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The range of fault slip behaviors near the trench at subduction plate boundaries is critical to know, as this is where the world's largest, most damaging tsunamis are generated. Our knowledge of these behaviors has remained largely incomplete, partially due to the challenging nature of crustal deformation measurements at offshore plate boundaries. Here we present detailed seafloor deformation observations made during an offshore slow-slip event (SSE) in September and October 2014, using a network of absolute pressure gauges deployed at the Hikurangi subduction margin offshore New Zealand. These data show the distribution of vertical seafloor deformation during the SSE and reveal direct evidence for SSEs occurring close to the trench (within 2 kilometers of the seafloor), where very low temperatures and pressures exist.

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S U M M A R YWe use seismic reflection data to map the geometry and character of the subduction interface in the Gisborne area of the Hikurangi subduction margin, New Zealand, which experiences repeated shallow (<15 km) slow slip events. The reflection character and geometry in this area is highly variable, which we interpret to be related to the subduction of seamounts and underthrust sediments. Three zones of high-amplitude interface reflectivity (HRZ-1, 2 and 3) are interpreted to be the result of fluid-rich sediments that have been entrained with subducting seamounts. The interface above the HRZ zones is shallower than the surrounding areas by 2-4 km, due to the warping of the interface to accommodate seamount subduction. These zones of high-amplitude reflectivity and shallower interface geometry correlate broadly with locations of recorded slow slip events from 2002 to 2008. We hypothesize that effective stresses on the interface may be lower along the northeast margin in areas of high-amplitude reflectivity due to; (1) the enhanced underthrusting of fluid-rich sediment, (2) reduced overburden stresses where the interface has been warped to shallower depths to accommodate seamount subduction and (3) potential fluid flow concentration effects leading to overpressure along these shallower interface corrugations. From our observations we propose localized reductions in effective stress caused by interface structural relief may be a potential factor in promoting shallow slow slip events.

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