Abstract

Typical plate boundary earthquakes in global subduction zone occurred along the contact between the crust in the upper plate and subducting slab. On 4 March 2021, a magnitude 8.1 earthquake struck the northern Kermadec, the largest known plate boundary event along the 1000-km long Kermadec subduction zone. It was preceded by a magnitude 7.4 foreshock at 1 hr 47 min earlier. Using teleseismic, geodetic and tsunami data, we find all substantial coseismic slip is located along the slab-mantle interface, with the foreshock nucleating near the downdip edge of the mainshock, and no significant shallow slip. Significant overlap of the slip regions between major foreshock and mainshock with a moderate number of small events occurred between them suggest a cascading faulting process. Extensive aftershocks locate updip along the slab-crust interface, which has hosted substantial prior moderate earthquake activity. The 200-km long slab-mantle interface in the northern Kermadec, the only region with major megathrust earthquakes along Kermadec subduction, has 11 M7+ deep megathrust earthquakes, including the 1976 M8 sequence. It provides a rare example of great rupture on the slab-mantle interface to understand frictional properties for olivine.