealand occurred as the result of reverse faulting in the Tonga-Kermadec subduction zone at a depth of ~22 km. The Tonga-Kermadec subduction zone 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 a result of reverse faulting, either by low angle thrust motion on a west-dipping fault or by high angle reverse motion on an east-dipping fault. The location, depth, and style of faulting are consistent with the earthquake having occurred on or near the west-dipping subducting plate interface between the Pacific and Australia plates.
The March 4 2021 M 8.1 earthquake was preceded ~107 minutes by an M 7.4 thrust earthquake located ~50 km west of the M 8.1. The proximity and timing of the two events indicates that the M 7.4 was likely a foreshock of the M 8.1 earthquake. In terms of seismic moment, the M 8.1 was ~11 x larger than the M 7.4 foreshock. Additionally, a M 7.3 oblique reverse faulting earthquake occurred ~6 hours prior to the M 8.1 and ~900 km to the south; however, the spatial and temporal gap between these earthquakes likely indicates that static stress changes induced by the earlier M 7.3 did not directly cause the M 7.4 or M 8.1 earthquakes.
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.
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 175 km long x 75 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. In the past century, 215 earthquakes >M 6 have occurred within 250 km of the M 8.1 earthquake, including the M 7.4 that occurred ~107 minutes prior. The largest previous event was a M 8 earthquake in January 1976, more than 100 km to the north, with no associated casualties or damage.
