Abstract
AbstractThe 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 (VS30around 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 (VSand 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 (rd).
Highlights
The Groningen gas field production in the Dutch province of Groningen started in 1963
This paper describes two geological models: the ‘Geological model for Site response in Groningen’ (GSG model) and the ‘Geological model for Liquefaction sensitivity in Groningen’ (GLG model) respectively
The Groningenspecific magnitude scaling factors (MSF) and rd relationships will be used in conjunction with the ‘unbiased’ Cyclic Resistance Ratio (CRR) curve to evaluate liquefaction potential for Groningen profiles where cone penetration tests (CPT) have been performed. These evaluations will be performed within the seismic hazard and risk framework for Groningen, with the results being in the form of regional Liquefaction Potential Index (LPI) maps for different mean annual probabilities of exceedance
Summary
The Groningen gas field production in the Dutch province of Groningen started in 1963. A vast incised valley system was carved east of the Hondsrug during deglaciation Incision of this Hunze valley system occurred up to 60 m depth and was followed by the infill with coarse-grained and gravelly sediments (Drente Formation, Schaarsbergen Member; Figs 1A, B, 2B). The top of this unit gradually changes to medium- and fine-grained sands. During the Weichselian period, in particular during the period 60,000–15,000 years ago, the Hunze valley system was largely infilled with local river sediments consisting of medium- and fine-grained sands and intercalated loam and peat layers (Boxtel Formation; Figs 1A, 2D). The GeoTOP model in the Groningen region was constructed using ∼42,300 digital borehole descriptions from DINO, the national Dutch subsurface database operated by the Geological Survey. The maximum lateral extent of the Drente Formation, Gieten Member (glacial till) unit and the procedure to identify the unit in the borehole descriptions were largely based on the large-scale and detailed till model for the north of the Netherlands (Vernes et al, 2013)
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