Characterisation of seismic site effects influenced by near-surface structures using 3D waveform modelling

Abstract

Seismic waves propagating from source to surface can be significantly affected by vertical and lateral heterogeneities of the near-surface geology. These modified waves can lead to local amplification or attenuation effects of the wave amplitude at the surface. In the assessment of seismic events and the evaluation of possible impacts on infrastructures, the analysis of such site effects is an important issue. In this study, the amplification behaviour of typical near-surface structures in Northern Germany is investigated depending on various parameters to assemble a catalogue of typical structures in the area, which can be used to quickly assess the impact of induced seismic events at the surface. The effect of incoming seismic waves on simple model structures as layer, channel and lens geometries is analysed using 3D waveform simulations with dominant signal frequencies between 1 and 10 Hz based on the spectral element method. The dimensions of the modelled structures range from tens of metres to a few hundred metres. The parameter study shows the dependency of the resulting amplitude at the surface on the impedance contrast, geometrical features and the dominant signal frequency, which are quantified as amplification (attenuation) factors. Simple layer structures reveal low amplification values ranging from 1 to 3.5 and smooth amplification curves, which become more complex for channel and lens type structures while the maximum amplification increases by a factor of two. Focusing and interference effects could be observed depending on the structure type itself as well on the edge shape of the structure. Furthermore, the results of the simple model simulations are used in combination with additional borehole, geological and geophysical data to generate a first large-scale model of the area of the natural gas fields in Northern Germany. Waveform simulations of the large-scale model illustrate the influence of near-surface structures on wave propagation and allow to calculate the amplification behaviour at the surface. These (regional) model simulations can be essential for the reliable assessment of site effects and minimising uncertainties in the interpolation of the ground velocity distribution.