The 2021 Mw 7.3 East Cape Earthquake: Triggered Rupture in Complex Faulting Revealed by Multi-Array Back-Projections

Abstract

On 04 March 2021 the Mw 7.3 East Cape, New Zealand earthquake occurred at the southern end of the Kermadec Trench. The United States Geological Survey W-phase solution shows an unexpected focal mechanism compared to historical earthquakes. We combine earthquake back-projections of 4 arrays to image the kinematic process of the Mw 7.3 earthquake. Our results reveal a bilateral rupture predominantly propagating upward at a slow speed of 1 km/s and a triggered rupture at ∼10 km, corresponding to the deep reverse faulting event at 70 km and a triggered shallow normal faulting event (∼20 km) resolved by previous subevent and finite fault inversions. The epicenter and a group of aftershocks are deeper than the bottom of the slab according to a regional tomography model. Such deep failure is possibly enabled by a delaminating lower crust of the Hikurangi plateau.