Shallow Megathrust Rupture during the 10 February 2021 Mw 7.7 Southeast Loyalty Islands Earthquake Sequence

Abstract

On 10 February 2021, an Mw 7.7 thrust earthquake ruptured the megathrust along the southeast Loyalty Islands within the strong bend in the plate boundary between the Australian plate and the North Fiji basin. The mainshock involved rupture with ∼50 s duration, with pure thrust slip concentrated in an east–west-trending slip patch with up to 4.2 m of slip extending from 10 to 25 km depth. Slip at depths <10 km is negligible on the curved fault surface, which conforms to the SLAB2 interface model. Static stress-drop estimates are ∼5.5 MPa, and the radiated energy is 2.38×10^15 J, with the moment-scaled value of 5.7×10^−6. The relatively shallow rupture from 10 to 25 km was moderately efficient in generating tsunami, with waves amplitudes up to 20 cm recorded in New Caledonia, New Zealand, Kermadec, and Fiji. Numerous M5+ normal-faulting aftershocks occur south of the trench, indicating effective stress change transfer from the megathrust to the bending flange of the Australian plate that is negotiating the bend in the trench. Highly productive sequences involving paired thrust and normal faulting have repeatedly occurred westward along the northwest-trending portion of the Loyalty Islands region, also indicating unusually efficient stress communication.