A developer planning a three-storey mixed-use building on Taupo Quay faced a common Whanganui challenge: the site sat on deep alluvial sediments near the river, and the standard NZS 4203 site subsoil classification didn't capture the rapid lateral change from dense gravels to soft silts across the property. The geotechnical report recommended a seismic microzonation study to map ground response at a block scale, not just at a single borehole. Our team deployed a dense array of MASW and microtremor recordings, integrated with existing CPT test logs, to build a high-resolution shear-wave velocity model. The output was a set of site period and amplification maps that allowed the structural engineer to zone the foundation system—piled under the west wing, mat foundations under the east—and justify the design to Whanganui District Council. Whanganui’s variable geology, from consolidated dune sands in Castlecliff to compressible floodplain deposits near the CBD, makes block-scale microzonation a practical tool for managing seismic risk efficiently.
In Whanganui’s river corridor, site period can shift from 0.2 to 0.8 seconds over less than 200 metres—microzonation captures that transition so the foundation design adapts block by block.
Process and scope
The subsurface beneath Whanganui ranges from the Pleistocene dune sands and shell beds of Castlecliff to the Holocene alluvial sands, silts, and peats of the Whanganui River floodplain. This lateral heterogeneity means that two boreholes 100 metres apart can record fundamentally different dynamic properties. A seismic microzonation study is a systematic procedure to map parameters such as fundamental site period (T0), average shear-wave velocity in the upper 30 metres (Vs30), and amplification factors across a site. We follow the New Zealand Geotechnical Society’s Module 4 guidelines for site amplification assessment, combining active-source
MASW surveys with passive-source horizontal-to-vertical spectral ratio (HVSR) measurements. For sites in liquefaction-prone areas, such as the lower river terraces, the microzonation dataset feeds directly into a
liquefaction assessment, delineating zones where ground improvement or deep foundations become mandatory. The deliverables include GIS-compatible raster maps and a detailed report documenting the site classification in accordance with NZS 1170.5, giving the design team a spatially continuous understanding of seismic demand.
Local geotechnical context
Whanganui’s climate compounds its seismic vulnerability. High annual rainfall and a shallow water table across the floodplain keep fine-grained soils saturated for much of the year, increasing pore pressure and extending the zone susceptible to cyclic softening during a moderate earthquake. A uniform site subclass from a desk study ignores these moisture-driven contrasts. Without microzonation, a developer risks designing to an averaged ground motion that under-represents the softest patches, where spectral acceleration can be 40% higher than the site-wide average. The economic consequence is either over-design in competent zones or under-design in weak pockets—both undesirable. The Whanganui District Plan encourages site-specific hazard assessment for buildings over 3000 square metres or with an Importance Level of 3, and microzonation provides the defensible, spatially explicit data that council peer reviewers expect. Mapping site response block by block also helps developers negotiate insurance terms by demonstrating that seismic demand has been quantified rather than assumed from generic code coefficients.
Quick answers
How much does a seismic microzonation study cost for a typical Whanganui site?
The cost ranges between NZ$6,400 and NZ$25,680 depending on site area, grid density, and whether CPT or borehole data are already available. A 2000-square-metre site with mixed alluvial soils and a 25-metre MASW grid typically falls near the middle of this range when integrated with existing geotechnical records.
Is microzonation required by the Whanganui District Council?
The council does not mandate microzonation universally, but it is frequently requested as part of the resource consent process for projects with an Importance Level of 3 or higher, or for large developments on the river floodplain where the technical category varies rapidly. Demonstrating a block-scale ground response analysis often accelerates the building consent review.
How long does the fieldwork and reporting take?
A typical microzonation survey for a 3000-square-metre site requires two to three days of field acquisition plus two to three weeks for processing, analysis, and report drafting. The timeline can extend if complementary intrusive testing, such as CPT or boreholes, is needed to calibrate the velocity model.
