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<br The subduction of Pacific plate beneath Australian plate at Hikurangi-Kermadec trench changes style to Australian plate subducting underneath Pacific plate at Puysegur trench. The transition takes place at New Zealand and dictates the regional seismicity record. I utilized earthquakes record from Havard centroid moment tensor (CMT) Catalogue (1977-2018) with historical earth- quakes record from GeoNet (1800-1977) to investigate the interplate properties. The study area is divided into areas of coherent deformation which has strain rate and velocity obtained. The results are compared with global strain rate model and MORVEL velocity model. CMT Cat- alogue yields generally low strain strain and velocity at North Island which reflects the slow slip (asesimic zone) patches at Hikurangi subduction trench. By contrast, high strain rate from CMT is found at South Island, where six earthquakes with Mw > 7 were recorded. The low seismicity and strain rate at Alpine fault reflects its current interseismic stage with potential of large earthquakes. I also found the deep earthquakes correspond to mineral phase transition within the subducting slab at ∼ 410km depth.

Table of Contents

  1. Background
  2. Results and Conclusion
  3. Final Remark

Background

New Zealand situates at the boundary of Australian and Pacific plates, where the convergence of Pacific and Australian plates forms six zones of deformation (Figure 2.1). The oblique conver- gence produces 1, 300km Kermadec trench which connects with the Tonga trench, forming the Tonga-Kermadec arc system. Kermadec arc is one of the most volcanically and hydrothermally active arc systems (Gamble et al. (2014)), producing ridges of active volcanos on the back-arc basin named Harve Trough (Delteik et al. (2002). South of the Kermadec arc, Hikurangi mar- gin formed as the 120 Myrs old Pacific oceanic lithosphere subducts beneath the Australian continental lithosphere. Together with Kermadec and Tonga trench, they form a 3000km long subduction system. The Oblique convergence partitioned in the North Island. Over geological time, relative motion occurs on the subduction thrust accommodate more than 80 % of the convergent component, upper plate reverse fault accommodate the rest 20% (Wallace et al. (2009)). Taupo rift zone formed in continental crust in association with the subduction and is the continuation of the Harve Trough back-arc basin(Villamor et al. (2017).

Results and Conclusion

Strain rate and velocity results differs significantly from the geodetic model, In the North Island, both strain rate and velocity are lower than the Global strain rate model (GSRM). The model results reflect the convergence of Australian and Pacific plate, whereas the CMT Catalogue shows regional deformation. The difference between CMT and GSRM results is dictated by the zones of slow slip patches (where plates are unlocked). At region where the slow-slip patches are present, the deformation is dominated by the upper plate deformation, hence GSRM and CMT yield different results. Due to frequent SSEs coupled with large slip displacement, large earthquakes are absent at Hikurangi trench. The subduction of seamounts at Kermadec trench elevated stress filed and induced brittle deformation, hence Mw7 earthquake is observed. Most strain rates at south Island have higher value from CMT Catalogue than GSRM. It cor- relates with the large earthquakes recorded, which contributed to more than 85 % of the total strain strain. The low CMT strain rate at Alpine region indicates the area is currently in inter- seismic stage, accumulating stress within plate interface and have potential of large earthquake. My strain rate results at deep subduction region correlate well with the internal deformation in subductng plate due to bending of the plate and phase transition at 410km. In addition, the ter- mination of Hikurangi slab at MFC and slab tearing at Puysegur-Alpine transition are not fully resolved by seismicity cross-sections. I recommend further study such as seismic tomography to fully understand these complexities.

Final Remark

This is a breif summary of my thesis, if you are interested in the methodology, and concepts, please get in touch.