Rheological Structure and Stress Triggered Megathrust Slip Constrained From the 2016 Mw 7.8 Kaikōura Crustal Earthquake

Abstract

Understanding postseismic processes following the 2016 Mw 7.8 Kaikōura earthquake remains challenging due to the time-dependent afterslip over the complex forearc system including crustal faults and megathrust, and the viscoelastic relaxation of the upper mantle. How the 2016 Mw 7.8 Kaikōura crustal earthquake interacts with the megathrust has yet to be better understood. Here we have derived the first 5-year postseismic displacements from Global Positioning System (GPS) time series of 75 stations to study postseismic processes through a three-dimensional viscoelastic finite element model. The optimal steady state viscosities of the crustal shear zone, megathrust shear zone, Australian upper mantle and Pacific upper mantle in the lowest-misfit model among test models are 10¹⁸ Pa s, 4 × 10¹⁷ Pa s, 2 × 10¹⁹ Pa s and 10²⁰ Pa s, respectively. The stress-driven afterslip within the first 5 years after the earthquake is up to 80 cm over crustal faults, and up to 70 cm over the megathrust. A Kapiti slow slip sequence is probably promoted with a shorter interval by the 2016 earthquake, and is up to ∼11 cm within the first year after the earthquake. Afterslip over crustal faults and the megathrust are both required to reproduce the first-order pattern of horizontal GPS observations. Coseismic rupture over the megathrust enhances shallow megathrust afterslip, which better fit the eastward postseismic displacement of sites near the rupture area. The southern end of Hikurangi megathrust may be activated during the 2016 earthquake and undergo continuous aseismic slip after the event.