Effects of subsurface heterogeneity on ground motion amplification in Groningen, the Netherlands

Gas extraction from the Groningen field has induced a substantial number of earthquakes that, despite their typically low magnitudes, produce notable ground motions at the surface due to their shallow depths of approximately 3 km. These ground motions pose risks to society and infrastructure. Therefore, an accurate ground motion simulation is essential for seismic hazard assessment. Previous studies have demonstrated that near-surface unconsolidated layers significantly influence ground motion amplification. However, less attention has been devoted to understanding the role of deeper structures. In the Groningen region, significant amplification and de-amplification effects are anticipated due to the complex subsurface, thickness variations across relatively short lateral distances, compositional heterogeneity within sedimentary sequences, and the presence of the Zechstein salt layer overlying the reservoir formation.

This study investigates how subsurface heterogeneity, both shallow and deep, affects seismic wave propagation and the corresponding ground motion observed at the surface. To analyze this, we employ 3D full waveform modeling using the spectral element method (SEM). First, to optimize mesh resolution, determined by the local S-wave velocity and the target design frequency, we conduct simulations across a range of frequencies and corresponding spatial resolutions to analyze their impact on wavefield accuracy and computational cost. Second, we simulate seismic wave propagation through a synthetic velocity model representative of the Groningen subsurface and compute Peak Ground Acceleration (PGA) for different earthquake scenarios using various Centroid Moment Tensor (CMT) source solutions. Since amplification effects are highly location-dependent, we evaluate multiple earthquake scenarios with varying source characteristics and locations. We then compare these results with PGA values computed for a homogeneous half-space model that preserves the bulk elastic properties of the realistic heterogeneous model, using identical earthquake sources. This comparison produces amplification factor maps that reveal distinct spatial patterns of amplification and de-amplification across the study region. To isolate the contributions of individual factors, we examine the influence of source frequency, the depth and thickness of velocity layers, the presence of velocity inversions within the stratigraphic sequence, and subsurface interface topography.

These tests allow us to identify how each parameter contributes to the resulting amplification and de-amplification patterns. This framework can provide physical explanations for the spatial distribution of observed ground motion variations, offering valuable insights that are instrumental for current and future seismic hazard assessments in areas of subsurface resource exploitation throughout the Netherlands.