Tectonic shortening in the subduction trench and outer wedge, southern Hikurangi margin, New Zealand

Subduction zones generate the largest and most devastating earthquakes and tsunamis on Earth as a result of seismic slip on the megathrust fault. In addition to being capable of generating magnitude 8+ earthquakes, megathrusts also accommodate plate convergence via aseismic creep processes including episodic slow slip events. Above the megathrust, a portion of the overall plate convergence is accommodated as finite permanent strain (shortening) via slip along upper plate faults, tectonic folding, and reduction of sediment porosity (compaction). The most seaward expression of tectonic shortening in a subduction zone is focused within the outer accretionary wedge, but can also extend seaward of the main frontal thrust into sediments of the trench. Quantifying the strain budget among these different processes is essential for a better understanding of the partitioning between permanent inelastic strain and elastic strain accumulation as part of the seismic cycle – and thus ultimately toward an improved picture of subduction zone behavior and tsunami hazard. In this study, we use exceptionally detailed seismic reflection depth imaging and P-wave velocities to characterize sediment compaction within the outer wedge and trench along a profile of the southern Hikurangi subduction margin. Complementing these data with new constraints on stratigraphy, lithology and sediment physical properties, we provide the first quantifications of tectonic shortening attributable to sediment compaction on the Hikurangi margin. Our results demonstrate a broad region of compaction that extends more than 15 km seaward of the outermost faults. Future work beyond this study will explore relationships between pore scale compaction, proto-thrust development and active creep near the trench, in an attempt to provide a holistic understanding of strain accumulation in the outer wedge and trench.