Morphometric and critical taper analysis of the Rock Garden region, Hikurangi Margin, New Zealand: Implications for slope stability and potential tsunami generation

New high-resolution swath bathymetry data show a complex seafloor morphology from the Rock Garden area, offshore Hikurangi Margin, that coincides with the subduction of a seamount presently located beneath the summit of Rock Garden. Another ridge-shaped lower plate feature is initially colliding with Rock Garden, forming a re-entrant at its seaward flank. The slopes of the accretionary ridges are steeper than 10° and often more than 20° regionally. Slumping mostly occurs on the trench-ward slopes, with individual slumps affecting areas up to several km2. Critical taper analysis, using realistic wedge geometries and fluid pressures scenarios, shows that much of the seaward slopes in the region are most likely outside the stability field and therefore subject to failure. The most prominent feature revealed by seafloor maps is the trench-ward flank of Rock Garden with a height of 1800 to 2000 m and an average slope of more than 10°. Extensional faults arranged in two sub-circular arcs indicate that Rock Garden may be on the verge of failure. Critical taper analysis also supports this claim and shows that if basal fluid pressure approaches lithostatic pressure, e.g. during a large Mw > 8 earthquakes, then a complete failure of the entire trench-ward flank of Rock Garden would potentially affect an area as large as 150 km2 and a rock volume of 150 to 170 km3. This worst case scenario would generate a tsunami wave some tens of meters high. Therefore, the observation that a number of seamounts are buried beneath the outer Hikurangi accretionary wedge suggests that a thorough assessment of these features needs to be undertaken and its results incorporated into tsunami hazard models for the East Coast of New Zealand's North Island.