Bunbury sits on the Swan Coastal Plain where the Leschenault Estuary and the Indian Ocean define a landscape of sandy dunes, estuarine silts, and shallow groundwater. This coastal setting amplifies seismic shaking because soft sediments trap and prolong shear waves. A seismic microzonation study in Bunbury must therefore resolve the transition from Pleistocene dune sands to Holocene tidal flat deposits, each with markedly different shear-wave velocity (VS30) profiles. Without site-specific classification, a project risks adopting design spectra that underestimate peak ground acceleration in the estuarine corridor. We integrate MASW and HVSR surveys with existing borehole data to produce hazard-consistent site classes that feed directly into AS/NZS 1170.4 load calculations.
Estuarine muds near the Leschenault Inlet can amplify seismic motion by a factor of 2 to 3 relative to adjacent limestone ridges.
Methodology and scope
- Multi-channel Analysis of Surface Waves (MASW) to map VS30 across 1-2 km grids;
- single-station HVSR to identify fundamental resonance frequencies (often 1.5–4 Hz in soft deposits);
- and standard penetration test (SPT) blow-counts for liquefaction screening following Youd-Idriss (2001).
Local considerations
The primary equipment for microzonation fieldwork in Bunbury is the towable MASW spread — 24 geophones at 2–5 m spacing coupled with a 5 kg sledgehammer source. We deploy this across road reserves and vacant lots, recording 3–5 stacks per shot to improve signal-to-noise ratio in windy coastal conditions. The greatest operational risk is cultural noise from nearby industrial zones or the Bunbury Outer Ring Road, which can mask the surface-wave dispersion curve below 5 Hz. We mitigate this by scheduling surveys during low-traffic windows and using longer receiver spreads (up to 70 m) to capture deeper velocity structure.
Applicable standards
AS 1726:2017 Geotechnical Site Investigations, AS/NZS 1170.4:2007 Structural Design Actions – Earthquake Actions, NEHRP Recommended Seismic Provisions (FEMA P-1050) – Site Class Definitions, AS 1289/D4428M-14 Standard Test Method for Crosshole Seismic Testing
Associated technical services
MASW & VS30 Profiling
Multi-channel surface-wave surveys along 500–2000 m transects to produce VS30 maps at 1:5,000 scale. Suitable for residential subdivisions and industrial lots in the Glen Iris and Pelican Point areas.
HVSR Resonance Analysis
Single-station ambient-noise recordings at 50–100 m grid spacing to identify fundamental resonance frequencies. Critical for differentiating between stiff-sand and soft-clay site classes.
Integrated Liquefaction Hazard Assessment
Combined SPT-based screening (Youd-Idriss 2001) with CPT-based cyclic resistance ratios. We map liquefaction potential index (LPI) for the estuarine fringe and recommend ground improvement strategies.
Typical parameters
Frequently asked questions
What is the difference between seismic microzonation and a standard geotechnical site investigation?
A standard site investigation focuses on bearing capacity and settlement for static loads. Seismic microzonation specifically characterises ground response during earthquakes — shear-wave velocity, resonance frequency, and amplification factors — to produce site-specific design spectra (AS 1170.4 site class). In Bunbury, the contrast between dune sands and estuarine silts makes this distinction critical.
How much does a seismic microzonation study for a typical Bunbury subdivision cost?
For a 2–3 hectare subdivision in the Bunbury area, a microzonation study typically ranges between AU$6,610 and AU$24,550, depending on the number of MASW transects, HVSR stations, and the depth of borehole correlation required. Larger developments or those near the Leschenault Inlet fall at the upper end due to the need for deeper velocity profiling.
Which areas of Bunbury have the highest seismic amplification potential?
The highest amplification occurs in the estuarine muds and tidal flat deposits along the Leschenault Inlet, particularly around the Bunbury Port, Koombana Bay, and the lower Collie River floodplain. These areas commonly exhibit VS30 below 200 m/s and fundamental frequencies between 1.5 and 3 Hz, placing them in NEHRP site class E.
What normatives govern seismic microzonation in Australia?
The primary standards are AS 1726:2017 for site investigation methods, AS/NZS 1170.4:2007 for earthquake actions, and the NEHRP seismic provisions (FEMA P-1050) for site class definitions. For liquefaction screening we follow the NCEER/Youd-Idriss (2001) procedures. All MASW work references AS 1289.
How long does a microzonation campaign take for a medium-sized Bunbury project?
A typical campaign for a 10–15 ha estate takes 3–5 field days for MASW and HVSR surveys, followed by 2–3 weeks for data processing, velocity inversion, and report preparation. Timelines extend if deep boreholes or CPTs are needed to calibrate the velocity model.