The contrast between Galway's eastern limestone terraces and the deep, soft alluvium flanking the River Corrib creates a textbook case for site-specific seismic hazard. A stiff Ballyvaughan limestone outcrop in Knocknacarra amplifies motion differently than the compressible estuarine clays near the Docks, where a long-period basin effect can double the spectral acceleration at 1.5 seconds. Our seismic microzonation work quantifies these differences through direct shear-wave velocity profiling and one-dimensional equivalent-linear site response, mapping how the 270-million-year-old bedrock and the post-glacial drift will each shake during a design earthquake. We deliver the ground-motion maps and amplification factors that structural engineers need to apply the Irish National Annex to IS EN 1998-1:2005, moving beyond the default Type 2 spectrum toward a project-specific elastic response spectrum.
Galway's soft estuarine clays can amplify the 475-year bedrock motion by a factor of 2.3 at periods matching 4-to-6-storey buildings, a site effect that the default code spectrum misses entirely.
Process and scope
In Galway, we often observe that the standard rock spectrum overestimates short-period demand on granite while underestimating mid-period amplification on the lacustrine clays between Terryland and the University Hospital campus. A proper
in situ campaign typically combines multichannel surface-wave testing with downhole seismic in at least two boreholes per geological unit, capturing the shear-wave velocity gradient from the weathered upper crust down to the intact rock. When the site straddles a buried channel feature, the velocity contrast across the bedrock interface can exceed 400 m/s within less than two metres, a transition that linear-equivalent codes like SHAKE or DEEPSOIL resolve poorly without site-specific modulus reduction curves. We therefore run laboratory
triaxial cyclic tests on undisturbed Shelby samples to calibrate the G/G
max and damping curves used in the non-linear site-response analysis. For sites near the shoreline where liquefiable silts may be present, the microzonation integrates a
liquefaction susceptibility screening based on the updated Boulanger & Idriss (2014) triggering procedure, ensuring the final hazard map flags zones where the factor of safety drops below 1.1 under the 475-year return period motion.
Local ground factors
A Geotomographie PS-logging string lowered into a cased borehole near the Spanish Arch picks up the sharp velocity jump where the Corrib alluvium meets the granite basement at 18 metres depth. That single interface controls how much energy gets trapped in the overlying soft layer. If the microzonation ignores a lateral pinch-out of that clay lens, the ground-motion prediction can miss a 40% variation in spectral acceleration between two pads only 60 metres apart. The consequence is a structure designed with the wrong foundation period, potentially drifting into resonance with the site period. Galway's glacial history has left chaotic stratigraphy: drumlin till, esker sand, and lacustrine silt can alternate in a single cross-section. Our approach maps the fundamental site period T0 = 4H/Vs across the footprint and flags any location where the soil class transitions, so the structural engineer can segment the foundation design accordingly.
Reference standards
IS EN 1998-1:2005 + Irish National Annex (Eurocode 8, seismic design), IS EN 1998-5:2005 (foundations, retaining structures), IS EN 1997-2:2007 + Irish National Annex (ground investigation, Eurocode 7), IAEG / ISSMGE guidelines for seismic microzonation mapping
Frequently asked questions
How much does a seismic microzonation study cost for a typical Galway development?
For a project site between 0.5 and 2 hectares with two to three boreholes and surface geophysics, the fee usually falls between €4,360 and €13,280. The spread depends on the number of geological units, whether laboratory cyclic testing is needed, and the complexity of the basin geometry requiring 2D modelling.
Do I need microzonation if Galway is a low-seismicity region?
Even under low seismicity, the Irish National Annex to Eurocode 8 requires site-specific assessment when soft soil deposits thicker than 10 m are present. Galway's estuarine clays and alluvial sands can amplify modest bedrock motion to levels that control the design of taller or period-sensitive structures, so a microzonation often becomes the rational basis for the geotechnical design report.
What is the difference between a seismic refraction survey and a full microzonation?
Seismic refraction gives a P-wave velocity model of the near-surface, which is useful for rippability and depth to bedrock, but it does not deliver the shear-wave velocity profile or the site amplification function. A microzonation integrates Vs measurements, laboratory dynamic soil properties, and site-response analysis to produce the ground-motion parameters that structural engineers input directly into their models.
