Crustal Structure of the Hikurangi Subduction Zone Revealed by Four Decades of Onshore-Offshore Seismic Data: Implications for the Dimensions and Slip Behavior of the Seismogenic Zone

Four decades of seismic reflection, onshore-offshore and ocean-bottom seismic data are integrated to constrain a high-resolution 3-D P-wave velocity model of the Hikurangi subduction zone. Our model shows wavespeeds in the offshore forearc to be 0.5–1 km/s higher in south Hikurangi than in the central and northern segments (V_P ≤ 4.5 km/s). Correlation with onshore geology and seismic reflection data sets suggest wavespeed variability in the overthrusting plate reflects the spatial distribution of Late Jurassic basement terranes. The crustal backstop is 25–35 km from the deformation front in south Hikurangi, but this distance abruptly increases to ∼105 km near Cape Turnagain. This change in backstop position coincides with the southern extent of shallow slow-slip, most of which occurs updip of the backstop along the central and northern margin. These relationships suggest the crustal backstop may impact the down-dip extent of shallow conditional stability on the megathrust and imply a high likelihood of near/trench-breaching rupture in south Hikurangi. North of Cape Turnagain, the more landward position of the backstop, in conjunction with a possible reduction in the depth of the brittle ductile transition, reduces the down-dip width of frictional locking between the southern (∼100 km) and central Hikurangi margin by up-to 50%. Abrupt transitions in overthrusting plate structure are resolved near Cook Strait, Gisborne and across the northern Raukumara Peninsula, and appear related to tectonic inheritance and the evolution of the Hikurangi margin. Extremely low forearc wavespeeds resolved north of Gisborne played a key role in producing long durations of long-period earthquake ground motions.