Nested regional-global seismic tomography and precise earthquake relocation along the Hikurangi subduction zone, New Zealand

• Published in: Geophysical journal international 2021-12, Vol.227 (3), p.1567-1590
• Main Authors: Aziz Zanjani, Farzaneh, Lin, Guoqing, Thurber, Clifford H
• Format: Article
• Language: English
• ISSN: 0956-540X; 1365-246X
• DOI: 10.1093/gji/ggab294

SUMMARY Seismic and geodetic examinations of the Hikurangi subduction zone (HSZ) indicate a remarkably diverse and complex system. Here, we investigate the 3-D P-wave velocity structure of the HSZ by applying an iterative, nested regional-global tomographic algorithm. The new model reveals enhanced details of seismic variations along the HSZ. We also relocate over 57 000 earthquakes using this newly developed 3-D model and then further improve the relative locations for 75 per cent of the seismicity using waveform cross-correlation. Double seismic zone characteristics, including occurrence, depth distribution and thickness change along the strike of the HSZ. An aseismic but fast Vp zone separates the upper and lower planes of seismicity in the southern and northern North Island. The upper plane of seismicity correlates with low Vp zones below the slab interface, indicating fluid-rich channels formed on top and/or within a dehydrated crust. A broad low Vp zone is resolved in the lower part of the subducting slab that could indicate hydrous mineral breakdown in the slab mantle. In the northern North Island and southern North Island, the lower plane of seismicity mostly correlates with the top of these low Vp zones. The comparison between the thermal model and the lower plane of seismicity in the northern North Island supports dehydration in the lower part of the slab. The mantle wedge of the Taupo volcanic zone (TVZ) is characterized by a low velocity zone underlying the volcanic front (fluid-driven partial melting), a fast velocity anomaly in the forearc mantle (a stagnant cold nose) and an underlying low velocity zone within the slab (fluids from dehydration). These arc-related anomalies are the strongest beneath the central TVZ with known extensive volcanism. The shallow seismicity (