The northern suburbs of Mackay, like Andergrove and Beaconsfield, sit on Quaternary alluvium from the Pioneer River, while the city centre and Mount Pleasant rest on older residual soils and partially weathered rock. This contrast in sediment thickness produces very different seismic site responses. A high-rise in the CBD might see natural periods around 0.3 s, whereas a structure on deep alluvium near the river could experience peaks above 0.6 s. That is where the HVSR microtremor survey becomes essential. The Nakamura method records ambient noise from a single three-component seismometer to identify the fundamental resonance frequency of the soil column. We combine this data with MASW-Vs30 profiles to map Vs30 values across the city and with amplification analysis to quantify expected spectral amplification for each zone.
A single HVSR station delivers the fundamental frequency within hours, cutting site investigation time by half compared to a full Vs30 array.
Methodology and scope
We deployed an HVSR microtremor survey on a 10-storey residential project in North Mackay, where the client needed a quick yet reliable estimate of site period before the structural design phase. The procedure follows three steps. First, we place the tromograph on undisturbed ground away from traffic or machinery. Second, we record ambient vibrations for 30 to 60 minutes at each station. Third, we apply the Nakamura transfer function to separate horizontal from vertical spectral ratio. The results give the fundamental frequency f0 and the corresponding amplification level. For sites with softer deposits, we also run a dilatometer test to derive constrained modulus and compare with the resonance period. On compacted fills, a plate load test helps validate the bearing layer stiffness.
Technical reference image — Mackay
Local considerations
Mackay sits approximately 50 km from the Whitsunday seismic zone, where earthquake magnitudes up to M5.5 have been recorded historically, such as the 1935 event near Proserpine. The Pioneer Valley's deep alluvial sediments can amplify long-period ground motions significantly. A site without an HVSR microtremor survey may underestimate the true resonance period by a factor of two. We have measured f0 values as low as 0.35 Hz on 40 m of soft clay near the river mouth. If the building's natural frequency coincides with that value, even a moderate tremor can cause structural distress. The Nakamura method provides the first screening of site class for AS 1170.4 compliance.
Rapid resonance screening for preliminary design. Ideal for small to medium projects where a full Vs30 array is not justified.
02
HVSR array with Vs30 correlation
Multi-station grid covering up to 2 km². Data processed to generate site period maps and Vs30 estimates for AS 1170.4 classification.
03
Combined HVSR + MASW survey
Integration of ambient noise with active source surface wave testing for high-resolution vs30/" data-interlink="1">shear wave velocity profiles down to 50 m depth.
Applicable standards
SESAME guidelines (2004) for HVSR acquisition and processing, AS 1170.4-2007 (Earthquake actions) – site class assignment, NEHRP site classification for Vs30 correlation
Frequently asked questions
How does HVSR differ from a MASW survey?
HVSR uses only ambient noise (no active source) and identifies the fundamental resonance frequency of the soil column. MASW requires a sledgehammer or vibroseis to generate surface waves and produces a 1D vs30/" data-interlink="1">shear wave velocity profile. HVSR is faster and cheaper for site period screening; MASW gives depth-specific stiffness data.
What is the typical cost range for an HVSR microtremor survey in Mackay?
For a standard single-station survey with basic processing, the cost ranges between AU$2,100 and AU$3,590 depending on site access and number of stations. A grid of 10 to 15 stations with full reporting sits at the upper end of that range.
Does the Nakamura method work on rocky sites?
Yes, but the H/V ratio on rock is typically flat with no clear peak. That indicates a high-frequency site (f0 > 10 Hz) where amplification is negligible. In Mackay, bedrock sites near Eungella show flat spectra, while alluvial sites produce clear peaks between 0.3 and 1.5 Hz.
