Crustal Structure of the Hikurangi Subduction Zone Revealed by Four Decades ​​of Onshore-Offshore Seismic Data

Abstract
Some subduction zones produce the largest earthquakes and tsunamis on Earth, while others slip freely. New Zealand's Hikurangi subduction zone exhibits profound differences in slip-behavior, transitioning from a region of strong fault locking in the south, to a region where the fault slips episodically in slow earthquakes further north. To understand what factors impact subduction zone slip behavior, we have integrated over 4 decades of seismic data to construct a high-resolution, 3-D image of the Hikurangi subduction zone. This image shows that relative to north Hikurangi, seismic wavespeeds in south Hikurangi are faster, and the region of fast wavespeeds extends further offshore toward the subduction trench. Correlations with onshore geology and seismic images suggest these changes likely reflect differences in the offshore (updip) extent of basement rocks within the overriding plate. We suggest the distribution of basement rocks overlying the subduction interface influences the strength and distribution of fault locking, and the offshore extension of rigid basement rocks in south Hikurangi may enable earthquakes to rupture further offshore. Wavespeed differences in the overthrusting plate also impact durations of earthquake ground shaking. Our results reinforce the importance of crustal-scale architecture in modulating earthquake and tsunami hazard at subduction zones.

Dan Bassett, Stuart Henrys, Brook Tozer, Harm van Avendonk, Andrew Gase, Nathan Bangs, Shuichi Kodaira, David Okaya, Katie Jacobs, Rupert Sutherland, Hannu Seebeck, Dan Barker, Gou Fujie, Ryuta Arai, Anya Seaward, Kimi Mochizuki, Martha Savage, Tim Stern, Thomas Luckie