Fluid upwelling across the Hikurangi subduction thrust during deep slow-slip earthquakes

Slow-slip events at global subduction zones relieve tectonic stress over days to years. Through slow-slip cycles, high fluid pressures observed at the top of subducting plates are thought to fluctuate, potentially due to the valving action of an impermeable layer near the plate interface. We model teleseismic scattering data at the Manawatu deep slow-slip patch at the Hikurangi margin in New Zealand and find high seismic P-to-S wave velocity ratios, VP/VS, in the upper ~5 km of the subducting Pacific Plate, reflecting sustained elevated fluid pressures that decrease during slow-slip and increase during inter-slow-slip periods. Within a ~ 3 km thick lower crustal layer of the overriding Australian Plate, decreasing VP/VS during inter-slow-slip periods reflects permeability reduction due to mineral precipitation. Increasing VP/VS during slow-slip reflects increasing permeability and crack density, facilitating upward fluid transfer through this layer. Our results suggest it acts as a valve to relieve high fluid pressures in the subducting slab. Seismic velocity changes near the subduction thrust of the Hikurangi subduction zone in New Zealand result from fluid pressure dynamics associated with deep slow-slip earthquakes, as revealed from receiver function analysis.