Coulomb stress and gravity changes associated with the 2016 Mw 7.8 Kaikoura Earthquake, New Zealand: Application for aftershock triggering and fault interaction process analysis

The Kaikoura earthquake on November 14, 2016 is one of the largest and most complex earthquakes in New Zealand since 1947. Despite the fact that it has ruptured about 12 separate faults, triggered 2132 aftershocks within one week of the mainshock and induced considerable stress changes, few studies have been conducted to comprensively investigate the characteristics. The current study examines the horizontal and vertical displacements as well as the stress and gravity changes, aftershock distributions and also find out whether these changes affect the surrounding regions along the complex fault systems. The study covers the entire area affected by the Kaikoura event, which includes the northern part of the South Island and the southern part of the North Island. The dislocation theory was employed to evaluate the coseismic slip model on the multiple faults. The displacement results revealed that the maximum horizontal displacement is about 6 m and the vertical about 2 m, which are reasonably consistent with earlier study findings. Besides, the stress and gravity changes are quite complicated and inhomogeneous as evidenced by our coseismic model, demonstrating the complexity of the Kaikoura earthquake as well. Almost all the aftershocks are distributed in places where the stress and gravity change are found to be significant. In order to investigate the stability of our stress change models, we applied different friction coefficients and receiver fault parameters. The results justify the friction coefficient (μ=0.4) and the receiver fault parameters (230°, 70°, 150°) are suitable to define good stress change estimates. According to the stress change results at 15 km depth, the northern parts of the mainshock region, Hundalee fault, Humps fault and Jordan thrust areas together with the Wellington area are closer to failure and situated in a seismic risk zone. The multidimensional analysis adopted in this paper is helpful for making decisions and applications of stress and gravity change models in assessing seismic hazards.