Slow wavespeeds and fluid overpressure in a region of shallow geodetic locking and slow slip, Hikurangi subduction margin, New Zealand

Abstract

Travel times recorded onshore from active-source marine seismic surveys were used to determine Hikurangi forearc wavespeeds. Ray-path midpoints sample forearc crust above the shallow (<10–12 km) subduction thrust. The southern region is locked to c. 30 km depth, exhibits slow slip at 30-45 km depth, and is similar to other subduction zones. Our 1D southern model has a rapid increase in seismic velocity (Vp) from the seabed to 4 km depth, consistent with rapidly-deposited clastic sediments, and near-constant Vp=5.0±0.2 km/s at depths of 4–10 km within the forearc. The northern region has slow-slip events at shallow depths of c. 5–15 km, beneath the volume sampled by our analysis. Average travel times at offsets of 20–80 km are >1 s more than at equivalent offsets in the south, and Vp increases with depth within the forearc from 3.5±0.1 km/s at 4 km depth to 4.5±0.2 km/s at 10 km depth. We interpret the low wavespeeds in terms of compaction disequilibrium and compare seismic inferences of anomalously-high porosity (>10%) with hydrostatic reference compaction models to show effective stress is low in both the north (27±10 Mpa) and south (36±14 MPa). In the south, pore space and conduits have a high chance of being localized on faults or sand layers, the wedge is clearly compressionally critical, and we suggest the higher seismic velocities recorded primarily reflect higher effective stress levels transmitted through the rock framework. The observed wedge geometry and our estimate of fluid pressure within the wedge (λ=0.87±0.05) suggest a weak overpressured subduction thrust. In the north, we suggest high fluid pressure is maintained by a large fluid inventory from subducting sediment, a pore-space geometry characterized by pervasively-fractured rock and mudstone, and a lower mean stress due to its stable and non-critical wedge geometry. We consider both end-member assumptions of convergent and tensile failure, the latter of which reveals the possibility of near-lithostatic fluid pressure on the north Hikurangi subduction interface, although this remains unproven. Our observations are consistent with high fluid pressures encountered in petroleum wells at depths <3 km and we observe a clear spatial correlation between the distribution of residual travel-times and the maximum depth of geodetic coupling and slow-slip. Our observations provide evidence for a causative link between high fluid pressure and anomalously-shallow slow slip events and depth of geodetic locking.