Characterisation of the Groningen subsurface for seismic hazard and risk modelling

Abstract

The shallow subsurface of Groningen, the Netherlands, is heterogeneous due to its formation in a Holocene tidal coastal setting on a periglacially and glacially inherited landscape with strong lateral variation in subsurface architecture. Soft sediments with low, small-strain shear wave velocities (V S30 around 200 m s −1 ) are known to amplify earthquake motions. Knowledge of the architecture and properties of the subsurface and the combined effect on the propagation of earthquake waves is imperative for the prediction of geohazards of ground shaking and liquefaction at the surface. In order to provide information for the seismic hazard and risk analysis, two geological models were constructed. The first is the 'Geological model for Site response in Groningen' (GSG model) and is based on the detailed 3D GeoTOP voxel model containing lithostratigraphy and lithoclass attributes.The GeoTOP model was combined with information from boreholes, cone penetration tests, regional digital geological and geohydrological models to cover the full range from the surface down to the base of the North Sea Supergroup (base Paleogene) at ∼800 m depth. The GSG model consists of a microzonation based on geology and a stack of soil stratigraphy for each of the 140,000 grid cells (100 m × 100 m) to which properties (V S and parameters relevant for nonlinear soil behaviour) were assigned. The GSG model serves as input to the site response calculations that feed into the Ground Motion Model. The second model is the 'Geological model for Liquefaction sensitivity in Groningen' (GLG). Generally, loosely packed sands might be susceptible to liquefaction upon earthquake shaking. In order to delineate zones of loosely packed sand in the first 40 m below the surface, GeoTOP was combined with relative densities inferred from a large cone penetration test database. The marine Naaldwijk and Eem Formations have the highest proportion of loosely packed sand (31% and 38%, respectively) and thus are considered to be the most vulnerable to liquefaction; other units contain 5-17% loosely packed sand. The GLG model serves as one of the inputs for further research on the liquefaction potential in Groningen, such as the development of region-specific magnitude scaling factors (MSF) and depth-stress reduction relationships (r d ).

Addressing limitations in existing ‘simplified’ liquefaction triggering evaluation procedures: application to induced seismicity in the Groningen gas field

“…The geological setting of Groningen, including detailed cross sections, is described in Kruiver et al (2017a), and the velocity model from the selected reference rock horizon (at ~ 800 m depth) to the ground surface is described in detail by Kruiver et al (2017b). An example of the resulting VS profiles is shown in Figure 9.…”

Developing a model for the prediction of ground motions due to earthquakes in the Groningen gas field

“…The work described in this paper has benefited from excellent databases available in the Groningen field. There is detailed characterisation of the shallow geology in the field (Kruiver et al, 2017a), which, in combination with numerous measurements, has enabled the development of reliable shear-wave velocity profiles that extend from the ground surface to the base of the North Sea supergroup at some 800m depth (Kruiver et al, 2017b). There are also extensive networks of seismic recording instruments in the field (Fig.…”

“…Using the velocity profiles from NS_B to the surface (Kruiver et al, 2017b) and input motions at this horizon, nonlinear site amplification factors were derived for all spectral accelerations (Rodriguez-Marek et al, 2017). Starting from a geological zonation of the field (Kruiver et al, 2017a), adjustments were made to the boundaries such that each zone represented a relatively consistent behaviour in terms of the amplification factors (Fig. 5).…”

From checking deterministic predictions to probabilities, scenarios and control loops for regulatory supervision

“…Of these, 10 events had a magnitude equal to or above 3.0, with 2 events equal to or above 3.5. Due to the soft Dutch soils (Kruiver et al, 2017), events with magnitudes as low as 1.8 have reportedly been felt. Events with magnitude 2.0 and higher have caused minor to severe non-structural damage.…”

“…In hindsight, a trend break occurred in 2003 after which seismicity occurred at a much higher rate. It is now recognised that the effects of the induced seismicity in Groningen are enhanced by its shallow source, a partly very fragile build environment never designed to sustain seismic ground motions and soft near-surface layers locally leading to upswing of the ground motions (Kruiver et al, 2017).…”