New Insights Into Hikurangi Subduction Inputs and Megathrust Host Rocks Spanning Along‐Margin Changes in Fault Slip Behavior

The seismic behavior of subduction megathrusts varies spatially and is influenced by the properties of subducting plates, including their sedimentary cover. Characterizing these subduction inputs is essential for understanding the mechanisms behind fault slip variability. The Hikurangi subduction margin exhibits well‐documented variations in fault slip ranging from large earthquakes to slow slip events and aseismic creeping. The variations coincide with along‐margin changes in convergence rate, trench sediment thickness, and accretionary wedge development. Here, we integrate regional seismic reflection profiles with drill cores from the International Ocean Discovery Program Expeditions 372 and 375, and with other seafloor samples. We find that the Hikurangi Trough hosts some of Earth's thickest trench sediments, at up to 10 km, including pelagic sediments and a >6 km thick siliciclastic trench wedge that is younger than previously thought. Seismic imaging reveals that the shallow megathrust depth increases by 3.5–4.5 km southward along the margin in parallel with increasing trench sediment thickness despite a “step up” of the fault's stratigraphic position to a different assemblage of host rocks. In contrast to the irregular northern megathrust associated with subducting seamounts and heterogeneous volcaniclastic and pelagic host rocks, the southern megathrust is smooth, hosted in calcareous pelagic rocks, and straddles inherited polygonal faults. These along‐strike variations in physical properties and megathrust geometry correspond to observed changes in fault slip behavior. Our findings provide new insights into the factors controlling fault slip variability, with implications for seismic hazard assessment at subduction zones worldwide and for understanding subduction margin evolution.