The 2021 MW 8.1 Kermadec Earthquake Sequence: Great Earthquake Rupture Along the Mantle/Slab Contact

Abstract

Most great earthquakes on subduction zone plate boundaries have large coseismic slip concentrated along the contact between the subducting slab and the upper plate crust. On 4 March 2021, a magnitude 7.4 foreshock struck 1 hr 47 min before a magnitude 8.1 earthquake along the northern Kermadec island arc. The mainshock is the largest well-documented underthrusting event along the ∼2,500-km long Tonga-Kermadec subduction zone. Using teleseismic, geodetic, and tsunami data, we find that all substantial coseismic slip in the mainshock is located along the mantle/slab interface at depths from 20 to 55 km, with the large foreshock nucleating near the down-dip edge. Smaller foreshocks and most aftershocks are located up-dip of the mainshock, where substantial prior moderate thrust earthquake activity had occurred. The upper plate crust is ∼17 km thick in northern Kermadec with only moderate-size events along the crust/slab interface. A 1976 sequence with MW values of 7.9, 7.8, 7.3, 7.0, and 7.0 that spanned the 2021 rupture zone also involved deep megathrust rupture along the mantle/slab contact, but distinct waveforms exclude repeating ruptures. Variable waveforms for eight deep M6.9+ thrusting earthquakes since 1990 suggest discrete slip patches distributed throughout the region. The ∼300-km long plate boundary in northern Kermadec is the only documented subduction zone region where the largest modeled interplate earthquakes have ruptured along the mantle/slab interface, suggesting that local frictional properties of the putatively hydrated mantle wedge may involve a dense distribution of Antigorite-rich patches with high slip rate velocity weakening behavior in this locale.