Newly observed, deep slow slip events at the central Hikurangi margin, New Zealand: Implications for downdip variability of slow slip and tremor, and relationship to seismic structure

We present previously undocumented deep (25-45 km depth), moderate-duration (2-3 months) slow slip events (SSEs) directly downdip of known shallow (< 15 km depth), short-term (2-3 weeks) SSEs at the central Hikurangi margin, New Zealand. The moderate-duration SSEs produce 2-6 mm of horizontal displacement at continuous Global Positioning System stations in 2006 and 2008, similar to Cascadia SSEs. They involve 2-5 cm of slip over similar to 5000 km(2) of the subduction interface. Tremor occurs just downdip of the deep SSE source area. We also present results from a large, shallow (< 12 km) SSE in 2013 just updip of the 2006/2008 SSEs. The location of deep SSEs between shallow, short-term SSEs and tremor indicates strong downdip variation of subduction interface slip behavior at central Hikurangi. The deep SSEs occur where Qp and Vp/Vs data suggest an accumulation of fluid-rich underplated sediment at the interface, consistent with the idea that SSEs occur under high fluid pressure.