The decade-long Machaze-Zinave aftershock sequence in the slowly straining Mozambique Rift

Abstract

Southern Mozambique is the southernmost expression of the continental East African Rift. Here, extension rates are low and rifting is achieved through normal faulting. Incipient rift environments provide an ideal location to investigate the role of reactivated pre-existing structures, aftershock sequences and fault interactions in rift development. In 2016 an Mw 5.6 earthquake occurred in the Zinave region of southern Mozambique, ∼10 km south-east of the Mw 7.0 2006 Machaze earthquake. We reanalyse ENVISAT InSAR observations of the Machaze earthquake, together with new Sentinel-1 Interferometric Synthetic Aperture Radar (InSAR) observations of the Zinave earthquake, and solve for uniform and distributed slip models for both events. We find that the Machaze earthquake occurred on a steeply dipping (∼75°) fault, in agreement with other studies, but that the Zinave earthquake occurred on an ∼60° dipping fault. The occurrence of the Zinave earthquake at the same depth as afterslip following the Machaze earthquake suggests laterally heterogeneous crustal frictional properties. The Machaze earthquake caused a Coulomb stress increase of ∼0.2MPa on the Zinave fault. The full >10 yr record of seismicity following the Machaze event can be fit by the Omori law, showing that the Zinave earthquake is part of a decade-long aftershock sequence, consistent with long-duration aftershock sequences in other slowly straining regions. Aftershocks represent a major hazard that needs to be considered if a large earthquake were to occur in the southern East African Rift system today.