Imaging the Subduction Decollement, Hikurangi Subduction Zone, New Zealand

Beneath the eastern coastline of North Island, New Zealand, the subducted Pacific plate dips at less than 3 degrees to the northwest and the subduction decollement is at a depth of less than 15-km. The active-source NIGHT and passive-source CNIPSE experiments carried out in 2001 image the shallow dipping decollement down to more than 6 s (twt) at which point it steepens landward, 120 km from the Hikurangi trench. This change in dip appears to be closely associated with the onset of seismogenesis in the subducted plate. Velocity inversion of CNIPSE earthquake times reveals the forearc to be a relatively low Vp (<5.5 km/s), high Vp/Vs (>1.85), high Poisson's ratio (>0.29) region overlying the 12-15 km thick subducted crust. In March-May 2005 a new industry-seismic survey, 05CM, was carried out offshore the east coast, seismically imaging the subducted plate. More than 278000 airgun shots were also recorded by temporary seismometer stations placed along the coastline, out to offsets of more than 100 km. The combined marine and offshore- onshore seismic data highlight an area of more than 400 sq.kms. with higher reflectivity on the subduction decollement up-dip of the up-dip limit of seismogenesis, which we further examine using AVO and finite- difference modelling.