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Research Papers

Issue date: 
Category: 
Seismology & geology
Paper number: 
3810

Postseismic deformation following the 2016 Kaikora Earthquake

By Sigrún Hreinsdóttir, GNS Science (EQC Funded project 18/754)

 

Non-Technical Abstract

Postseismic deformation from a large magnitude earthquake will affect the region for months to years with different processes acting on different temporal and spatial scales. Afterslip on crustal faults can dominate the near field motions for weeks to months whereas viscoelastic response of the lower crust and upper mantle dominates in longer term and affects wider region. In addition transient deformation due to pore fluid flow has been observed in the weeks following large earthquakes.


The Top of the South Island (TOPS) GNSS network has been measured every 8 years since the 1990’s, augmenting the sparse continuous GNSS network, to study active deformation of the region. The Marlborough Fault Zone in the TOPS area is a complex region with multiple faults transferring the plate boundary from the Hikurangi subduction zone in the north to the Alpine Fault in the south.


In the weeks following the Kaikoura earthquake over 80 TOPS GNSS sites, mostly located within 50 km of the fault ruptures, were measured to estimate co-seismic deformation during the earthquake and lay the groundwork for future post-seismic studies. GNS Science and University of Otago set up 15 additional semi continuous GNSS stations to monitor post seismic deformation and GeoNet installed six new GNSS stations in the region. In March 2017 additional 58 campaign sites were measured with funding from NHRP. In 2018 we re-measured the entire TOPS GNSS network allowing us to evaluate and study the first 15 months of post seismic deformation in the region. The next GNSS campaign for the region is scheduled for 2020.


The measurements of the TOPS GNSS network 15 months after the Kaikoura earthquake allows us better understand the temporal decay and spatial pattern of the different postseismic processes currently affecting the region. The GNSS data along with InSAR data are being used to constrain models of the underlying crustal and mantle processes and to evaluate stress transfer on active faults in the region and the subduction interface. The post seismic deformation affects seismic hazard on other faults in central New Zealand in both the short and long term, loading some faults and unloading others.


The main purpose of this EQC proposal was to allow us to remeasure the TOPS GNSS network 15 months after the Kaikoura earthquake and operate semi continuous GNSS stations. The semi continuous and continuous GNSS data collected in 2018 show that the postseismic deformation following the Kaikoura earthquake is decaying rapidly with time. Halfway through 2018 GNSS stations in the southern North Island appear to go back to background deformation and sites in the South Island change direction, mostly toward the east coast. Sites in the epicentral region which previously had little postseismic deformation now appear to be moving southwest and sites in the Cape Campbell region are moving more easterly. Preliminary modelling of the data suggests that this marks the end of the Kapiti slow slip event and afterslip on the subduction interface, with the plate interface locking up again. The pattern of deformation in the South Island after middle of 2018 suggests that afterslip on the offshore Kaikoura and the Needles fault could still be ongoing but further modelling is required to explore viscoelastic processes that could be acting in the region.


The data collected during this project has allowed us to evaluate afterslip on faults that ruptured during the earthquake as well as the plate interface. We will continue to refine models and explore possible effects of mantle relaxation on the longer-term postseismic deformation. The postseismic deformation influences seismic hazard on other faults in central New Zealand both in the short and long term, loading some faults and unloading others. Better understanding of the postseismic processes will lead to enhanced assessment and modelling of New Zealand’s geohazard risk. The measurements are also helping us understanding the role of the Hikurangi interface beneath the northern South Island which is currently not fully understood and directly impacts how large earthquakes we could expect on the Hikurangi interface.

 

Technical Abstract

The M7.8 Kaikōura earthquake, took place in the northeastern South Island of New Zealand on 14 November 2016. It ruptured within a complex tectonic region, where the subduction along the Hikurangi margin to the north transitions to strike-slip and collision along the Alpine Fault in the south. The earthquake ruptured over 170 km with significant slip along at least 12 major crustal faults and possibly portions of the subduction plate interface beneath the region. In 2018 the Top of the South Island GNSS network was re measures to capture the first 15 months of postseismic response of the region. In addition, several semi continuous GNSS stations were operated to augment the sparse continuous GNSS network. Postseismic deformation following the Kaikōura earthquake shows a large-scale northeast-directed movement with the most rapid deformation observed in the region of Cape Campbell (northeast tip of South Island), coinciding with a large cluster of aftershocks, decaying with time. The overall pattern of deformation is consistent with rapidly decaying slip on a low-angle source at depth, inferred to be the subduction interface beneath the Marlborough region, as well as significant afterslip on crustal faults that ruptured during the earthquake, in particular the Jordan Thrust/Kekerengu and the Needles fault. Majority of the afterslip relating to the subduction interface had decayed off by middle of 2018 with a total moment release equal to a Mw7.4 event. Sites close to the fault rupture still have significant movement suggesting ongoing afterslip on crustal faults.

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