Cardiff sits on a mix of glacial till, alluvial silts, and estuarine clays overlying Mercia Mudstone, with groundwater typically encountered between 2 m and 5 m depth across the city centre. This layered geology means seismic waves from distant sources can amplify significantly depending on the stiffness contrast between soft surface deposits and the bedrock below. A proper site response analysis in Cardiff must capture those impedance contrasts using shear-wave velocity profiles (VS30) and dynamic soil parameters. Before running the numerical model, we often recommend a microtremor HVSR survey to identify the fundamental natural frequency of the soil column, which gives a quick reality check on resonance risks for mid-rise buildings.
For Cardiff's alluvial deposits, ignoring soil amplification can underestimate spectral accelerations by 30 to 50 percent at mid-periods.