Spatial and temporal stress field changes in the focal area of the 2016 Kaikōura earthquake, New Zealand: A multi-fault process interpretation

Abstract

To understand the stress controls on the occurrence of a multi-fault rupture, we estimated the crustal stress between April 2013 to December 2018, i.e., before and after the Mw7.8 Kaikōura earthquake that occurred in New Zealand on 13 November 2016. We used both the focal mechanism solutions from the temporary seismic networks and the GeoNet moment tensor solutions and selected the solutions that differed significantly from the mainshock fault planes and rakes. Then, we performed stress tensor inversions for the selected focal mechanism solutions. Using the stress tensor inversion results, we also calculated the slip tendency. Prior to the Kaikōura earthquake, the stress regime was the strike-slip type, and the maximum eigenvalue of the stress tensor (σ1) was oriented WNW–ESE. The stress field orientation did not change significantly after the earthquake. This suggests that the stress change during the Kaikōura earthquake was too small to alter the stress orientations, implying that there may have been large differential stress prior to the Kaikōura earthquake. However, the average stress ratio in different clusters changed in two different patterns after the earthquake, suggesting possible changes in the magnitude of different components of the stress tensor, or of pore pressure in different regions. A high slip tendency was observed at the hypocentre, while a low slip tendency was observed at the northern end of the Kaikōura earthquake faults. This may suggest that the stress orientation and the stress ratio controlled the initiation and the end of the multi-fault rupture. These results corroborate previous fault propagation models.