Deep, Deeper, Deepest-NL - Imaging the Crust and Upper Mantle Structure of The Netherlands
Stephen Akinremi is a PhD student in the department Department of Applied Earth Sciences. (Co)Promotors are prof.dr. M. van der Meijde and dr. I.E.A.M. Fadel from the faculty of Geo-Information Science and Earth Observation (ITC), University of Twente and dr. E. Ruigrok from the Utrecht University.
Although the surface of the Netherlands appears flat, its subsurface is surprisingly complex. This hidden geology controls earthquake nucleation and propagation, influences deep geothermal potential, and determines suitability for large-scale applications such as CO2 storage. While the shallow subsurface (~5 km) is well mapped, deeper structure remains poorly known, yet crucial for understanding regional geology, resources exploration, and risk assessment.
To address this gap, this thesis imaged the crust and upper mantle velocity structure of the Netherlands using seismological data. A temporary array of 25 broadband seismometers filled data coverage gaps and improved imaging resolution from 50 km to 25 km. Receiver function (RF) method was used to retrieve the velocities (Vp/Vs) and depths of crustal discontinuities (deep sedimentary layers and crust-mantle boundary). Across sedimentary regions—such as the Netherlands, crust–mantle boundary signals can be masked by complex sedimentary phases. A new method based on waveform-fitting was developed to recover reliable crustal structure from RFs beneath sedimentary sequence. Complementary surface-wave tomography—Rayleigh and Love dispersion from ambient noise (2–30 s) and teleseismic earthquakes (20–120 s)—was used to constrain shear wave velocity (Vs) structure to 200 km depth.
The RF results show that crustal thickness varies by 8 km across the country, with relatively thicker crust (36 km) beneath structural highs in the south and thinner (28 km) beneath basins and platforms; the Vs model reproduces this pattern. Thinned crust beneath basins reflects rift-related stretching, while thicker crust suggests relative tectonic stability.
Two tectono-compositional domains emerge: RF-inferred bulk basement Vp/Vs is relatively lower (~1.68) beneath structural highs—consistent with more felsic composition—than beneath basins and platforms (~1.77). Elevated mid–deep crustal Vs beneath highs supports this interpretation: the southern Netherlands rests on more felsic Avalonian crust, while other areas reflect Caledonian-reworked terranes.
The RF and Vs models imaged deep-penetrating faults to ~30 km depth, demonstrating that major faults fundamentally shape present-day crustal architecture. Displacement of the Feldbiss and Peel Boundary faults is greater within the Roer Valley Graben (RVG) in southeast Netherlands, consistent with concentrated seismicity and hypocenters at 5–20 km.
A modest 2–3% reduction in Vs beneath RVG may indicate a weak upwelling or buoyant asthenosphere with limited magmatic input. The absence of strong mantle anomalies suggests the RVG rift is largely passive.
