Updating the Wellington Basin Model: Seismic and Gravity Surveys in Wellington City

Multichannel active-source seismic surveys image Wellington Basin at two sites using P-, SV-, and SH-waves. Sky Stadium is near where gravity studies suggest the basin is deepest; and Waitangi Park straddles the recently discovered active Aotea Fault. A sledgehammer plate source and arrays of single-component geophones were used to generate and record polarised seismic waves. We conclude that for hammer and plate methods, P-waves are optimal for anticipated sediment thicknesses >350 m depth, SV-waves can image up to 400 m depth, and SH-waves provide enhanced imaging of shallow sediments at <100 m depth in Wellington Basin. We present a new depth to basement map of Wellington Basin based on new seismic surveys, existing boreholes, and new gravity density models that incorporate 185 new measurements. At Wellington Stadium, high-amplitude high-continuity SV-wave reflections are recorded to ~600 ms two-way travel time (TWTT) (~ 240 m depth), with packets of less coherent reflection events at 1100 ms TWTT and 1300 ms TWTT that may include multiples of shallower events. The deepest SV-wave reflections at 1300 ms TWTT yield a depth to basement of 485 ± 80 m, if they are considered primary reflections. P-wave reflections have high-amplitude and continuity above 460 ms TWTT, and there are only isolated discontinuous events at larger TWTT. The base of P-wave reflectivity is interpreted as the base of sediment. Using refraction velocities, the depth to basement is calculated to be at 525 ± 80 m depth and is consistent with SV-wave data. The P-wave-derived depth estimate is incorporated into 2.75D gravity models that account for lateral out-of-plane extent of the basin. The onshore maximum sediment thickness is ~550 m just east of Sky Stadium, and ~475 m depth in northern Pipitea (approximately twice the depth of previous estimates). At Waitangi Park, a new gravity survey (20 m grid) is used to orient a seismic transect perpendicular to Aotea Fault and model depth to basement. SV-wave reflections are observed to 800 ms TWTT, corresponding to a depth of 220 ± 25 m. Coherent sub horizontal reflections are truncated by faults and sediment thickness is less toward the east: a series of tilted blocks offset by steeply-dipping reverse faults accommodate ~215 m of cumulative fault throw. An SH-wave survey resolves shallow (<400 ms TWTT) fault structures with improved resolution: we interpret sedimentary layers offset by ~10 shallow fault strands and a broad shallow anticline. The last dip-slip fault rupture is inferred to have been at 5952 ± 2500 years BP. Gravity models (2.75D) suggest the Aotea Fault has a low-density weathered greywacke zone between fresh bedrock and Quaternary sediments. The new Wellington Basin model steepens the northwestern basin edge from ~30° to near vertical, increasing estimated basin depths in northern Pipitea by >100% (+200 m), and drawing attention to the possible significance of the Lambton Fault. A steeper basin edge may imply stronger basin-edge ground motion amplification in Thorndon and Pipitea. The thicker sedimentary sequence implies resonance at lower frequencies, that likely will result in greater duration of shaking, and may produce focussing effects. A 50 m wide active Aotea Fault zone mapped at Waitangi Park has implications for local zoning and avoidance of a broad surface rupture hazard in eastern Te Aro.