The Spatiotemporal properties of afterslip following the 2016 Kaikoura earthquake in New Zealand from GPS observations: Its complementary and inherited patterns with coseismic slip

Abstract

Summary The Mw 7.8 Kaikoura earthquake, which occurred on November 13, 2016, ruptured a complex system of strike-slip and reverse faults in northeastern South Island, New Zealand. However, the postseismic afterslip behavior and its relationship to the coseismic slip remain incompletely understood. This study investigates the spatiotemporal characteristics of afterslip following the mainshock by using four years of position data from 58 continuous GPS (cGPS) stations, considering the viscoelastic relaxation. Meanwhile, this study considers the contributions of crustal and the interface faults when exploring the combined effect of afterslip and viscoelastic relaxation. Results reveal substantial coseismic deformation northeastern of the epicenter, and postseismic displacements exhibit a continuation of the northeastward evolution. The primary coseismic slip occurred along the Kekerengu and Jordan Thrust faults, while secondary slip was accommodated by the Humps fault and the shallow subduction interface. Two primary afterslip zones are identified: one extending downdip from the secondary coseismic slip areas, and the other adjacent to shallow primary coseismic slip areas near the seismogenic Needles and Hope faults. The afterslip distribution exhibits a spatially complementary pattern to the coseismic slip areas, suggesting that velocity-strengthening zones may have hindered coseismic rupture propagation. The total seismic moment released by afterslip is estimated at ∼2.51×1020 N·m (Mw ∼7.53), approximately 30% of the coseismic moment. Meanwhile, about 80% of the postseismic seismic moment is ascribed to the slip along the southern subduction interface, suggesting the subduction fault plays an important role during postseismic slip. Temporal evolution modeling highlights that roughly 55% of the total afterslip moment was released within the initial three months. Postseismic afterslip dominated during the first month following the earthquake, with a slip rate of ∼10 mm/day. This rate subsequently decreased to ∼5 mm/day over the following two to three months, indicating that the majority of postseismic afterslip occurred shortly after the mainshock. In contrast to the earlier afterslip stages, the latter stages show continued movement along the Needles fault and the subduction interface. Cumulative peak slips have reached 2 cm since mid-2018, with fault slip rates decreasing to approximately 0.6-1.0 mm/day. This indicates ongoing afterslip at shallow faults and the subduction interface, with a steady slip rate over time. Importantly, the cumulative Coulomb stress changes induced by both coseismic slip and afterslip have increased the earthquake hazards potential near the Wellington fault, a densely populated region warranting further investigation.