Seismic Microzonation in Wellington: Understanding Site-Specific Ground Response

A mid-rise development on The Terrace hit a snag last season. The geotechnical report flagged competent greywacke bedrock at 8 metres, but the seismic response spectrum across the site varied by nearly 40% between the southern and northern boundaries. This is Wellington. The combination of steep terrain, reclaimed land along Lambton Quay, and the active Wellington Fault running through the Hutt Valley creates a patchwork of subsoil conditions that no generic seismic zone factor from NZS 1170.5 can capture. We use seismic refraction and multichannel surface wave testing to map shear-wave velocity profiles across the site, then feed that data into one-dimensional and two-dimensional site response models. The output is a microzonation map that shows how ground motion amplifies differently across even a single building footprint. For critical structures near the fault, we layer in CPT testing to capture thin liquefiable silt seams that borehole SPT data often miss.

A 40% variation in spectral acceleration across a single Wellington site is not unusual when basin-edge effects and shallow bedrock interact.

Our approach and scope

Wellington's geography creates a specific set of amplification patterns that repeat across the city. The flat land around the CBD is mostly Holocene-age alluvium and marine sediment, while the hills of Kelburn and Karori sit on weathered greywacke with a shallow impedance contrast. Each of these conditions produces a different spectral acceleration at the surface. Our microzonation studies measure three things: the fundamental site period from horizontal-to-vertical spectral ratios, the average shear-wave velocity in the upper 30 metres, and the depth to the seismic bedrock where Vs exceeds 760 m/s. On a recent project in Te Aro, the site period jumped from 0.4 seconds on the hill side to 1.1 seconds on the basin side—a shift that moves the peak spectral response directly into the natural period range of 8- to 12-storey structures. We combine these measurements with MASW surveys to build a continuous Vs profile across the entire site, then correlate it with borehole logs to confirm the stratigraphic boundaries. The final microzonation map becomes the basis for selecting design spectra, determining liquefaction triggering, and siting critical infrastructure away from the strongest amplification zones.

Local ground factors

NZS 3404 and the NZGS Module 4 guidelines make site-specific seismic hazard assessment mandatory for structures classified as Importance Level 3 and above. In Wellington, this is not a box-ticking exercise. The 2016 Kaikōura earthquake demonstrated that basin resonance in the Te Aro and Thorndon areas can sustain shaking durations well beyond what the standard design spectrum assumes. A microzonation study that ignores the two-dimensional basin geometry—or that relies on a single Vs30 value applied uniformly across a large site—gives a false sense of precision. The real risk is differential ground motion: two foundation elements 30 metres apart experiencing out-of-phase shaking because one sits on shallow rock and the other on 25 metres of soft sediment. This can induce torsional response in the superstructure that the structural engineer never designed for. Our approach quantifies that differential motion explicitly, so the design team can decide whether to stiffen the structure, deepen the foundations to reach uniform bearing, or incorporate seismic isolation at the base.

Typical values

Parameter	Typical value
Vs30 (average shear-wave velocity upper 30 m)	150–760 m/s (site class B to E per NZS 1170.5)
Fundamental site period (T0)	0.1–2.0 s (measured via HVSR)
Depth to seismic bedrock (Vs > 760 m/s)	5–80 m (variable across Wellington basin)
Peak ground acceleration (PGA) at bedrock	0.40–0.65 g (500-year return, Wellington Fault scenario)
Liquefaction severity index (LSI)	0–45 (mapped per NZGS Module 4)
Spectral acceleration amplification factor	1.0–3.5 (site-dependent, short-period range)
Mapping resolution	10–50 m grid spacing (adaptable to site size)

Complementary services

Site Response Analysis and Spectral Mapping

We build 1D and 2D ground response models using measured shear-wave velocity profiles and modulus reduction curves calibrated to Wellington basin soils. The output includes site-specific design spectra, amplification factor contours, and maps of fundamental site period that feed directly into the structural engineer's seismic load definition.

Liquefaction Triggering and Lateral Spread Assessment

Using CPT, SPT, and Vs data from the microzonation grid, we apply the Boulanger and Idriss (2014) triggering procedure to map liquefaction severity index across the site. For waterfront and reclamation zones like CentrePort, we add lateral spreading displacement estimates using the empirical models of Youd et al. (2002) calibrated to Wellington's gravel-mixed alluvial deposits.

Common questions

When does the Wellington City Council require a seismic microzonation study?

The Council follows the MBIE guidelines and NZS 1170.5, which require site-specific seismic hazard analysis for Importance Level 3 and 4 structures (schools, hospitals, emergency response facilities, and buildings with more than 300 occupants). Any development on reclaimed land—which covers much of the CBD east of Lambton Quay—should include microzonation due to the liquefaction and amplification risks specific to Wellington's harbour sediments. Some resource consent conditions also mandate it for large excavations near the fault trace.

What is the typical cost range for a seismic microzonation study in Wellington?

A complete microzonation study for a typical central Wellington development site ranges from NZ$7.250 to NZ$29.320, depending on the grid density, number of measurement points, depth of investigation, and whether 2D basin modeling is required. The scope includes field surveys (MASW, seismic refraction, or downhole Vs logging), data processing, site response analysis, and the final microzonation map with design spectra.

How do you account for the Wellington Fault proximity in the microzonation?

The Wellington Fault runs from Cook Strait through the Hutt Valley and along the western side of the harbour. For sites within 2 km of the mapped fault trace, we incorporate near-fault directivity effects into the ground motion selection. This means we use pulse-like recorded ground motions from similar strike-slip events worldwide, scaled to the NZS 1170.5 hazard spectrum, and run them through the site response model. The result is a design spectrum that captures the velocity pulse characteristic of near-source Wellington Fault rupture, which standard probabilistic hazard models can smooth out.