Subduction thrust valving action revealed from time-lapse imaging of deep Hikurangi slow slip events

Abstract Slow slip events (SSEs) at subduction zones relieve tectonic stress over periods ranging from days to years. A pervasive feature in SSE regions worldwide is the presence of high fluid pressures in the subducting slab, which are thought to fluctuate during SSEs through the fault-valving action of an impermeable layer at the plate interface. Yet, geophysical observations of this fault-valving behaviour remain sparse. Here we show, by applying timelapse receiver function analysis at the Manawatu deep slow slip region, that VP/VS in the top ~7.3 km of the subducting Pacific Plate increases up to 2.40 during inter-SSE periods and decreases down to 2.20 during SSE periods, indicating sustained high fluid pressures across slip cycles. In a ~2 km thick layer above, VP/VS fluctuates in the opposite sense, decreasing to ~1.4 during inter-SSE to >2.0 during SSEs. We interpret these results to indicate a reduction in fluid saturation due to decreased crack density and mineral precipitation between SSE cycles, followed by increased crack density and fluid saturation induced by slip on the subduction thrust. Our work shows that this thin layer relieves high fluid pressures in the subducting slab by acting as a valve.