Double Benioff Zones Along the Hikurangi Subduction Zone, New Zealand, Based on Nested Regional-Global Seismic Tomography and Precise Earthquake Relocation

Date: 4/24/2019

Time: 05:15 PM

Room: Cascade II

The exact mechanism for the genesis of the Double Benioff Zone (DBZ) and intermediate-depth earthquakes is an unresolved question. Several hypotheses have been proposed including dehydration-embrittlement and thermal shear instability. Here, we investigate the variation of the DBZs along the strike of Hikurangi trench, New Zealand. We obtain local and regional seismic data from the Geonet for over 70,000 events in magnitude range of 2 to 5 between 2013 and July 2017. In addition, we download teleseismic P arrival times reported by the International Seismological Center for the distance range between 30 and 90 degrees. We apply the teletomoDD algorithm (Pesicek et al., 2014), which uses a nested regional-global parameterization with a finely gridded regional model embedded in a coarser global model. We obtain a 3-D P-wave velocity model based on 16,000 master events from the regional dataset and 5,660 events from the global dataset and use this velocity model to relocate the whole dataset. A differential-time relocation method based on waveform cross-correlation is then used to improve the accuracy of relative locations. Our results show a clear DBZ with separated upper and lower planes of seismicity in the north part of the Hikurangi subduction zone. Spatial distribution of seismicity in accordance with zones of low P-wave velocities supports the dehydration of the subducting slab. The slab is torn at about 80 km depth and the DBZ disappears below this depth, where the dip of the subducting slab increases. In the central North Island, there is no clear DBZ and the seismicity is more concentrated in the upper part of the subducting slab that may suggest a different degree of hydration. The DBZ reappears in the southern part of the North Island, where the layer separation is smaller compared to the DBZ in the northern part. These observations imply along strike changes in the geometrical, thermal, and/or hydration state of the subducting slab.

Presenting Author: Farzaneh Aziz Zanjani

Authors